Human Reproduction and Homeostasis

Human Reproductive System

Male Reproductive System

Structure	Function
Testes	Produce sperm (in seminiferous tubules) and testosterone (in Leydig/interstitial cells)
Scrotum	Holds testes outside the body cavity at approximately 2-3 degrees C below core body temperature for viable sperm production
Epididymis	Coiled tube on the surface of each testis; stores and matures sperm
Vas deferens	Muscular tube that carries sperm from the epididymis to the urethra during ejaculation
Seminal vesicles	Produce seminal fluid rich in fructose (energy source for sperm) and prostaglandins
Prostate gland	Produces alkaline fluid that neutralises vaginal acidity; contains enzymes to liquefy semen
Bulbourethral gland (Cowper's gland)	Produces mucus for lubrication and to neutralise residual urine in urethra
Urethra	Shared passageway for semen (during ejaculation) and urine (during urination)
Penis	Erectile organ that delivers semen into the vagina during intercourse

Semen composition:

• Sperm (approximately 5% of volume)
• Seminal vesicle fluid (approximately 60%) -- fructose, prostaglandins
• Prostate fluid (approximately 30%) -- alkaline, citrate, enzymes
• Bulbourethral fluid (small volume) -- mucus

Sperm structure:

Part	Description
Head	Contains the haploid nucleus (23 chromosomes); acrosome (vesicle containing hydrolytic enzymes to penetrate the zona pellucida of the egg)
Middle piece	Packed with mitochondria arranged in a helix; produce ATP via aerobic respiration to power tail movement
Tail (flagellum)	Whip-like structure for locomotion; propels the sperm through the female reproductive tract

Female Reproductive System

Structure	Function
Ovaries	Produce ova (eggs) in follicles; produce hormones oestrogen and progesterone
Oviduct (fallopian tube)	Carries the ovum from the ovary to the uterus; site of fertilisation; lined with ciliated epithelium to move the ovum
Uterus (womb)	Site of implantation and embryonic/fetal development; thick, muscular walls
Endometrium	Inner lining of the uterus; thickens and becomes vascularised during the menstrual cycle; shed during menstruation if no implantation
Cervix	Ring of muscle at the lower end of the uterus; produces mucus that changes consistency during the cycle
Vagina	Receives the penis during intercourse; birth canal during delivery; acidic environment (pH approximately 4) to inhibit pathogens
Vulva	External female genitalia

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Gametogenesis

Spermatogenesis

Spermatogenesis is the production of sperm in the seminiferous tubules of the testes. It begins at puberty and continues throughout life.

Process:

1. Mitosis of germ cells: Spermatogonia (diploid, 2n = 46) divide by mitosis to produce more spermatogonia (maintaining the stem cell population) and primary spermatocytes.

2. Meiosis I: Each primary spermatocyte (2n) undergoes meiosis I to produce two secondary spermatocytes (n, each with 23 chromosomes consisting of two chromatids).

3. Meiosis II: Each secondary spermatocyte undergoes meiosis II to produce two spermatids (n, each with 23 single-chromatid chromosomes). Total: 4 spermatids from one primary spermatocyte.

4. Spermiogenesis: Spermatids undergo differentiation to become mature spermatozoa. The nucleus condenses, the acrosome forms, mitochondria concentrate in the middle piece, and the tail develops.

5. Spermiation: Mature sperm are released from the Sertoli cells (nurse cells) into the lumen of the seminiferous tubule.

Role of Sertoli cells:

• Provide nourishment for developing sperm
• Phagocytose excess cytoplasm shed by developing sperm
• Form the blood-testis barrier (prevents immune system from attacking sperm, which express unique surface antigens)
• Produce inhibin (negative feedback on FSH secretion)

Duration: Approximately 74 days from spermatogonium to mature sperm.

Oogenesis

Oogenesis is the production of ova (eggs) in the ovaries. It begins during foetal development and is completed at fertilisation.

Process:

1. Foetal development: Oogonia (diploid, 2n = 46) divide by mitosis to produce primary oocytes (2n). Primary oocytes begin meiosis I but arrest at prophase I. A female is born with all the primary oocytes she will ever have (approximately 1-2 million). Each primary oocyte is surrounded by a layer of follicular cells, forming a primordial follicle.

2. Puberty to menopause: Each month (during the menstrual cycle), one (or occasionally more) primary oocyte resumes meiosis I, producing a secondary oocyte (n) and the first polar body (n). The secondary oocyte receives almost all the cytoplasm (unequal cytokinesis). The polar body is small and eventually degenerates.

3. Ovulation: The secondary oocyte begins meiosis II but arrests at metaphase II. It is released from the ovary (ovulation).

4. Fertilisation: If the secondary oocyte is fertilised by a sperm, meiosis II is completed, producing the ovum (n) and a second polar body. The sperm nucleus fuses with the ovum nucleus to form the zygote (2n).

Duration: Oogenesis from oogonium to ovulation takes approximately 12-50 years (arrested for decades). Meiosis II is completed in minutes to hours after fertilisation.

Comparison of Spermatogenesis and Oogenesis

Feature	Spermatogenesis	Oogenesis
Location	Seminiferous tubules of testes	Ovaries
Timing	Begins at puberty; continuous throughout life	Begins during foetal development; arrested at prophase I
Number of gametes produced	4 sperm from one primary spermatocyte	1 ovum from one primary oocyte (+ 2-3 polar bodies)
Cytokinesis	Equal (four equal-sized cells)	Unequal (one large ovum, small polar bodies)
Completion	Completed in approximately 74 days	Completed only at fertilisation (meiosis II)
Onset of meiosis II	Immediate (no arrest)	Arrested at metaphase II until fertilisation
Result	Four functional sperm	One functional ovum
Mitochondrial contribution	Mitochondria in middle piece of sperm	Ovum provides all mitochondria to zygote

Worked Example

Starting with one primary spermatocyte and one primary oocyte, both of which complete meiosis, how many functional gametes are produced in total? Explain the difference in outcomes.

Solution

• Spermatogenesis: One primary spermatocyte undergoes meiosis I (producing 2 secondary spermatocytes) then meiosis II (producing 4 spermatids). All 4 spermatids differentiate into functional sperm. Total: 4 functional gametes.

• Oogenesis: One primary oocyte undergoes meiosis I (producing 1 secondary oocyte + 1 polar body). The secondary oocyte undergoes meiosis II (producing 1 ovum + 1 second polar body). The first polar body may also divide. Total: 1 functional gamete (ovum) + 2--3 polar bodies.

• Difference: Cytokinesis is equal in spermatogenesis (4 equal-sized cells) but unequal in oogenesis (almost all cytoplasm goes to the ovum). The ovum needs abundant cytoplasm and nutrients to support early embryonic development, so polar bodies receive minimal cytoplasm and degenerate.

• Arrest points in oogenesis: Unlike spermatogenesis where meiosis proceeds without arrest, oogenesis has two arrest points: prophase I (from foetal development until puberty, lasting decades) and metaphase II (from ovulation until fertilisation, lasting approximately 24 hours or until the secondary oocyte degenerates). This means a single cycle of oogenesis can span decades, while spermatogenesis takes approximately 74 days.

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The Menstrual Cycle and Hormonal Control

Overview

The menstrual cycle is approximately 28 days and involves coordinated changes in the ovaries and the uterus, controlled by four hormones.

The Four Key Hormones

Hormone	Source	Function
FSH	Anterior pituitary gland	Stimulates follicle development in the ovary; stimulates the ovaries to produce oestrogen
LH	Anterior pituitary gland	Triggers ovulation (LH surge on day 14); stimulates the corpus luteum to develop and secrete progesterone
Oestrogen	Developing follicle (ovary)	Stimulates endometrium to thicken and become vascularised; inhibits FSH at low levels (negative feedback); stimulates LH at high levels (positive feedback)
Progesterone	Corpus luteum (ovary)	Maintains the thickened endometrium; inhibits FSH and LH secretion (negative feedback)

Phases of the Menstrual Cycle

1. Menstruation (Days 1-5):

• If implantation has not occurred, the corpus luteum degenerates
• Progesterone and oestrogen levels drop sharply
• The endometrium (thickened lining) is shed, accompanied by bleeding
• FSH levels begin to rise

2. Follicular phase (Days 1-13):

• FSH stimulates the development of several follicles in the ovary
• The follicles produce oestrogen
• Oestrogen stimulates the endometrium to thicken (proliferative phase)
• Low oestrogen levels exert negative feedback on the pituitary, keeping FSH and LH relatively low
• As the follicle matures and oestrogen rises above a threshold, it switches to positive feedback on the pituitary
• High oestrogen triggers a surge in LH (and a smaller surge in FSH)

3. Ovulation (Day 14):

• The LH surge causes the mature follicle (Graafian follicle) to rupture
• The secondary oocyte is released from the ovary into the oviduct
• The fimbriae of the oviduct sweep the ovum into the tube

4. Luteal phase (Days 15-28):

• The ruptured follicle transforms into the corpus luteum
• The corpus luteum secretes progesterone (and some oestrogen)
• Progesterone maintains the thickened, vascularised endometrium (secretory phase)
• Progesterone inhibits FSH and LH (negative feedback), preventing new follicle development
• If fertilisation does not occur: the corpus luteum degenerates (approximately day 25-28), progesterone drops, and the cycle restarts
• If fertilisation occurs: the embryo produces hCG (human chorionic gonadotropin), which maintains the corpus luteum

Hormonal Interactions

Negative feedback (low oestrogen and progesterone):

• Low levels of oestrogen inhibit FSH and LH secretion from the anterior pituitary
• Progesterone inhibits FSH and LH secretion
• This prevents multiple ovulations and ensures only one follicle develops

Positive feedback (high oestrogen):

• When oestrogen concentration rises above a critical threshold (around day 12-13), it stimulates (rather than inhibits) the anterior pituitary to secrete LH
• This creates the LH surge that triggers ovulation
• The LH surge is a self-amplifying cycle: more LH causes more oestrogen release, which causes more LH release, until ovulation occurs and the follicle is ruptured (breaking the feedback loop)

info

The switch from negative to positive feedback by oestrogen is a critical concept. At low concentrations, oestrogen inhibits the pituitary (negative feedback). At high concentrations sustained for approximately 36 hours, oestrogen stimulates the pituitary (positive feedback), causing the LH surge. The DSE frequently tests this distinction.

Worked Example: Hormone Levels at Day 20

A blood test shows the following hormone levels in a 28-day menstrual cycle:

• Oestrogen: moderate
• Progesterone: high
• FSH: low
• LH: low

What stage of the menstrual cycle is this, and explain the relationships between these hormone levels?

Solution

This is the luteal phase (days 15-28). The high progesterone is produced by the corpus luteum, which formed after ovulation. The moderate oestrogen is also secreted by the corpus luteum.

The high progesterone (and oestrogen) exerts negative feedback on the anterior pituitary, inhibiting FSH and LH secretion. This prevents new follicle development and ensures no second ovulation occurs during the current cycle.

Key relationships:

• High progesterone indicates the corpus luteum is active
• Low FSH and LH are a consequence of negative feedback from progesterone and oestrogen
• This contrasts with the follicular phase, where FSH is rising and stimulating follicle development, and with day 14, where the oestrogen-triggered LH surge causes ovulation

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Fertilisation and Implantation

Fertilisation

Fertilisation is the fusion of a sperm nucleus with an ovum nucleus, forming a diploid zygote (2n = 46). It occurs in the oviduct (ampulla region), typically within 12-24 hours after ovulation.

Process:

1. Capacitation: Sperm undergo biochemical changes in the female reproductive tract (removal of cholesterol from the sperm membrane increases membrane fluidity and prepares the sperm for the acrosome reaction).

2. Chemotaxis: Sperm are attracted to chemical signals released by the ovum and surrounding follicular cells.

3. Acrosome reaction: The sperm binds to the zona pellucida (glycoprotein layer surrounding the ovum). The acrosome releases hydrolytic enzymes (acrosin, hyaluronidase) that digest the zona pellucida, allowing the sperm to penetrate.

4. Penetration of the zona pellucida: The sperm penetrates the zona pellucida and reaches the oocyte membrane (vitelline membrane).

5. Fusion of membranes: The sperm membrane fuses with the oocyte membrane.

6. Cortical reaction: The ovum releases cortical granules (vesicles) that modify the zona pellucida, making it impermeable to other sperm. This prevents polyspermy (fertilisation by more than one sperm).

7. Completion of meiosis II: The secondary oocyte completes meiosis II, producing the ovum and the second polar body.

8. Fusion of nuclei: The sperm nucleus and ovum nucleus fuse, forming the diploid zygote (2n = 46).

Early Development and Implantation

1. Cleavage divisions: The zygote divides by mitosis as it travels along the oviduct towards the uterus. Cell divisions are rapid but the total volume does not increase (cleavage).

2. Morula: By approximately day 3-4, the embryo is a solid ball of approximately 16 cells (morula).

3. Blastocyst: By approximately day 5-6, a fluid-filled cavity (blastocoel) forms inside, creating the blastocyst. The blastocyst has two parts:

   • Trophoblast: Outer layer of cells (will form the placenta)
   • Inner cell mass: Group of cells at one end (will form the embryo)

4. Implantation: The blastocyst implants into the thickened, vascularised endometrium (approximately day 6-7 after fertilisation). The trophoblast cells secrete enzymes that digest the endometrium, allowing the blastocyst to embed.

Placenta and Umbilical Cord

Placenta structure:

The placenta is a temporary organ formed from both maternal (endometrium) and foetal (trophoblast) tissues. It is the interface for exchange of substances between maternal and foetal blood.

Key features:

• Chorionic villi: finger-like projections of the trophoblast that extend into the maternal blood-filled spaces (intervillous spaces). This maximises the surface area for exchange.
• Maternal blood and foetal blood do NOT mix (they are separated by the placental barrier).
• The placental barrier consists of: foetal capillary endothelium, connective tissue, and trophoblast epithelium.

Functions of the placenta:

Function	Description
Gas exchange	$\mathrm{O}_2$ diffuses from maternal blood to foetal blood; $\mathrm{CO}_2$ diffuses from foetal blood to maternal blood
Nutrient transfer	Glucose, amino acids, fatty acids, vitamins, and minerals diffuse from maternal to foetal blood
Waste removal	Urea, uric acid, and creatinine diffuse from foetal blood to maternal blood for excretion
Hormone production	Produces progesterone (maintains endometrium), oestrogen, hCG (maintains corpus luteum), relaxin, and human placental lactogen
Antibody transfer	IgG antibodies cross the placenta, providing passive immunity to the foetus
Barrier	Prevents some pathogens and harmful substances from reaching the foetus (though alcohol, drugs, and some viruses can cross)

Umbilical cord:

• Contains two umbilical arteries (carry deoxygenated blood from the foetus to the placenta) and one umbilical vein (carries oxygenated blood from the placenta to the foetus)
• Wrapped in Wharton's jelly (connective tissue that protects the blood vessels from compression)

warning

A critical point for DSE: the umbilical ARTERY carries deoxygenated blood (away from the foetus to the placenta) and the umbilical VEIN carries oxygenated blood (from the placenta to the foetus). This is the OPPOSITE of the naming convention in the systemic circulation, where arteries carry oxygenated blood. The naming is based on direction of flow relative to the heart: arteries carry blood away from the heart, veins carry blood towards it. The umbilical arteries carry blood away from the foetal heart.

Worked Example

A pregnant woman's blood glucose is consistently elevated at 200 mg/100 cm cubed due to uncontrolled gestational diabetes. Explain the effects on the foetus, with reference to placental exchange and foetal hormonal responses.

Solution

1. Glucose transfer across the placenta: Glucose crosses the placenta from maternal blood to foetal blood by facilitated diffusion down its concentration gradient. With maternal blood glucose at 200 mg/100 cm cubed and normal foetal blood glucose at approximately 90 mg/100 cm cubed, the steep gradient drives excessive glucose transfer to the foetus.

2. Foetal blood glucose rises: The foetus receives more glucose than normal, causing foetal hyperglycaemia.

3. Foetal insulin response: The foetal pancreas responds to high blood glucose by secreting insulin (beta cells of the islets of Langerhans). Insulin stimulates foetal cells to take up glucose and convert it to glycogen and fat.

4. Effects on the foetus:

   • Macrosomia (excessive growth): High glucose plus high insulin promotes fat deposition and growth. The baby may be born significantly larger than normal (birth weight > 4 kg).
   • Neonatal hypoglycaemia: After delivery, the maternal glucose supply is cut off, but the baby continues producing high insulin temporarily. This causes a rapid drop in blood glucose after birth, which can be dangerous if untreated.
   • Respiratory distress: High insulin can inhibit surfactant production in foetal lungs, leading to breathing difficulties after birth.

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Homeostasis Principles

Definition

Homeostasis is the maintenance of a constant internal environment within narrow limits, despite changes in the external environment. It is essential for the proper functioning of enzymes and cellular processes.

Key conditions regulated by homeostasis:

• Body temperature (approximately 37 degrees C)
• Blood glucose concentration (approximately 90 mg/100 cm cubed)
• Blood water potential (osmotic balance)
• Blood pH (approximately 7.35-7.45)
• $\mathrm{CO}_2$ concentration in the blood
• Ion concentrations (Na$^+$, K$^+$, Ca$^{2+}$)

Negative Feedback

Negative feedback is the primary mechanism of homeostasis. When a parameter deviates from its set point, the body initiates a response that reverses the deviation and restores the parameter to its normal range.

General pattern:

1. Stimulus: A change in the internal environment (e.g., body temperature rises)
2. Receptor: Detects the change and sends signals to a control centre
3. Control centre: Processes the information and sends signals to effectors
4. Effector: Carries out a response that counteracts the change
5. Return to normal: The parameter returns to the set point, and the response is switched off

The term "negative" refers to the fact that the response opposes (negates) the original stimulus.

info

Homeostasis maintains parameters within a normal range, not at a single fixed value. The set point is a target value, and the actual value fluctuates within narrow limits around it. In DSE exam answers, write "maintained within narrow limits" rather than "kept exactly constant."

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Thermoregulation

The Thermoregulatory Centre

The hypothalamus contains the body's thermoregulatory centre. It receives input from:

• Central thermoreceptors: In the hypothalamus itself, monitoring blood temperature
• Peripheral thermoreceptors: In the skin (dermis) and mucous membranes, monitoring external temperature

Responses to Heat (When Body Temperature Rises)

Physiological responses (involuntary):

Response	Mechanism
Vasodilation	Arterioles supplying the skin dilate; more blood flows near the surface; heat is lost by radiation
Sweating	Sweat glands secrete sweat onto the skin surface; water evaporates, absorbing latent heat from the body (approximately 2.4 kJ per gram of water evaporated)
Increased respiration	Breathing rate increases; more heat is lost through exhaled air
Decreased metabolic rate	Body reduces heat production by lowering the rate of metabolic reactions
Piloerection (relaxed)	Hair lies flat, reducing the insulating air layer near the skin (less significant in humans than in furry animals)

Responses to Cold (When Body Temperature Drops)

Response	Mechananism
Vasoconstriction	Arterioles supplying the skin constrict; less blood flows near the surface; heat is conserved
Shivering	Rapid, involuntary muscle contractions; generates heat from increased metabolic rate
Increased metabolic rate	Adrenaline and thyroxine are released; increases cellular respiration and heat production
Piloerection (erect)	Hair stands erect, trapping a layer of insulating air near the skin (goosebumps in humans)
Behavioural responses	Curling up to reduce surface area; putting on clothes; seeking warmth

Temperature Regulation Summary Table

Condition	Receptor Signal	Effectors Activated	Response	Effect on Temperature
Too hot	Thermoreceptors	Skin arterioles, sweat glands, muscles	Vasodilation, sweating, reduced metabolism	Temperature decreases
Too cold	Thermoreceptors	Skin arterioles, skeletal muscles, adrenal medulla	Vasoconstriction, shivering, increased thyroxine/adrenaline	Temperature increases

Worked Example

On a hot summer day, a person exercises vigorously outdoors and their body temperature rises to 39.5 degrees C. Describe the negative feedback mechanisms that restore body temperature to normal, and explain why sweating alone may not be sufficient during intense exercise.

Solution

Negative feedback responses to the rise in body temperature:

1. Detection: Central thermoreceptors in the hypothalamus detect the rise in blood temperature (above the set point of 37 degrees C). Peripheral thermoreceptors in the skin also detect the high external temperature.

2. Vasodilation: The hypothalamus sends nerve impulses to the arterioles supplying the skin, causing them to dilate. More blood flows near the skin surface, and heat is lost from the blood by radiation to the surrounding air.

3. Sweating: The hypothalamus stimulates sweat glands to secrete sweat onto the skin surface. As water in sweat evaporates, it absorbs latent heat of vaporisation (approximately 2.4 kJ per gram) from the body surface, cooling the skin and the blood flowing through it.

4. Increased respiration: Breathing rate increases, so more heat is lost from the body in exhaled warm, moist air.

5. Decreased metabolic rate: The hypothalamus reduces the stimulation of metabolic heat production, reducing the body's internal heat generation.

6. Return to normal: As body temperature falls back towards 37 degrees C, the thermoreceptors detect the change and the hypothalamus reduces its signals to the effectors. Vasodilation decreases, sweating slows, and body temperature stabilises.

Why sweating alone may not be sufficient during intense exercise:

During vigorous exercise, skeletal muscle respiration generates large amounts of heat as a by-product. This heat production can exceed the rate of heat loss through sweating alone, especially in humid conditions where evaporation is slower (the air is already saturated with water vapour). The body may also be losing water and electrolytes through sweat faster than they are replaced, eventually reducing sweat production and the ability to cool. In extreme cases, this can lead to heat stroke (body temperature exceeds 40 degrees C, which is a medical emergency).

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Blood Glucose Regulation

Normal Blood Glucose Level

Blood glucose is maintained at approximately 90 mg/100 cm cubed (approximately 5 mmol/L). This concentration must be tightly regulated because:

• The brain relies almost exclusively on glucose as its energy source
• Excessively high blood glucose (hyperglycaemia) damages blood vessels, nerves, and kidneys
• Excessively low blood glucose (hypoglycaemia) impairs brain function, leading to confusion, coma, and death

Hormonal Control

Insulin (produced by beta cells of the islets of Langerhans in the pancreas):

• Released when blood glucose rises above the set point (e.g., after a meal)
• Effects:
  • Increases the permeability of cell membranes to glucose (especially liver and muscle cells)
  • Stimulates glycogenesis (conversion of glucose to glycogen for storage in the liver and muscles)
  • Increases the rate of glucose respiration in cells
  • Inhibits gluconeogenesis (conversion of amino acids/fats to glucose in the liver)
  • Inhibits glycogenolysis (breakdown of glycogen to glucose)
• Net effect: Lowers blood glucose concentration

Glucagon (produced by alpha cells of the islets of Langerhans in the pancreas):

• Released when blood glucose falls below the set point (e.g., between meals, during exercise)
• Effects:
  • Stimulates glycogenolysis (conversion of glycogen to glucose in the liver)
  • Stimulates gluconeogenesis (conversion of amino acids and glycerol to glucose in the liver)
• Net effect: Raises blood glucose concentration

Adrenaline (produced by the adrenal medulla in the "fight or flight" response):

• Released during stress, exercise, or danger
• Stimulates glycogenolysis in the liver and muscles
• Raises blood glucose rapidly to provide energy for action

Negative Feedback Loop for Blood Glucose

When blood glucose rises (e.g., after eating carbohydrates):

1. Glucose is absorbed from the small intestine into the blood
2. Blood glucose concentration rises above 90 mg/100 cm cubed
3. Beta cells in the islets of Langerhans detect the rise
4. Beta cells secrete insulin into the blood
5. Insulin stimulates liver and muscle cells to take up glucose and convert it to glycogen
6. Blood glucose concentration decreases back towards normal
7. As blood glucose falls, insulin secretion decreases (negative feedback)

When blood glucose falls (e.g., during fasting or exercise):

1. Cells continue to respire glucose, and no new glucose is absorbed from the gut
2. Blood glucose concentration falls below 90 mg/100 cm cubed
3. Alpha cells in the islets of Langerhans detect the fall
4. Alpha cells secrete glucagon into the blood
5. Glucagon stimulates the liver to convert glycogen to glucose (glycogenolysis) and to produce glucose from amino acids (gluconeogenesis)
6. Blood glucose concentration increases back towards normal
7. As blood glucose rises, glucagon secretion decreases (negative feedback)

Glycogenesis, Glycogenolysis, and Gluconeogenesis

Process	Definition	Location	Hormone
Glycogenesis	Conversion of glucose to glycogen (storage)	Liver, muscles	Insulin
Glycogenolysis	Conversion of glycogen to glucose-6-phosphate (then to free glucose in the liver)	Liver, muscles	Glucagon, adrenaline
Gluconeogenesis	Synthesis of glucose from non-carbohydrate precursors (amino acids, lactate, glycerol)	Liver only	Glucagon

warning

A critical distinction: glycogenolysis occurs in both liver and muscle, but only the liver can release free glucose into the blood. Muscle glycogenolysis produces glucose-6-phosphate, which is used directly by the muscle for respiration. Muscle lacks the enzyme glucose-6-phosphatase, which is required to convert glucose-6-phosphate to free glucose. The DSE often tests this distinction.

Worked Example

A student runs a 400-metre sprint. During the race, her blood glucose initially rises slightly, then begins to decrease. Explain the hormonal and metabolic changes that occur before, during, and after the race.

Solution

Before the race (anticipation):

• The sympathetic nervous system is activated by anticipation of exercise.
• Adrenaline is released from the adrenal medulla.
• Adrenaline stimulates glycogenolysis in the liver and muscles.
• The liver releases glucose into the blood, causing a slight rise in blood glucose.

During the race (intense exercise):

• Muscle cells take up glucose and use it for aerobic and anaerobic respiration.
• Blood glucose decreases as muscles consume glucose faster than the liver supplies it.
• As blood glucose drops below the set point, alpha cells detect the change and secrete glucagon.
• Glucagon stimulates glycogenolysis and gluconeogenesis in the liver to replenish blood glucose.
• Adrenaline continues to promote glycogenolysis.
• Insulin secretion is suppressed (adrenaline and sympathetic activity inhibit insulin release).

After the race (recovery):

• Adrenaline levels drop as sympathetic nervous system activity decreases.
• Glucagon continues to maintain blood glucose as glycogen stores are replenished.
• As blood glucose rises, insulin secretion resumes, promoting glycogenesis in the liver and muscles.
• Blood glucose returns to the normal range as uptake and production are rebalanced.

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Osmoregulation

The Kidneys

The kidneys are the primary osmoregulatory organs. They regulate blood water potential by adjusting the volume and concentration of urine produced.

Gross structure of the kidney:

Region	Description
Cortex	Outer region; contains Bowman's capsules, proximal and distal convoluted tubules
Medulla	Inner region; contains the loops of Henle and collecting ducts; highly concentrated tissue fluid
Renal pelvis	Central cavity; collects urine from the collecting ducts
Ureter	Carries urine from the renal pelvis to the bladder

The Nephron

The nephron is the functional unit of the kidney. Each kidney contains approximately one million nephrons.

Structure:

Component	Location	Function
Renal (Bowman's) capsule	Cortex	Cup-shaped structure surrounding the glomerulus; receives filtrate
Glomerulus	Cortex	Knot of capillaries; site of ultrafiltration
Proximal convoluted tubule (PCT)	Cortex	Reabsorption of all glucose, all amino acids, most water, most Na$^+$, Cl$^-$ ions
Loop of Henle	Medulla	Countercurrent multiplier; creates a sodium gradient in the medulla for water reabsorption
Distal convoluted tubule (DCT)	Cortex	Selective reabsorption of ions; water permeability controlled by ADH
Collecting duct	Medulla	Final water reabsorption controlled by ADH; delivers urine to the renal pelvis

Ultrafiltration

Ultrafiltration occurs at the glomerulus and Bowman's capsule.

Mechanism:

1. The afferent arteriole (supplying the glomerulus) has a wider diameter than the efferent arteriole (draining it).
2. This creates high hydrostatic pressure in the glomerular capillaries.
3. This pressure forces small molecules through the filtration barrier (capillary endothelium, basement membrane, and podocytes of the Bowman's capsule) into the capsular space.
4. Large molecules (proteins, blood cells) and medium molecules cannot pass through the filter and remain in the blood.

What is filtered (in the glomerular filtrate):

Water, glucose, amino acids, urea, salts (Na$^+$, K$^+$, Cl$^-$, Ca$^{2+}$, HCO$_3^-$), vitamins, small waste products.

What is NOT filtered:

Large proteins (e.g., albumin), red blood cells, white blood cells, platelets.

Glomerular filtration rate (GFR): Approximately 125 cm cubed/min, or approximately 180 litres per day. Since the average daily urine output is approximately 1.5 litres, approximately 99% of the filtrate is reabsorbed.

Selective Reabsorption

Proximal convoluted tubule (PCT):

The PCT reabsorbs approximately 85% of the glomerular filtrate.

Substance	Percentage Reabsorbed	Mechanism
Glucose	100%	Co-transport with Na$^+$ (secondary active transport)
Amino acids	100%	Co-transport with Na$^+$ (secondary active transport)
Water	Approximately 85%	Osmosis (follows reabsorbed solutes)
Na$^+$	Approximately 85%	Active transport and co-transport
Cl$^-$	Approximately 85%	Following Na$^+$ electrochemical gradient
Urea	Approximately 50%	Diffusion (partially reabsorbed)

The PCT has numerous microvilli (increasing surface area) and many mitochondria (providing ATP for active transport).

Loop of Henle:

The loop of Henle acts as a countercurrent multiplier, maintaining a high solute concentration (high osmotic gradient) in the medulla.

Limb	Permeability	Effect
Descending limb	Permeable to water; impermeable to salts	Water moves out by osmosis into the increasingly concentrated medulla; filtrate becomes more concentrated
Ascending limb	Impermeable to water; actively transports Na$^+$ and Cl$^-$ out	Na$^+$ and Cl$^-$ are pumped into the medullary tissue fluid; filtrate becomes more dilute

The countercurrent flow (descending and ascending limbs running in opposite directions) amplifies the osmotic gradient. The medullary tissue fluid reaches approximately 1200 mOsm/kg at the bottom of the loop (compared to approximately 300 mOsm/kg in the cortex).

Collecting duct:

Water reabsorption in the collecting duct is controlled by antidiuretic hormone (ADH).

The Role of ADH

Antidiuretic hormone (ADH, vasopressin) is produced by neurosecretory cells in the hypothalamus and released from the posterior pituitary gland.

Negative feedback mechanism:

1. Stimulus: Blood water potential decreases (blood becomes too concentrated, e.g., after sweating or insufficient water intake)
2. Receptor: Osmoreceptors in the hypothalamus detect the decrease in water potential
3. Control centre: The hypothalamus stimulates the posterior pituitary to release ADH into the blood
4. Effector: ADH binds to receptors on the collecting duct cells, triggering the insertion of aquaporin (water channel) proteins into the cell membrane. The collecting duct becomes more permeable to water.
5. Response: Water is reabsorbed from the collecting duct into the concentrated medullary tissue fluid by osmosis. Urine becomes more concentrated and lower in volume.
6. Return to normal: Blood water potential increases back to normal. The osmoreceptors are no longer stimulated. ADH secretion decreases. The collecting duct becomes less permeable to water. Less water is reabsorbed, and urine becomes more dilute.

Condition	Blood Water Potential	ADH Level	Urine Volume	Urine Concentration
Dehydrated	Low	High	Low	High
Normal hydration	Normal	Normal	Normal	Normal
Excess water intake	High	Low	High	Low

Worked Example

A person takes part in a marathon on a hot day. They drink water at regular intervals but still become mildly dehydrated by the end of the race. Compare the osmoregulatory responses that would occur at three points: (a) at the start of the race when well hydrated, (b) mid-race when mildly dehydrated, and (c) after the race when they drink a large volume of water.

Solution

(a) At the start -- well hydrated (normal blood water potential):

• Blood water potential is normal; osmoreceptors in the hypothalamus are stimulated at a baseline level.
• ADH is released at a moderate level from the posterior pituitary.
• The collecting duct has moderate water permeability (moderate number of aquaporin channels).
• Urine is produced at a normal rate with normal concentration.
• Sweat production is high (due to exercise and heat), causing gradual water loss.

(b) Mid-race -- mildly dehydrated (low blood water potential):

• Water loss through sweat exceeds water intake, reducing blood volume and increasing blood solute concentration (lowering blood water potential).
• Osmoreceptors in the hypothalamus detect the decrease in blood water potential and increase their firing rate.
• The hypothalamus stimulates the posterior pituitary to release more ADH.
• ADH binds to receptors on collecting duct cells, triggering insertion of more aquaporin channels into the cell membrane.
• More water is reabsorbed from the collecting duct by osmosis into the concentrated medullary tissue fluid.
• Urine volume decreases (oliguria) and urine becomes more concentrated (darker yellow).
• Thirst is also stimulated, motivating the person to drink more.

(c) After the race -- excess water intake (high blood water potential):

• The person drinks a large volume of water, which is absorbed from the small intestine into the blood.
• Blood water potential increases (blood becomes more dilute).
• Osmoreceptors in the hypothalamus detect the increase in blood water potential and decrease their firing rate.
• The hypothalamus reduces stimulation of the posterior pituitary; ADH secretion decreases.
• With less ADH, collecting duct cells remove aquaporin channels from their membranes.
• The collecting duct becomes less permeable to water; less water is reabsorbed.
• A large volume of dilute urine is produced (diuresis) to excrete the excess water.
• Blood water potential gradually returns to normal.

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Excretion

Definition

Excretion is the removal of metabolic waste products from the body. These are substances produced by metabolic reactions that, if allowed to accumulate, would be toxic or harmful.

Key distinction: Excretion is NOT the same as egestion. Egestion (defaecation) is the removal of undigested food (faeces) from the body. Faeces have never been part of the body's metabolism, so their removal is not excretion.

Excretory Products

Substance	Source of Production	Organ of Excretion	Method of Excretion
$\mathrm{CO}_2$	Cellular respiration	Lungs	Exhaled during breathing
Urea	Deamination of excess amino acids in the liver	Kidneys	Filtered from blood, excreted in urine
Water	Various metabolic reactions	Kidneys, skin, lungs	Urine, sweat, exhaled breath
Mineral salts (Na$^+$, K$^+$)	Various	Kidneys, skin	Urine, sweat
Bile pigments (bilirubin)	Breakdown of haemoglobin	Liver (via bile, excreted in faeces)	Stored in gall bladder, released into intestine
Uric acid	Breakdown of nucleic acids	Kidneys	Excreted in urine

The Liver in Excretion

The liver plays a central role in processing and detoxifying substances:

1. Deamination: Excess amino acids cannot be stored. The liver removes the amino group ($\mathrm{NH}_2$) from amino acids, converting it to ammonia ($\mathrm{NH}_3$, highly toxic), which is then converted to urea (less toxic) via the ornithine cycle in the liver:

$\mathrm{Amino acid} \to \mathrm{Keto acid} + \mathrm{NH}_3$

$2\mathrm{NH}_3 + \mathrm{CO}_2 \to \mathrm{CO(NH}_2)_2 + \mathrm{H}_2\mathrm{O}$

2. Detoxification: The liver converts harmful substances into less harmful ones:
   • Alcohol (ethanol) is oxidised to ethanal (by alcohol dehydrogenase), then to ethanoic acid, then to acetyl CoA
   • Hydrogen peroxide (toxic by-product of metabolism) is broken down by catalase into water and oxygen

The Skin as an Excretory Organ

The skin excretes water, urea, salts, and lactic acid through sweat. Sweat is produced by sweat glands in the dermis and reaches the skin surface through pores. However, the skin is a minor excretory organ compared to the kidneys -- its primary function is thermoregulation.

Worked Example

A person with liver failure has a reduced ability to carry out deamination and detoxification. Explain the specific consequences for the composition of their blood, and predict the symptoms that would result.

Solution

Consequences for blood composition:

1. Elevated blood urea (uraemia): Without adequate deamination, excess amino acids cannot be properly processed. The ornithine cycle is impaired, so ammonia ($\mathrm{NH}_3$) is not efficiently converted to urea. Toxic ammonia accumulates in the blood, and even the urea that is produced may not be adequately excreted. Both ammonia and urea levels rise in the blood.

2. Accumulation of toxins: Without proper detoxification, harmful substances that would normally be broken down by the liver accumulate. This includes alcohol, drugs, and other metabolic by-products that the liver would normally process.

3. Reduced plasma protein synthesis: The liver synthesises many plasma proteins (e.g., albumin). Liver failure reduces protein synthesis, lowering plasma protein concentration and reducing blood osmotic pressure (leading to tissue fluid accumulation/oedema).

4. Reduced bile production: The liver produces bile, which contains bile pigments (bilirubin) from haemoglobin breakdown. Liver failure reduces bile production, causing bilirubin to accumulate in the blood (jaundice -- yellowing of the skin and eyes).

5. Altered blood glucose: The liver plays a key role in blood glucose regulation (glycogenesis, glycogenolysis, gluconeogenesis). Liver failure impairs these processes, causing unstable blood glucose levels.

Predicted symptoms:

• Jaundice: Yellowing of the skin and sclera (whites of the eyes) due to bilirubin accumulation in the blood.
• Confusion and neurological symptoms (hepatic encephalopathy): Ammonia is highly toxic to the brain. Elevated blood ammonia crosses the blood-brain barrier and impairs brain function, causing confusion, drowsiness, and in severe cases, coma.
• Oedema: Reduced plasma protein (especially albumin) lowers blood osmotic pressure, causing fluid to accumulate in tissues (swelling in the legs and abdomen).
• Fatigue: Impaired glucose regulation and accumulation of toxins reduce energy availability and increase feelings of tiredness.
• Excessive bleeding: The liver synthesises clotting factors. Liver failure reduces their production, leading to impaired blood clotting and easy bruising.

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Common Pitfalls

1. Confusing excretion and egestion: This is one of the most common DSE errors. Egestion (removal of faeces) is not excretion. Faeces have never been absorbed into the body's metabolic processes. $\mathrm{CO}_2$ from respiration and urea from deamination are excretory products.

2. Writing that the umbilical artery carries oxygenated blood: The umbilical ARTERY carries deoxygenated blood from the foetus to the placenta. The umbilical VEIN carries oxygenated blood from the placenta to the foetus. The naming is based on direction of blood flow relative to the heart.

3. Confusing insulin and glucagon: Insulin LOWERS blood glucose (by promoting uptake and storage). Glucagon RAISES blood glucose (by promoting glycogenolysis and gluconeogenesis). Insulin is from beta cells; glucagon is from alpha cells.

4. Stating that muscle releases glucose from glycogen: Muscle cells can break down glycogen to glucose-6-phosphate for their own use, but they CANNOT release free glucose into the blood. Only the liver can do this because only the liver has glucose-6-phosphatase.

5. Confusing the roles of the descending and ascending limbs of the Loop of Henle: The descending limb is permeable to water (water moves out) but not to salts. The ascending limb is impermeable to water but actively transports Na$^+$ and Cl$^-$ out. This asymmetry is essential for creating the medullary concentration gradient.

6. Writing that ADH is produced by the posterior pituitary: ADH is PRODUCED by neurosecretory cells in the HYPOTHALAMUS. It is STORED and RELEASED by the posterior pituitary. This distinction matters in exam answers.

7. Forgetting that oestrogen has both negative and positive feedback effects: At low concentrations, oestrogen inhibits FSH and LH (negative feedback). At high sustained concentrations, oestrogen stimulates LH release (positive feedback, causing the LH surge). The DSE frequently requires both to be mentioned.

8. Writing that the placenta allows maternal and foetal blood to mix: Maternal and foetal blood are separated by the placental barrier (chorionic villi). They do NOT mix. Exchange occurs by diffusion across the barrier.

9. Confusing oogenesis timing: Oogenesis begins during foetal development (primary oocytes arrest at prophase I). It is only completed at fertilisation (meiosis II). Spermatogenesis begins at puberty and is continuous.

10. Writing that the kidney filters urea completely: Only approximately 50% of urea is reabsorbed in the PCT. The rest remains in the filtrate and is excreted in urine. If the kidney were to reabsorb all urea, it would accumulate to toxic levels.

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Problem Set

The following problems cover the full range of topics in this chapter. Attempt each problem before revealing the solution. If your answer differs significantly from the solution, use the cross-reference to revisit the relevant theory section. Pay particular attention to the Common Pitfalls section above, as DSE examiners frequently test these points.

Problem 1: Sperm Structure and Function

Explain how the structure of a sperm cell is adapted for its function of reaching and fertilising the ovum. Refer to at least three specific parts of the sperm in your answer.

Solution

• Head (acrosome): The acrosome is a vesicle at the tip of the head containing hydrolytic enzymes (acrosin, hyaluronidase). These enzymes digest the zona pellucida surrounding the ovum, allowing the sperm to penetrate and reach the oocyte membrane. The haploid nucleus (23 chromosomes) in the head delivers the paternal genetic material. The streamlined, oval shape of the head reduces resistance as the sperm swims through the female reproductive tract.

• Middle piece (mitochondria): The middle piece is packed with mitochondria arranged in a helix. These mitochondria carry out aerobic respiration to produce large amounts of ATP, which powers the movement of the tail. The high energy supply is essential because the sperm must swim through the cervix, uterus, and oviduct to reach the ovum -- a journey that can take several hours. The centriole at the base of the head also originates from the middle piece.

• Tail (flagellum): The long, whip-like flagellum propels the sperm through the female reproductive tract by undulating movements. The tail's lashing action is driven by ATP from the mitochondria in the middle piece, enabling the sperm to travel the considerable distance to the site of fertilisation in the oviduct. The tail contains microtubules arranged in a 9+2 arrangement (9 pairs surrounding a central pair), which slide past each other to produce the bending motion.

If you get this wrong, revise: Male Reproductive System

Problem 2: Oogenesis Timing

A female baby is born with approximately 1--2 million primary oocytes, but only about 400 will be ovulated during her entire reproductive lifetime. Explain why most primary oocytes never complete meiosis, and describe when meiosis is actually completed for those that are ovulated.

Solution

• Why most never complete meiosis: All primary oocytes are formed during foetal development and arrest at prophase I. Each month from puberty to menopause (approximately 30--40 years), typically only one primary oocyte resumes meiosis and is ovulated. The vast majority of primary oocytes undergo atresia (degeneration) and never resume meiosis. Only approximately 400 out of 1--2 million are ovulated over a lifetime. This is an important contrast with spermatogenesis, which is continuous and produces millions of sperm daily from puberty onwards.

• When meiosis is completed: Meiosis I is completed just before ovulation, producing a secondary oocyte and the first polar body. The secondary oocyte begins meiosis II but arrests at metaphase II. Meiosis II is only completed if fertilisation occurs -- the entry of a sperm triggers completion of meiosis II, producing the ovum and the second polar body. If no sperm fertilises the secondary oocyte within approximately 24 hours, it degenerates without completing meiosis.

• Biological significance of this arrangement: The long arrest at prophase I means that by the time a woman reaches menopause, her remaining oocytes have been in meiotic arrest for decades. This prolonged arrest is thought to contribute to the increased incidence of chromosomal abnormalities (e.g., Down syndrome) in babies born to older mothers, as the meiotic machinery has had more time to accumulate errors.

If you get this wrong, revise: Oogenesis and Comparison of Spermatogenesis and Oogenesis

Problem 3: Menstrual Cycle Hormone Levels

On day 20 of a normal 28-day menstrual cycle, a blood test shows high progesterone and moderate oestrogen levels. Explain the source and roles of these hormones at this stage, and describe what would happen to their levels if fertilisation does not occur.

Solution

• Source on day 20 (luteal phase): Progesterone is produced by the corpus luteum (the structure formed from the ruptured follicle after ovulation on day 14). Moderate oestrogen is also produced by the corpus luteum.

• Roles on day 20:

  • Progesterone maintains the thickened, vascularised endometrium (secretory phase), keeping it ready for potential implantation.
  • Progesterone inhibits FSH and LH secretion via negative feedback, preventing new follicle development and further ovulation.
  • Oestrogen helps maintain the endometrium alongside progesterone.

• If fertilisation does not occur:

  • The corpus luteum degenerates around day 25--28 (it has a lifespan of approximately 12--14 days without hCG). This degeneration is called luteolysis.
  • Progesterone and oestrogen levels drop sharply as the corpus luteum degenerates.
  • The drop in progesterone causes the thickened endometrium to break down and be shed (menstruation, days 1--5 of the next cycle).
  • The drop in progesterone and oestrogen removes the negative feedback on FSH, so FSH levels begin to rise, initiating follicle development for the next cycle.
  • The cycle begins again with menstruation marking day 1.

• If fertilisation does occur (for contrast):

  • The embryo implants in the endometrium around day 6--7.
  • The implanted embryo (trophoblast cells) produces hCG (human chorionic gonadotropin).
  • hCG maintains the corpus luteum, preventing its degeneration.
  • The corpus luteum continues to secrete progesterone and oestrogen, maintaining the endometrium throughout pregnancy.
  • This is the basis of pregnancy tests, which detect hCG in urine.

If you get this wrong, revise: Phases of the Menstrual Cycle and Hormonal Interactions

Problem 4: Fertilisation and the Cortical Reaction

Describe the sequence of events from the moment a sperm reaches the zona pellucida to the formation of the diploid zygote. Explain why the cortical reaction is essential.

Solution

1. Acrosome reaction: The sperm binds to specific receptors (ZP3 glycoproteins) on the zona pellucida. This binding triggers the acrosome to release hydrolytic enzymes (acrosin, hyaluronidase) that digest the zona pellucida, creating a path for the sperm to penetrate.

2. Penetration: The sperm penetrates the zona pellucida using a combination of enzyme action and mechanical force from the lashing tail. It reaches the vitelline membrane (oocyte membrane).

3. Membrane fusion: The sperm membrane fuses with the oocyte membrane. The sperm nucleus and centriole begin to enter the oocyte cytoplasm.

4. Cortical reaction (critical step): Contact between the sperm and oocyte membranes triggers a wave of calcium release ($\mathrm{Ca}^{2+}$) across the ovum. This calcium wave causes cortical granules (vesicles just beneath the membrane) to fuse with the oocyte membrane and release their contents into the space between the vitelline membrane and the zona pellucida. The enzymes from the cortical granules:

   • Cross-link glycoproteins in the zona pellucida, hardening it (the zona reaction).
   • Remove the ZP3 receptors, preventing any additional sperm from binding.
   • These changes make the zona pellucida impermeable to other sperm.

5. Why the cortical reaction is essential: It prevents polyspermy -- fertilisation by more than one sperm. If polyspermy occurred, the zygote would have more than 46 chromosomes (triploidy or polyploidy), which is typically lethal. The cortical reaction is a fast block (chemical) to polyspermy, complementing the initial electrical block caused by depolarisation of the oocyte membrane upon sperm entry.

6. Completion of meiosis II: The calcium wave also triggers the secondary oocyte to complete meiosis II, producing the ovum (with the maternal haploid nucleus) and the second polar body.

7. Nuclear fusion: The sperm nucleus and ovum nucleus migrate towards each other, their nuclear envelopes break down, and the chromosomes intermingle. The diploid zygote (2n = 46) is formed, containing genetic material from both parents.

If you get this wrong, revise: Fertilisation

Problem 5: Placental Exchange

Explain why maternal and foetal blood do not mix in the placenta. Describe the structure of the placental barrier and explain why it is important that the two blood supplies remain separate.

Solution

• Structure of the placental barrier: Maternal blood and foetal blood are separated by the placental barrier, which consists of: (1) trophoblast epithelium (outer layer of chorionic villi), (2) connective tissue, and (3) foetal capillary endothelium. Exchange occurs by diffusion across this thin barrier. Substances move from maternal blood in the intervillous spaces, through the barrier, into the foetal capillaries inside the chorionic villi (and vice versa).

• Why they do not mix:

  • The foetal and maternal circulations are completely separate closed systems.
  • The chorionic villi are bathed in maternal blood, but the foetal blood flows inside capillaries within the villi, separated by the placental barrier.

• Why separation is important:

  • Blood group incompatibility: If maternal and foetal blood mixed, the mother's immune system could attack foetal red blood cells (e.g., in Rh incompatibility, where an Rh-negative mother carries an Rh-positive foetus). The barrier limits this interaction, although small amounts of foetal blood cells can occasionally cross, potentially sensitising the mother's immune system.
  • Pressure differences: Maternal blood pressure is much higher than foetal blood pressure. Mixing could damage delicate foetal blood vessels and cause haemorrhage.
  • Selective exchange: The barrier allows selective passage of small molecules (nutrients, gases, waste, antibodies) while preventing passage of large molecules, blood cells, and many pathogens. This protects the foetus from many infections while still allowing beneficial substances to pass.
  • Maintaining concentration gradients: Because the blood supplies do not mix, concentration gradients for diffusion are maintained. If blood mixed, the concentrations would equalise and efficient exchange would be impossible.

If you get this wrong, revise: Placenta and Umbilical Cord

Problem 6: Thermoregulation in the Cold

A person is standing outside on a cold winter day. Describe the involuntary physiological responses that occur to maintain body temperature, and explain how each response helps to raise or conserve heat.

Solution

1. Stimulus: Peripheral thermoreceptors in the skin detect the drop in external temperature. Central thermoreceptors in the hypothalamus detect the drop in blood temperature.

2. Vasoconstriction: Arterioles supplying the skin constrict (narrow), reducing blood flow near the body surface. Less heat is lost by radiation from the skin surface. More blood is diverted to deeper organs where heat is conserved. The skin may appear pale and feel cold to the touch.

3. Shivering: Rapid, involuntary contractions of skeletal muscles generate heat through increased metabolic rate (respiration releases heat as a by-product). Shivering can increase heat production by up to five times the resting rate. This is a significant source of additional heat production and one of the most important thermoregulatory responses in cold conditions.

4. Increased metabolic rate: The hypothalamus stimulates the adrenal medulla to release adrenaline and the thyroid gland to release thyroxine. These hormones increase the basal metabolic rate of cells throughout the body, increasing heat production from cellular respiration. Thyroxine acts more slowly (over hours to days) but provides a sustained increase in metabolic heat production.

5. Piloerection: Hair erector muscles contract, causing body hairs to stand erect. This traps a layer of still air next to the skin, which acts as an insulating layer (reducing heat loss by convection). Air is a poor conductor of heat, so this trapped layer significantly reduces heat loss. This response is more effective in furry animals than in humans, but the mechanism is conserved.

6. Behavioural responses (although the question asks about involuntary physiological responses): Note that behavioural responses such as curling up, putting on clothes, and seeking warmth are also important thermoregulatory mechanisms, but these are voluntary (controlled by the cerebral cortex) rather than involuntary (controlled by the hypothalamus).

7. Negative feedback: As body temperature returns to 37 degrees C, the thermoreceptors detect the change and the hypothalamus reduces its signals to the effectors, reducing the thermoregulatory responses.

If you get this wrong, revise: Responses to Cold

Problem 7: Insulin and Glucagon Comparison

Compare and contrast the roles of insulin and glucagon in regulating blood glucose concentration. For each hormone, state the cell type that produces it, the stimulus for its secretion, and the specific metabolic pathways it affects.

Solution

Feature	Insulin	Glucagon
Producing cell	Beta cells of islets of Langerhans (pancreas)	Alpha cells of islets of Langerhans (pancreas)
Stimulus	Blood glucose rises above set point (e.g., after a meal)	Blood glucose falls below set point (e.g., between meals)
Pathways stimulated	Glycogenesis (glucose to glycogen); glucose respiration	Glycogenolysis (glycogen to glucose); gluconeogenesis
Pathways inhibited	Glycogenolysis; gluconeogenesis	Glycogenesis
Net effect	Lowers blood glucose	Raises blood glucose
Target organs	Liver, muscle cells	Liver only

Similarities: Both are produced by the islets of Langerhans in the pancreas. Both are involved in negative feedback loops to maintain blood glucose at approximately 90 mg/100 cm cubed. Both act on the liver.

Key differences: Insulin lowers blood glucose by promoting storage and utilisation; glucagon raises it by promoting release from stores and synthesis from non-carbohydrate precursors. Insulin acts on both liver and muscle; glucagon acts primarily on the liver (muscle lacks glucagon receptors).

Antagonistic relationship: Insulin and glucagon are antagonistic hormones -- they have opposing effects on blood glucose. When one is high, the other is low. After a meal, insulin is high and glucagon is low (insulin dominance). Between meals or during fasting, glucagon is high and insulin is low (glucagon dominance). This antagonism ensures precise control of blood glucose concentration from both directions.

Additional note on adrenaline: Adrenaline (from the adrenal medulla) has a similar effect to glucagon (stimulates glycogenolysis) but is released in response to stress or exercise rather than blood glucose concentration directly. Adrenaline acts on both liver and muscle, whereas glucagon acts only on the liver.

If you get this wrong, revise: Hormonal Control and Glycogenesis, Glycogenolysis, and Gluconeogenesis

Problem 8: Type 1 Diabetes

A person with Type 1 diabetes forgets to take their insulin injection before eating a large meal rich in carbohydrates. Explain the sequence of events that follows and why this situation is dangerous.

Solution

1. Carbohydrate digestion and absorption: The meal is digested and glucose is absorbed from the small intestine into the blood. Blood glucose concentration rises significantly above the normal set point.

2. No insulin response: In Type 1 diabetes, the beta cells of the islets of Langerhans have been destroyed (autoimmune destruction), so no insulin is secreted regardless of blood glucose levels. Without insulin:

   • Cells cannot take up glucose efficiently (glucose uptake requires insulin-stimulated GLUT4 transporter translocation in muscle and fat cells).
   • Glycogenesis does not occur (glucose is not converted to glycogen for storage).
   • Cells continue to break down fat and protein for energy instead of using glucose.

3. Hyperglycaemia: Blood glucose concentration continues to rise, potentially reaching dangerous levels (well above the renal threshold of 180 mg/100 cm cubed).

4. Glycosuria and polyuria: The kidneys cannot reabsorb all the glucose. Excess glucose appears in the urine (glycosuria). Glucose in the filtrate lowers the water potential of the tubular fluid, reducing water reabsorption by osmosis. Large volumes of dilute urine are produced (polyuria).

5. Dehydration and polydipsia: Excessive water loss through polyuria leads to dehydration, triggering excessive thirst (polydipsia).

6. Ketoacidosis (dangerous complication): As cells cannot use glucose, the body breaks down fat for energy (lipolysis). Fat breakdown produces fatty acids, which are converted in the liver to ketones (e.g., acetoacetic acid, acetone). Ketones are acidic and accumulate in the blood, lowering blood pH below the normal range of 7.35--7.45. This condition is called diabetic ketoacidosis (DKA). Symptoms include deep, rapid breathing (Kussmaul breathing, as the body attempts to exhale $\mathrm{CO}_2$ to raise blood pH), sweet-smelling breath (acetone on the breath), nausea, vomiting, abdominal pain, and dehydration. Severe ketoacidosis can lead to coma and death if untreated.

7. Long-term complications of repeated hyperglycaemia: Even if acute ketoacidosis does not occur, chronically elevated blood glucose damages blood vessels (causing atherosclerosis, increasing the risk of heart attack and stroke), damages small blood vessels in the retina (diabetic retinopathy, leading to blindness), damages the glomeruli in the kidneys (diabetic nephropathy, leading to kidney failure), and damages peripheral nerves (diabetic neuropathy, causing numbness, tingling, and poor wound healing, particularly in the feet). These long-term complications emphasise the importance of maintaining blood glucose within the normal range through proper insulin therapy.

If you get this wrong, revise: Diabetes Mellitus and Negative Feedback Loop for Blood Glucose

Problem 9: Loop of Henle and ADH

Explain how the Loop of Henle creates and maintains a concentration gradient in the medulla of the kidney. Then explain how this gradient is used by ADH to regulate water reabsorption.

Solution

Creating the medullary gradient (countercurrent multiplier):

1. Descending limb: The descending limb is permeable to water but impermeable to salts. As filtrate flows down, water moves out by osmosis into the increasingly concentrated medullary tissue fluid. The filtrate becomes progressively more concentrated as it descends. By the bottom of the loop, the filtrate concentration approaches that of the medullary tissue fluid (approximately 1200 mOsm/kg).

2. Ascending limb (thin segment): The thin ascending limb is permeable to salts (Na$^+$ and Cl$^-$) but impermeable to water. Na$^+$ and Cl$^-$ diffuse passively out into the medullary tissue fluid.

3. Ascending limb (thick segment): The thick ascending limb actively transports Na$^+$ and Cl$^-$ out of the filtrate into the medullary tissue fluid (using ATP from many mitochondria in the cells of the thick ascending limb). This active transport is the primary driver of the countercurrent multiplier. The filtrate becomes progressively more dilute as it ascends.

4. Countercurrent effect: Because the descending and ascending limbs run in opposite directions (counter-current arrangement), the active transport of salts out of the ascending limb continually adds to the concentration of the medullary tissue fluid at every level. The water loss from the descending limb also concentrates the medullary tissue fluid. This arrangement multiplies (amplifies) the osmotic gradient, creating a gradient from approximately 300 mOsm/kg at the cortex to approximately 1200 mOsm/kg at the bottom of the medulla. The longer the Loop of Henle, the steeper the gradient that can be maintained (desert animals like kangaroo rats have very long loops).

How ADH uses this gradient:

5. When ADH is present (high blood concentration/low water potential), it binds to receptors on the collecting duct cells and triggers a second messenger (cAMP) cascade, leading to the insertion of aquaporin (water channel) proteins into the luminal membrane of the collecting duct cells.

6. As filtrate flows down the collecting duct through the medulla, water moves out by osmosis into the concentrated medullary tissue fluid (down the water potential gradient created by the Loop of Henle). The water enters the vasa recta (capillaries surrounding the nephron) and is returned to the bloodstream.

7. The steeper the medullary gradient (more concentrated medulla), the more water can be reabsorbed from the collecting duct, producing more concentrated urine. Without a medullary gradient, ADH would have no effect -- there would be no osmotic gradient to drive water reabsorption.

8. Without ADH, the collecting duct remains impermeable to water (aquaporins are removed from the membrane and stored in vesicles inside the cells), and dilute urine is produced regardless of the medullary gradient. The gradient is maintained regardless of ADH because the Loop of Henle operates continuously.

If you get this wrong, revise: Selective Reabsorption -- Loop of Henle and The Role of ADH

Problem 10: Excretion vs Egestion

A student claims: "Defaecation is a form of excretion because waste material is being removed from the body." Evaluate this claim with reference to the definitions of excretion and egestion, and give specific examples of true excretory products.

Solution

The claim is incorrect. Excretion and egestion are fundamentally different processes:

• Excretion is the removal of metabolic waste products -- substances that have been produced by metabolic reactions within the body cells. These substances have been part of the body's internal metabolism. Examples include:

  • $\mathrm{CO}_2$ (produced by cellular respiration, excreted by lungs)
  • Urea (produced by deamination of excess amino acids in the liver, excreted by kidneys)
  • Bile pigments/bilirubin (produced by breakdown of haemoglobin, excreted via the liver into the intestine)
  • Water and mineral salts (by-products of various metabolic reactions)

• Egestion (defaecation) is the removal of undigested food material (faeces) from the body through the anus. Faeces consist of material that was never absorbed into the body -- it passed through the digestive system as a tube (the gut lumen is technically outside the body). Since faeces were never part of the body's metabolic processes, their removal is not excretion.

• Why this distinction matters: In exam questions, confusing egestion with excretion would be marked wrong. For example, stating that the large intestine is an excretory organ because it eliminates faeces would be incorrect. The large intestine absorbs water and salts but does not excrete metabolic waste -- the substances in faeces (e.g., fibre, dead bacteria, undigested food) were never metabolised by the body.

• Common exam traps:

  • The anus is NOT an excretory organ -- it is involved in egestion.
  • The lungs ARE excretory organs because they remove $\mathrm{CO}_2$ (a metabolic waste product of respiration).

• The skin IS an excretory organ (minor) because it removes water, urea, salts, and lactic acid (metabolic waste products) in sweat. However, the skin's PRIMARY function is thermoregulation, not excretion.

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Placenta and Foetal Development

Placental Structure and Exchange

The placenta is a temporary organ that forms during pregnancy, facilitating exchange between maternal and foetal blood. Maternal and foetal blood do NOT mix -- they are separated by the placental barrier.

Structure:

Component	Description
Chorionic villi	Finger-like projections of the chorion (outer layer of the blastocyst) that project into the intervillous spaces filled with maternal blood
Intervillous spaces	Spaces filled with maternal blood that bathe the chorionic villi
Placental barrier	Thin membrane (trophoblast epithelium, connective tissue, foetal capillary endothelium) across which exchange occurs
Umbilical artery	Carries deoxygenated blood and waste from the foetus to the placenta (two arteries)
Umbilical vein	Carries oxygenated blood and nutrients from the placenta to the foetus (one vein)

Substances exchanged across the placenta:

Substance	Direction	Method of Exchange
Oxygen ($\mathrm{O}_2$)	Maternal to foetal	Simple diffusion
Carbon dioxide ($\mathrm{CO}_2$)	Foetal to maternal	Simple diffusion
Glucose	Maternal to foetal	Facilitated diffusion
Amino acids	Maternal to foetal	Active transport
Antibodies (IgG)	Maternal to foetal	Active transport (provides passive immunity)
Urea, waste	Foetal to maternal	Simple diffusion
Alcohol, drugs, viruses	Maternal to foetal	Simple diffusion (can harm the foetus)
Maternal antibodies (IgG)	Maternal to foetal	Active transport (provides passive immunity for the first few months)
Red blood cells	NO exchange	Too large to cross the barrier

Functions of the placenta:

1. Nutrient exchange: Glucose, amino acids, fatty acids, vitamins, and minerals pass from mother to foetus
2. Gas exchange: $\mathrm{O}_2$ passes from mother to foetus; $\mathrm{CO}_2$ passes from foetus to mother
3. Waste removal: Urea and other waste products pass from foetus to mother for excretion
4. Hormone production: Produces progesterone (maintains pregnancy), oestrogen, and hCG (detected in pregnancy tests)
5. Immune protection: Maternal IgG antibodies cross the placenta, providing passive immunity to the foetus
6. Barrier function: The placental barrier prevents many (but not all) pathogens and harmful substances from reaching the foetus

Foetal Circulation

The foetal circulatory system has adaptations that differ from the adult system because the foetus does not use its lungs or digestive system.

Adaptation	Description
Ductus venosus	Bypasses the liver; shunts blood from the umbilical vein directly to the inferior vena cava
Foramen ovale	Opening between the right and left atria; shunts blood from the right atrium to the left atrium, bypassing the lungs
Ductus arteriosus	Connects the pulmonary artery to the aorta; shunts blood away from the lungs
High haemoglobin concentration	Foetal haemoglobin has a higher affinity for $\mathrm{O}_2$ than adult haemoglobin, facilitating oxygen transfer from the mother

After birth, the ductus venosus, foramen ovale, and ductus arteriosus close as the baby begins to breathe and the placental circulation ceases.

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Methods of Contraception

Comparison of Contraceptive Methods

Method	Type	Mechanism	Effectiveness (typical use)
Male condom	Barrier	Physical barrier preventing sperm reaching the egg	~85%
Female condom	Barrier	Physical barrier worn inside the vagina	~79%
Combined oral contraceptive pill	Hormonal	Contains oestrogen and progesterone; inhibits ovulation; thickens cervical mucus; thins endometrium	~91%
Progestogen-only pill (mini-pill)	Hormonal	Thickens cervical mucus; may inhibit ovulation in some women	~91%
Intrauterine device (IUD)	Barrier/Hormonal	Copper IUD: toxic to sperm, prevents implantation. Hormonal IUD: thickens mucus, thins endometrium	>99%
Injection (Depo-Provera)	Hormonal	Slow-release progesterone; inhibits ovulation; thickens cervical mucus	~94%
Implant (e.g., Nexplanon)	Hormonal	Subdermal rod releasing progesterone over 3 years	>99%
Male sterilisation (vasectomy)	Surgical	Vas deferens cut and tied; sperm cannot reach semen	>99.9%
Female sterilisation (tubal ligation)	Surgical	Oviducts cut and tied; egg cannot reach uterus; sperm cannot reach egg	>99.5%
Rhythm method (calendar)	Behavioural	Avoiding intercourse during the fertile window (around ovulation)	~76%
Withdrawal (coitus interruptus)	Behavioural	Penis removed before ejaculation	~78%
Emergency contraception	Hormonal	High-dose progesterone (or ulipristal acetate); taken within 72 hours of unprotected intercourse; prevents/delays ovulation	~85%

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Infertility and Assisted Reproductive Technology

Causes of Infertility

Cause	Male/Female	Description
Low sperm count	Male	Fewer sperm in the ejaculate reduces the probability of fertilisation
Poor sperm motility	Male	Sperm cannot swim effectively to reach the egg
Blocked oviducts (e.g., PID)	Female	Scarring from pelvic inflammatory disease (often caused by chlamydia or gonorrhoea) prevents the egg and sperm from meeting
Failure to ovulate	Female	Hormonal imbalances (e.g., PCOS, low FSH/LH) prevent egg release
Endometriosis	Female	Endometrial tissue grows outside the uterus; can block oviducts and impair ovulation
Age-related decline	Both	Egg quality and quantity decline with age; sperm quality declines with age
Uterine abnormalities	Female	Fibroids, polyps, or structural abnormalities prevent implantation

Assisted Reproductive Technology (ART)

1. In vitro fertilisation (IVF):

1. The woman is given FSH injections to stimulate multiple follicle development (superovulation)
2. Mature eggs are collected from the ovaries (laparoscopy or ultrasound-guided needle)
3. Sperm is collected from the male partner
4. Eggs and sperm are mixed in a laboratory dish (in vitro = "in glass")
5. Fertilised eggs (zygotes) are allowed to develop into embryos (2-8 cell stage)
6. One or two embryos are transferred into the uterus (embryo transfer)
7. Remaining viable embryos can be frozen (cryopreserved) for future use

Success rate: Approximately 25-30% per cycle (varies with age and cause of infertility)

2. Intracytoplasmic sperm injection (ICSI):

• A single sperm is injected directly into the egg cytoplasm
• Used when male infertility is the cause (very low sperm count, poor motility, abnormal sperm morphology)
• Otherwise similar procedure to IVF

3. Artificial insemination (AI):

• Sperm is collected, processed, and inserted directly into the uterus (intrauterine insemination, IUI)
• Can use partner's sperm or donor sperm
• Used for mild male infertility, unexplained infertility, or when using donor sperm

Ethical considerations of ART:

Issue	Arguments For	Arguments Against
IVF for infertile couples	Helps couples have children who otherwise could not	Expensive; not universally accessible; multiple births risk
Designer babies	Could eliminate genetic diseases (PGD)	Could lead to selection for non-medical traits; eugenics concerns
Surrogacy	Allows couples who cannot carry a pregnancy	Exploitation concerns; legal complexity; emotional impact
Embryo research	Could lead to treatments for diseases	Destruction of embryos is ethically contentious for some
Storage of embryos	Allows future use; reduces need for repeated IVF	What happens to stored embryos if couple separates or dies?

Preimplantation Genetic Diagnosis (PGD)

PGD is a technique used alongside IVF to screen embryos for specific genetic disorders before implantation:

1. Embryos are created by IVF
2. At the 8-cell stage, one or two cells are removed (blastomere biopsy)
3. The DNA of these cells is tested for specific genetic conditions (e.g., cystic fibrosis, sickle cell disease, Huntington's disease, chromosomal abnormalities)
4. Only embryos without the genetic condition are selected for transfer

PGD allows parents who are carriers of genetic disorders to have children without passing on the condition, without the ethical dilemma of terminating an affected pregnancy.

tip

Diagnostic Test Ready to test your understanding of Human Reproduction and Homeostasis? The diagnostic test contains the hardest questions within the DSE specification for this topic, each with a full worked solution.

Unit tests probe edge cases and common misconceptions. Integration tests combine Human Reproduction and Homeostasis with other biology topics to test synthesis under exam conditions.

See Diagnostic Guide for instructions on self-marking and building a personal test matrix.

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The Menstrual Cycle in Detail

Hormonal Control

The menstrual cycle is controlled by four main hormones, involving a complex interplay of positive and negative feedback:

Hormone	Source	Function
FSH (Follicle-Stimulating Hormone)	Anterior pituitary	Stimulates follicle development in the ovary; stimulates oestrogen production by the follicle cells (granulosa cells)
LH (Luteinising Hormone)	Anterior pituitary	Triggers ovulation (release of the egg from the follicle); stimulates the corpus luteum to develop and produce progesterone
Oestrogen	Developing follicle (granulosa cells)	Stimulates proliferation of the endometrium (uterine lining); inhibits FSH production (negative feedback at low levels); stimulates LH surge (positive feedback at high levels, just before ovulation)
Progesterone	Corpus luteum	Maintains the thick, glandular endometrium; inhibits FSH and LH production (negative feedback); suppresses uterine contractions

Phases of the Menstrual Cycle

Phase	Days	Ovarian Events	Uterine Events	Hormone Levels
Menstruation	Days 1-5	The corpus luteum from the previous cycle degenerates (if fertilisation did not occur); progesterone and oestrogen levels drop	The thickened endometrium breaks down and is shed along with blood through the vagina	Low oestrogen, low progesterone
Follicular phase	Days 1-13	Several follicles begin to develop; one becomes dominant and continues to mature; the egg within the dominant follicle completes meiosis I	The endometrium begins to thicken and proliferate (stimulated by rising oestrogen)	Rising oestrogen (from follicle); FSH initially rises then falls (inhibited by oestrogen)
Ovulation	Day 14 (approximately)	The mature follicle ruptures, releasing the secondary oocyte (egg) into the fallopian tube	Endometrium continues to thicken	Peak in oestrogen triggers LH surge (positive feedback); LH peak triggers ovulation
Luteal phase	Days 15-28	The ruptured follicle develops into the corpus luteum, which secretes progesterone (and some oestrogen)	Endometrium reaches maximum thickness; becomes secretory (glands produce glycogen-rich secretions to support a potential embryo)	Rising progesterone (from corpus luteum); progesterone inhibits FSH and LH (negative feedback)

What Happens If Fertilisation Occurs vs Does Not Occur

Scenario	Events
Fertilisation occurs	The embryo implants in the endometrium (~day 6-7 after fertilisation); the developing placenta produces hCG (human chorionic gonadotrophin), which maintains the corpus luteum; the corpus luteum continues to produce progesterone and oestrogen, maintaining the endometrium; menstruation does NOT occur; hCG is detected by pregnancy tests
Fertilisation does NOT occur	The corpus luteum degenerates after approximately 14 days (day 28); progesterone and oestrogen levels drop sharply; the endometrium breaks down; menstruation begins (day 1 of the next cycle); the drop in progesterone and oestrogen removes the negative feedback on the anterior pituitary; FSH begins to rise again, starting a new cycle

Feedback Mechanisms

Feedback Type	Description	Timing in Cycle
Negative feedback (oestrogen on FSH)	Rising oestrogen inhibits FSH secretion from the anterior pituitary; this prevents multiple follicles from developing simultaneously	Throughout follicular phase (low-medium oestrogen)
Positive feedback (oestrogen on LH)	When oestrogen reaches a critical HIGH threshold (around day 12-13), it switches from negative to positive feedback; high oestrogen stimulates a surge in LH secretion from the anterior pituitary	Just before ovulation (day 12-13)
Negative feedback (progesterone on FSH and LH)	Progesterone inhibits both FSH and LH secretion; this prevents the development of a new follicle and another ovulation while a potential pregnancy is being established	Throughout luteal phase

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Methods of Studying Reproduction

Infertility Investigation

Investigation	Description	What It Detects
Semen analysis	Sample of semen is analysed for sperm count, motility (movement), morphology (shape), and volume	Male infertility factors
Hormone assays	Blood tests measuring levels of FSH, LH, oestrogen, progesterone, testosterone, prolactin	Hormonal imbalances
Ovulation tracking	Monitoring basal body temperature (slight rise after ovulation due to progesterone); urine LH tests; ultrasound monitoring of follicles	Whether and when ovulation occurs
Hysterosalpingography	X-ray imaging of the uterus and fallopian tubes after injection of a contrast dye	Blocked fallopian tubes; uterine abnormalities
Laparoscopy	A small camera is inserted through a keyhole incision to examine the pelvic organs	Endometriosis; pelvic adhesions; ovarian cysts
Ovarian reserve testing	Blood test for anti-Mullerian hormone (AMH); antral follicle count via ultrasound	Number and quality of remaining eggs

Assisted Reproductive Technology (ART)

IVF (In Vitro Fertilisation) detailed steps:

Step	Description
1. Ovarian stimulation	The woman is given hormonal drugs (FSH injections) to stimulate the development of multiple follicles (instead of just one)
2. Egg retrieval	Mature eggs are collected from the ovaries using a needle guided by ultrasound (transvaginal oocyte retrieval)
3. Sperm collection	A semen sample is collected from the male partner; sperm are prepared (washed and concentrated)
4. Fertilisation	Eggs and sperm are mixed in a culture dish (conventional IVF) or a single sperm is injected directly into each egg (ICSI -- intracytoplasmic sperm injection); fertilisation occurs in the laboratory
5. Embryo culture	Fertilised eggs (zygotes) are cultured for 3-5 days, developing into embryos; the best-quality embryos are selected
6. Embryo transfer	One or two embryos are transferred into the woman's uterus using a thin catheter
7. Pregnancy test	A blood test for hCG is performed approximately 2 weeks after embryo transfer to determine if implantation was successful

Success rates: IVF success depends on the woman's age (approximately 30-40% per cycle for women under 35; decreasing to less than 5% for women over 42).

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Common Pitfalls

• Oestrogen has BOTH negative and positive feedback effects on the anterior pituitary. At LOW to MODERATE levels, oestrogen inhibits FSH (negative feedback). At HIGH levels (just before ovulation), oestrogen STIMULATES LH secretion (positive feedback). This dual role is frequently tested in DSE exams
• The corpus luteum is maintained by hCG during pregnancy, NOT by FSH or LH. hCG is produced by the developing embryo/placenta and has a similar structure and function to LH
• Menstruation occurs because progesterone and oestrogen levels DROP, not because they rise. The drop in hormone levels causes the endometrium to break down. If progesterone is maintained (by hCG from a pregnancy), the endometrium is maintained and menstruation does not occur
• The menstrual cycle is typically described as 28 days, but this is an average. Normal cycles range from 21-35 days. The luteal phase is relatively constant (14 days); variation in cycle length is mainly due to variation in the follicular phase

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Endocrine System Overview

Comparison of Nervous and Endocrine Systems

Feature	Nervous System	Endocrine System
Signal type	Electrical impulses (nerve impulses)	Chemical signals (hormones)
Transmission speed	Very fast (milliseconds)	Slower (seconds to hours to days)
Duration of effect	Short-lived (impulses stop when stimulus is removed)	Longer-lasting (hormones persist in the blood until broken down or excreted)
Target specificity	Very specific (nerve impulses travel along specific neurons to specific target cells)	Can be widespread (hormones are carried in the blood to all tissues) but only affect cells with the appropriate receptors
Method of communication	Neurons (synapses between cells)	Bloodstream (hormones secreted into blood, travel throughout the body)
Response	Movement, secretion, sensation	Growth, metabolism, reproduction, homeostasis
Example	Reflex arc (withdrawal from hot object)	ADH regulating water balance; insulin and glucagon regulating blood glucose

Major Endocrine Glands and Hormones

Gland	Hormone(s)	Function(s)	Target
Hypothalamus	Releasing hormones (e.g., GnRH, TRH, CRH, GHRH); ADH (stored in posterior pituitary); oxytocin (stored in posterior pituitary)	Controls the pituitary gland; regulates homeostasis (water balance, temperature); controls circadian rhythms	Anterior pituitary; posterior pituitary
Anterior pituitary	FSH, LH, ACTH, TSH, GH, prolactin	Stimulates other endocrine glands; promotes growth; stimulates milk production	Gonads, adrenal cortex, thyroid, whole body, mammary glands
Posterior pituitary	ADH (vasopressin), oxytocin	Water reabsorption in kidneys; uterine contractions during labour; milk ejection during breastfeeding	Kidneys; uterus; mammary glands
Thyroid	T3 (triiodothyronine), T4 (thyroxine), calcitonin	Increases basal metabolic rate; stimulates growth and development; regulates calcium levels (calcitonin lowers blood calcium)	Most body cells; bones
Parathyroid glands	Parathyroid hormone (PTH)	Increases blood calcium concentration (stimulates osteoclasts to break down bone; increases calcium reabsorption in kidneys; activates vitamin D)	Bones; kidneys; intestine
Adrenal cortex	Cortisol, aldosterone, androgens	Cortisol: stress response; increases blood glucose; suppresses immune system. Aldosterone: increases sodium reabsorption in kidneys (regulates blood pressure and blood volume)	Most body cells; kidneys
Adrenal medulla	Adrenaline (epinephrine), noradrenaline (norepinephrine)	Fight-or-flight response: increases heart rate, blood pressure, blood glucose; dilates airways; dilates pupils; diverts blood to muscles	Heart, blood vessels, liver, lungs, eyes
Pancreas (islets)	Insulin ($\beta$ cells), glucagon ($\alpha$ cells), somatostatin ($\delta$ cells)	Insulin: lowers blood glucose. Glucagon: raises blood glucose. Somatostatin: inhibits insulin and glucagon secretion	Liver, muscle, fat cells
Ovaries	Oestrogen, progesterone, inhibin	Female secondary sexual characteristics; regulates menstrual cycle; maintains pregnancy	Uterus, breasts, hypothalamus/pituitary
Testes	Testosterone, inhibin	Male secondary sexual characteristics; spermatogenesis; muscle growth; bone density	Most body cells; hypothalamus/pituitary
Pineal gland	Melatonin	Regulates circadian rhythm (sleep-wake cycle); produced in response to darkness	Brain (suprachiasmatic nucleus)

Thyroid Disorders

Disorder	Cause	Symptoms	Treatment
Hyperthyroidism	Overproduction of T3/T4 (e.g., Graves' disease -- autoimmune stimulation of thyroid; toxic nodular goitre)	Weight loss; rapid heart rate (tachycardia); anxiety; tremor; sweating; heat intolerance; exophthalmos (bulging eyes in Graves' disease)	Anti-thyroid drugs (carbimazole); radioactive iodine treatment; surgical removal of thyroid
Hypothyroidism	Underproduction of T3/T4 (e.g., Hashimoto's thyroiditis -- autoimmune destruction of thyroid; iodine deficiency)	Weight gain; fatigue; cold intolerance; dry skin; constipation; depression; goitre (enlarged thyroid)	Thyroxine replacement therapy (daily oral tablet); lifelong treatment
Cretinism	Congenital hypothyroidism (thyroid deficiency from birth)	Stunted physical growth; intellectual disability if not treated promptly	Early detection by newborn screening; thyroxine treatment from birth

Adrenal Disorders

Disorder	Cause	Symptoms	Treatment
Cushing's syndrome	Excess cortisol (e.g., prolonged corticosteroid use; ACTH-secreting pituitary tumour; adrenal tumour)	Weight gain (especially face, trunk); "moon face"; thin skin; easy bruising; high blood pressure; muscle weakness; mood changes	Treat underlying cause; reduce corticosteroid dose; surgery to remove tumour
Addison's disease	Insufficient cortisol and aldosterone (autoimmune destruction of adrenal cortex)	Weight loss; fatigue; low blood pressure; darkening of skin (hyperpigmentation); salt craving; hypoglycaemia	Lifelong corticosteroid and mineralocorticoid replacement therapy (hydrocortisone and fludrocortisone)
Conn's syndrome	Excess aldosterone (adrenal tumour)	High blood pressure; muscle weakness; frequent urination; low blood potassium (hypokalaemia)	Surgery to remove tumour; aldosterone antagonists (spironolactone)

Feedback Mechanisms in the Endocrine System

TSH-T3/T4 negative feedback:

1. Hypothalamus releases TRH (thyrotropin-releasing hormone)
2. TRH stimulates the anterior pituitary to release TSH (thyroid-stimulating hormone)
3. TSH stimulates the thyroid gland to produce T3 and T4
4. T3 and T4 inhibit the hypothalamus (reducing TRH) and the anterior pituitary (reducing TSH) -- negative feedback
5. If T3/T4 levels are too high, TRH and TSH production are suppressed, reducing thyroid hormone production
6. If T3/T4 levels are too low, TRH and TSH production increase, stimulating more thyroid hormone production

Stress response (HPA axis):

1. Stress detected by the hypothalamus
2. Hypothalamus releases CRH (corticotropin-releasing hormone)
3. CRH stimulates the anterior pituitary to release ACTH (adrenocorticotropic hormone)
4. ACTH stimulates the adrenal cortex to release cortisol
5. Cortisol increases blood glucose (gluconeogenesis), suppresses the immune system, and reduces inflammation
6. Cortisol inhibits the hypothalamus and anterior pituitary (negative feedback), reducing CRH and ACTH production

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Common Pitfalls

• The posterior pituitary does NOT produce hormones; it STORES and RELEASES hormones produced by the hypothalamus. ADH and oxytocin are synthesised in the hypothalamus and transported down nerve fibres to the posterior pituitary, where they are stored until needed
• Thyroxine (T4) is produced in much larger quantities than T3, but T3 is the ACTIVE form. T4 is converted to T3 in target tissues by the enzyme deiodinase
• Cortisol INCREASES blood glucose (by stimulating gluconeogenesis), it does not decrease it. Students sometimes confuse cortisol with insulin. Cortisol is a catabolic hormone that breaks down proteins and fats to provide glucose
• Aldosterone acts on the KIDNEYS to increase sodium reabsorption (not on all body cells). Water follows sodium by osmosis, so aldosterone indirectly increases water reabsorption and blood volume/blood pressure
• Adrenaline is produced by the adrenal MEDULLA (not cortex); cortisol is produced by the adrenal CORTEX (not medulla). The adrenal medulla is stimulated by the sympathetic nervous system (fight-or-flight); the adrenal cortex is stimulated by ACTH from the anterior pituitary**