Human Physiology | DSE - Wyatt's Notes

A balanced diet provides all essential nutrients in the correct proportions to maintain health. The Seven classes of food are:

Basal Metabolic Rate (BMR) is the minimum energy required to maintain vital functions at rest. Total Daily energy expenditure depends on:

$\mathrm{Total Energy} = \mathrm{BMR} + \mathrm{Physical Activity} + \mathrm{SDA (Specific Dynamic Action)}$

Average daily energy requirements:

A 65 kg male office worker has a BMR of approximately 7000 kJ/day. His physical activity accounts for 3000 kJ/day and SDA accounts for 1000 kJ/day.

(a) Calculate his total daily energy requirement.

(b) If he starts marathon training and his physical activity doubles, what is his new total energy requirement?

(c) Explain why athletes often consume high-carbohydrate diets before competition.

Undernutrition: Insufficient intake of calories or specific nutrients.

Overnutrition: Excessive intake leading to obesity, cardiovascular disease, Type 2 diabetes.

The digestive system is a muscular tube running from mouth to anus, with accessory glands (salivary Glands, liver, pancreas) secreting enzymes into the tract.

Mechanical digestion:

• Teeth cut, tear, and grind food into smaller pieces (increases surface area)
• Tongue manipulates food and mixes it with saliva

Chemical digestion:

• Salivary glands produce saliva containing:
• Salivary amylase: Breaks down starch into maltose (optimal pH ~6.8)
• Mucus: Lubricates food for swallowing
• Water: Dissolves food molecules for taste

$\mathrm{Starch} \xrightarrow{\mathrm{salivary amylase}} \mathrm{Maltose}$

• Gastric glands in the stomach wall secrete gastric juice containing:
• Hydrochloric acid (HCl): pH ~2; denatures proteins; activates pepsinogen to pepsin; kills bacteria
• Pepsinogen: Inactive precursor; activated to pepsin by HCl; digests protein into polypeptides
• Mucus: Protects the stomach lining from acid and enzymes

$\mathrm{Proteins} \xrightarrow{\mathrm{pepsin (pH 2)}} \mathrm{Polypeptides}$

The stomach churns food with gastric juice to form chyme, a semi-liquid acidic mixture. The Pyloric sphincter controls the release of chyme into the duodenum.

The small intestine is the primary site of both digestion and absorption. It is divided into:

1. Duodenum: Where most digestion occurs; receives bile and pancreatic juice
2. Jejunum and Ileum: Where most absorption occurs

Bile (produced by the liver, stored in the gall bladder):

• Emulsifies fats — breaks large fat droplets into smaller droplets (increases surface area for lipase)
• Contains no enzymes — it is not a digestive enzyme
• Neutralises stomach acid (alkaline, pH ~8)
• Contains bile pigments (bilirubin from haemoglobin breakdown) excreted in faeces

Pancreatic juice (produced by the pancreas):

• Pancreatic amylase: Starch to maltose
• Trypsin: Proteins/polypeptides to smaller peptides
• Lipase: Fats to fatty acids and glycerol

Intestinal enzymes (produced by cells on the villi):

A student eats a meal containing starch, protein, and fat. Trace the complete digestion of each nutrient from the mouth to the small intestine, naming the enzyme(s), substrate(s), and product(s) at each stage.

The small intestine is adapted for absorption through structural features:

Villi: Finger-like projections of the intestinal wall that increase surface area. Each villus Contains:

• A dense network of blood capillaries (absorb glucose, amino acids — carried to liver via hepatic portal vein)
• A lacteal (lymphatic capillary) that absorbs fatty acids and glycerol (re-formed into triglycerides, packaged into chylomicrons, transported via the lymphatic system)

Microvilli: Tiny projections on the epithelial cells of the villi, further increasing surface Area.

Features for efficient absorption:

1. Large surface area (villi + microvilli)
2. Thin walls (single layer of epithelial cells)
3. Dense blood capillary network (maintains concentration gradient)
4. Lacteal for lipid absorption
5. Short diffusion distance

Absorption mechanisms:

• Diffusion: From high to low concentration (e.g., fatty acids, glycerol)
• Facilitated diffusion: Via carrier proteins (e.g., fructose)
• Active transport: Against concentration gradient, requires ATP (e.g., glucose, amino acids)
• Co-transport: Na$^+$ gradient drives glucose/amino acid uptake via SGLT1 transporter

• Colon: Absorbs water and mineral ions from remaining indigestible material
• Caecum/Appendix: Vestigial in humans; contains gut bacteria that synthesise some vitamins (e.g., vitamin K, some B vitamins)
• Rectum: Stores faeces until defaecation

Faeces consist of undigested food (mainly dietary fibre), dead cells, bacteria, bile pigments, and Water.

Inhalation (inspiration):

1. External intercostal muscles contract
2. Ribs move up and out
3. Diaphragm contracts and flattens
4. Volume of thorax increases
5. Pressure in lungs decreases below atmospheric pressure
6. Air rushes in

$P_{\mathrm{lung}} \lt P_{\mathrm{atm}} \implies \mathrm{air flows in}$

Exhalation (expiration) — at rest (passive):

1. External intercostal muscles relax
2. Ribs move down and in
3. Diaphragm relaxes and returns to dome shape
4. Volume of thorax decreases
5. Pressure in lungs increases above atmospheric pressure
6. Air is pushed out

$P_{\mathrm{lung}} \gt P_{\mathrm{atm}} \implies \mathrm{air flows out}$

Forced exhalation: Internal intercostal muscles and abdominal muscles contract actively.

Features of alveoli adapted for efficient gas exchange:

1. Large surface area: Millions of alveoli provide enormous total surface area (~70 m$^2$)
2. Thin walls: Alveolar epithelium is one cell thick
3. Dense capillary network: Each alveolus surrounded by capillaries; capillary walls are also one cell thick
4. Short diffusion path: Total distance between air and blood is ~1 micrometre
5. Moist surface: Gases dissolve before diffusing; moisture maintains the diffusion gradient
6. Maintained concentration gradient: Continuous blood flow and ventilation refresh the gradient

Diffusion of gases:

$\mathrm{O}_2 \mathrm{ (alveolar air, } pO_2 \approx 13.3 \mathrm{ kPa)} \to \mathrm{O}_2 \mathrm{ (blood, } pO_2 \approx 5.3 \mathrm{ kPa)}$

$\mathrm{CO}_2 \mathrm{ (blood, } pCO_2 \approx 6.0 \mathrm{ kPa)} \to \mathrm{CO}_2 \mathrm{ (alveolar air, } pCO_2 \approx 5.3 \mathrm{ kPa)}$

Oxygen diffuses from alveolar air into the blood; carbon dioxide diffuses from blood into alveolar Air. Both movements are down their respective partial pressure gradients.

Asthma:

• Inflammation and constriction of bronchioles
• Excess mucus production
• Symptoms: wheezing, shortness of breath, coughing
• Triggers: allergens, exercise, cold air, stress
• Treatment: bronchodilator inhalers (relax smooth muscle), steroid inhalers (reduce inflammation)

Chronic Obstructive Pulmonary Disease (COPD):

• Includes chronic bronchitis and emphysema
• Chronic bronchitis: inflammation and mucus hypersecretion in bronchi; “smoker”s cough”
• Emphysema: destruction of alveolar walls; reduced surface area for gas exchange; loss of elasticity
• Symptoms: persistent cough, breathlessness, fatigue
• Primary cause: smoking

Tuberculosis (TB):

• Caused by bacterium Mycobacterium tuberculosis
• Transmitted by droplet infection
• Infected lung tissue is destroyed by white blood cells, forming cavities
• Symptoms: persistent cough, blood-tinged sputum, weight loss, night sweats
• Treatment: long-term antibiotics (6-9 months)

Lung cancer:

• Uncontrolled cell division in lung tissue
• Strongly linked to smoking (carcinogens in tar)
• Symptoms: persistent cough, coughing up blood, chest pain, weight loss
• Treatment: surgery, chemotherapy, radiotherapy

Carbon monoxide reduces the oxygen-carrying capacity of blood because it binds to haemoglobin Forming carboxyhaemoglobin, which is stable and does not readily release the CO.

Humans have a closed, double circulatory system:

1. Pulmonary circulation: Right ventricle to lungs to left atrium (deoxygenated blood to lungs for gas exchange, then oxygenated blood returns)
2. Systemic circulation: Left ventricle to body to right atrium (oxygenated blood to tissues, deoxygenated blood returns)

The heart is a four-chambered muscular pump.

Chambers:

• Right atrium: Receives deoxygenated blood from the body via the vena cava
• Right ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery
• Left atrium: Receives oxygenated blood from the lungs via the pulmonary vein
• Left ventricle: Pumps oxygenated blood to the body via the aorta

Valves:

Valves open and close due to pressure differences. They ensure one-way blood flow.

Why the left ventricle has a thicker wall:

The left ventricle pumps blood to the entire body (systemic circulation) against high resistance, so It needs to generate much higher pressure than the right ventricle, which only pumps blood to the Nearby lungs (pulmonary circulation).

1. Atrial systole: Atria contract; blood is forced through AV valves into ventricles
2. Ventricular systole: Ventricles contract; AV valves close (“lub” sound); semilunar valves open; blood is ejected into arteries
3. Diastole: Heart muscle relaxes; semilunar valves close (“dub” sound); blood flows into atria from veins

Blood consists of:

1. Plasma (55%): Liquid portion containing water, dissolved substances (glucose, amino acids, urea, hormones), plasma proteins (albumin, fibrinogen, globulins)
2. Red blood cells (erythrocytes, ~44%): Transport oxygen via haemoglobin
3. White blood cells (leucocytes, ~1%): Defence against pathogens
4. Platelets (thrombocytes, ~0.5%): Blood clotting

Red blood cells:

• Biconcave disc shape: increases surface area to volume ratio for gas exchange
• No nucleus: more space for haemoglobin
• Contain haemoglobin: iron-containing protein that binds oxygen
• Flexible: can squeeze through narrow capillaries
• Produced in red bone marrow; destroyed in the spleen and liver (lifespan ~120 days)

White blood cells:

• Phagocytes (neutrophils, macrophages): Engulf and digest pathogens by phagocytosis
• Lymphocytes: Produce antibodies; recognise specific antigens

Platelets:

• Cell fragments (no nucleus) produced in bone marrow
• Involved in blood clotting:

1. Platelets accumulate at the wound site
2. Thromboplastin is released
3. Thromboplastin converts prothrombin to thrombin (with calcium ions)
4. Thrombin converts soluble fibrinogen to insoluble fibrin
5. Fibrin forms a mesh that traps red blood cells, forming a clot

Blood groups are determined by antigens on the surface of red blood cells and antibodies in The plasma.

Universal donor: O (no antigens on RBC surface)

Universal recipient: AB (no antibodies in plasma)

Rhesus factor (RhD): An additional antigen. Rh+ individuals have the D antigen; Rh- do not. An Rh- mother carrying an Rh+ foetus may produce anti-D antibodies during a second pregnancy (haemolytic disease of the newborn).

A hospital has the following blood supplies: 5 units of type A, 3 units of type B, 2 units of type AB, and 8 units of type O. Three patients arrive simultaneously:

• Patient 1: Blood group A, needs 2 units
• Patient 2: Blood group B, needs 1 unit
• Patient 3: Blood group AB, needs 2 units

(a) Which blood types can each patient safely receive?

(b) Can all three patients be treated with the available supply? Justify your answer.

Each haemoglobin molecule can carry up to 4 oxygen molecules:

$\mathrm{Hb} + 4\mathrm{O}_2 \rightleftharpoons \mathrm{HbO}_8$

In the lungs (high $pO_2$), oxygen binds to haemoglobin (loading). In the tissues (low $pO_2$), Oxygen dissociates from haemoglobin (unloading).

The oxygen dissociation curve is an S-shaped (sigmoid) curve showing the relationship between the Partial pressure of oxygen ($pO_2$) and the percentage saturation of haemoglobin.

$\mathrm{Percentage saturation of Hb vs } pO_2$

Key points:

• At high $pO_2$ (lungs): haemoglobin is nearly fully saturated (~97%)
• At low $pO_2$ (tissues): haemoglobin releases oxygen
• The steep part of the curve means small changes in $pO_2$ cause large changes in oxygen unloading

Factors shifting the curve:

A right shift = lower affinity for O$_2$ = more O$_2$ released to tissues.

A left shift = higher affinity for O$_2$ = less O$_2$ released to tissues.

• Excretion: Removal of metabolic waste products (e.g., CO$_2$Urea, excess water) from the body. This is distinct from egestion.
• Egestion: Removal of undigested food (faeces) from the body. Faeces have never been part of metabolism, so egestion is NOT excretion.

Structure:

• Outer region: cortex (contains Bowman”s capsules, convoluted tubules)
• Inner region: medulla (contains loops of Henle, collecting ducts)
• Central cavity: renal pelvis (collects urine)
• Ureter: Carries urine from kidney to bladder
• Bladder: Stores urine
• Urethra: Releases urine from the body

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

Components:

1. Renal (Bowman’s) capsule: Cup-shaped structure surrounding the glomerulus
2. Glomerulus: Knot of capillaries inside the Bowman’s capsule
3. Proximal convoluted tubule (PCT): Highly coiled tube after the capsule
4. Loop of Henle: U-shaped tube extending into the medulla
5. Distal convoluted tubule (DCT): Coiled tube in the cortex
6. Collecting duct: Receives fluid from several nephrons; passes through the medulla

Ultrafiltration occurs at the glomerulus and Bowman’s capsule.

Mechanism:

• Blood enters the glomerulus via the afferent arteriole (wider) and leaves via the efferent arteriole (narrower)
• The narrower efferent arteriole creates high hydrostatic pressure in the glomerulus
• This pressure forces small molecules (water, glucose, amino acids, urea, ions) through the capillary wall and the podocytes of the Bowman’s capsule into the capsular space
• Large molecules (proteins, blood cells) remain in the blood

What is filtered: Water, glucose, amino acids, urea, salts, vitamins (all small solutes)

What is NOT filtered: Large proteins, red blood cells, white blood cells, platelets

The filtrate produced is called glomerular filtrate, which is similar in composition to blood Plasma minus proteins.

As the filtrate passes along the nephron, useful substances are reabsorbed back into the blood.

Proximal Convoluted Tubule (PCT):

• All glucose is reabsorbed (by active transport and co-transport)
• All amino acids are reabsorbed (by active transport)
• Most water (by osmosis, following the reabsorption of solutes)
• Most salts/ions (Na$^+$K$^+$Cl$^-$) by active transport
• The PCT reabsorbs approximately 85% of the filtrate

Loop of Henle:

• Descending limb: Permeable to water but not to salts. Water moves out by osmosis into the increasingly concentrated medullary tissue fluid.
• Ascending limb: Impermeable to water but actively transports Na$^+$ and Cl$^-$ out into the medullary tissue fluid. This creates a sodium gradient in the medulla.
• The counter-current multiplier mechanism maintains a high solute concentration in the medulla, which is essential for water reabsorption from the collecting duct.

Distal Convoluted Tubule (DCT):

• Selective reabsorption of ions (Na$^+$K$^+$Ca$^{2+}$) regulated by hormones
• Water permeability controlled by ADH

Collecting duct:

• Water reabsorption controlled by antidiuretic hormone (ADH)
• In the presence of ADH, the collecting duct becomes more permeable to water, and water is reabsorbed into the concentrated medullary tissue fluid by osmosis
• Produces concentrated urine (small volume)

Osmoregulation is the control of water balance in the body.

Mechanism (negative feedback):

1. Blood water potential decreases (blood becomes more concentrated, e.g., after sweating or not drinking)
2. Osmoreceptors in the hypothalamus detect the decrease in water potential
3. The hypothalamus stimulates the posterior pituitary gland to release more ADH into the blood
4. ADH increases the permeability of the collecting duct to water (by inserting aquaporin channels)
5. More water is reabsorbed from the collecting duct into the blood
6. Urine becomes more concentrated and lower in volume
7. Blood water potential returns to normal; osmoreceptors are no longer stimulated; ADH secretion decreases

A person drinks 2 litres of water within 30 minutes on a hot day after exercising. Describe the sequence of physiological events that restores their blood water potential to normal.

The nervous system has two main divisions:

1. Central Nervous System (CNS): Brain and spinal cord — processes information and coordinates responses
2. Peripheral Nervous System (PNS): All nerves outside the CNS — transmits signals between the CNS and the rest of the body

The PNS is further divided into:

• Somatic nervous system: Controls voluntary actions (conscious)
• Autonomic nervous system: Controls involuntary actions (unconscious)
• Sympathetic: “Fight or flight” — increases heart rate, dilates pupils, inhibits digestion
• Parasympathetic: “Rest and digest” — decreases heart rate, constricts pupils, stimulates digestion

Neurons are specialised cells that transmit electrical impulses.

Structure of a motor neuron:

• Cell body: Contains nucleus and organelles
• Dendrites: Receive impulses from other neurons
• Axon: Long fibre that carries impulses away from the cell body; insulated by myelin sheath (made by Schwann cells)
• Myelin sheath: Fatty layer that insulates the axon; allows saltatory conduction (impulses jump between Nodes of Ranvier), increasing the speed of transmission
• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated
• Motor end plates: Connections to muscle fibres at the neuromuscular junction

Resting potential: The inside of the neuron is negatively charged relative to the outside (~-70 MV). This is maintained by the sodium-potassium pump (pumps 3 Na$^+$ out for every 2 K$^+$ in) and The permeability of the membrane to K$^+$.

Action potential:

1. Stimulus causes voltage-gated Na$^+$ channels to open
2. Na$^+$ rushes in, depolarising the membrane (inside becomes positive, ~+40 mV)
3. Voltage-gated K$^+$ channels open
4. K$^+$ rushes out, repolarising the membrane
5. The Na$^+/K^+$ pump restores the resting potential (refractory period)

Propagation: The action potential travels along the axon. In myelinated neurons, it jumps Between Nodes of Ranvier (saltatory conduction), which is faster than continuous conduction in Unmyelinated neurons.

A synapse is the junction between two neurons or between a neuron and an effector.

Structure:

• Pre-synaptic neuron: Terminal knob containing synaptic vesicles filled with neurotransmitter
• Synaptic cleft: Gap (~20 nm) between the two neurons
• Post-synaptic neuron: Membrane contains receptor proteins specific to the neurotransmitter

Transmission across a synapse:

1. Action potential arrives at the pre-synaptic terminal
2. Voltage-gated Ca$^{2+}$ channels open; Ca$^{2+}$ enters the terminal
3. Ca$^{2+}$ causes synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitter into the synaptic cleft (exocytosis)
4. Neurotransmitter diffuses across the synaptic cleft
5. Neurotransmitter binds to specific receptors on the post-synaptic membrane
6. This triggers ion channels to open, generating a new action potential in the post-synaptic neuron
7. The neurotransmitter is broken down by enzymes or reabsorbed (taken up by the pre-synaptic neuron)

Key properties of synapses:

• Unidirectional: Impulses travel in one direction only (pre- to post-synaptic)
• Slow: Transmission is slower than along an axon (diffusion takes time)
• Summation: Multiple impulses may be needed to trigger an action potential (spatial summation from multiple neurons; temporal summation from rapid firing of one neuron)
• Inhibition: Some neurotransmitters are inhibitory (e.g., GABA), making the post-synaptic neuron less likely to fire

A reflex arc is the pathway taken by nerve impulses in an automatic, involuntary response (reflex).

Components:

1. Receptor: Detects the stimulus
2. Sensory neuron: Transmits impulse to the CNS
3. Relay neuron: In the spinal cord (or brain); connects sensory to motor neuron
4. Motor neuron: Transmits impulse from CNS to effector
5. Effector: Muscle or gland that produces the response

Example: Withdrawal reflex

• Touch a hot object
• Pain receptors in the skin detect heat
• Sensory neuron transmits impulse to the spinal cord
• Relay neuron passes impulse to motor neuron
• Motor neuron stimulates biceps to contract (withdraw hand) and triceps to relax
• The action is involuntary and rapid; the brain is informed afterwards

Structure:

Accommodation is the process of changing the shape of the lens to focus on objects at different Distances.

Focusing on a distant object:

1. Light rays from a distant object are nearly parallel
2. Ciliary muscles relax
3. Suspensory ligaments are pulled taut
4. Lens becomes thin (less curved)
5. Light is focused on the retina

Focusing on a near object:

1. Light rays from a near object are diverging
2. Ciliary muscles contract
3. Suspensory ligaments slacken
4. Lens becomes fat (more curved, more powerful refraction)
5. Light is focused on the retina

Structure:

1. Sound waves are collected by the pinna and directed along the ear canal
2. Sound waves cause the eardrum to vibrate
3. Vibrations are transmitted and amplified by the ossicles (malleus, incus, stapes)
4. The stapes pushes against the oval window, creating pressure waves in the cochlear fluid
5. Pressure waves cause the basilar membrane in the cochlea to vibrate
6. Hair cells on the basilar membrane are bent, generating nerve impulses
7. Impulses travel along the auditory nerve to the brain, where sound is interpreted
8. The round window allows the cochlear fluid to dissipate the pressure waves

The skin contains various sensory receptors:

The endocrine system consists of endocrine glands that secrete hormones directly into the Bloodstream. Hormones are chemical messengers that travel in the blood to target organs.

Blood glucose concentration is normally maintained at approximately 90 mg/100 cm$^3$.

If blood glucose rises (e.g., after a meal):

1. Detected by beta ($\beta$) cells in the islets of Langerhans (pancreas)
2. $\beta$ cells secrete more insulin
3. Insulin stimulates:

• Liver and muscle cells to take up glucose
• Conversion of glucose to glycogen (glycogenesis) in liver and muscles
• Increased rate of glucose respiration in cells

1. Blood glucose level decreases back to normal

If blood glucose falls (e.g., between meals or during exercise):

1. Detected by alpha ($\alpha$) cells in the islets of Langerhans (pancreas)
2. $\alpha$ cells secrete more glucagon
3. Glucagon stimulates:

• Conversion of glycogen to glucose (glycogenolysis) in the liver
• Conversion of amino acids and fats to glucose (gluconeogenesis) in the liver

1. Blood glucose level increases back to normal

This is an example of negative feedback.

After a 12-hour fast, a student’s blood glucose concentration is 85 mg/100 cm$^3$. They then drink a solution containing 50 g of glucose. Describe the hormonal response over the next 3 hours.

Type 1 Diabetes (insulin-dependent):

• Autoimmune destruction of $\beta$ cells in the pancreas
• No insulin is produced
• develops in childhood
• Treated with insulin injections

Type 2 Diabetes (non-insulin-dependent):

• Body cells become less responsive to insulin (insulin resistance)
• $\beta$ cells may produce less insulin over time
• develops in adults; linked to obesity and lifestyle
• Treated with dietary control, exercise, and oral medication; insulin may be needed later

Symptoms of diabetes:

• High blood glucose concentration (hyperglycaemia)
• Glucose in urine (kidneys cannot reabsorb all glucose above the renal threshold of ~180 mg/100 cm$^3$)
• Frequent urination (excess water follows glucose osmotically)
• Excessive thirst (dehydration from frequent urination)
• Weight loss (body breaks down fat and protein for energy)
• Fatigue

Sperm structure:

• Head: Contains the haploid nucleus (23 chromosomes) and the acrosome (contains enzymes to penetrate the egg)
• Middle piece: Packed with mitochondria to provide ATP for tail movement
• Tail (flagellum): Whiplash movement propels the sperm

The menstrual cycle is approximately 28 days and is controlled by four hormones:

1. FSH (Follicle-Stimulating Hormone): Produced by the anterior pituitary gland; stimulates the development of a follicle in the ovary; stimulates the ovaries to produce oestrogen
2. Oestrogen: Produced by the developing follicle; causes the endometrium to thicken; inhibits FSH production (negative feedback); stimulates LH release (positive feedback when level is high enough)
3. LH (Luteinising Hormone): Produced by the anterior pituitary gland; triggers ovulation (day 14); stimulates the remains of the follicle to develop into the corpus luteum
4. Progesterone: Produced by the corpus luteum; maintains the thickened endometrium; inhibits FSH and LH production

1. Sperm are deposited in the vagina during intercourse
2. Sperm swim through the cervix and uterus into the oviduct
3. One sperm penetrates the zona pellucida of the ovum (acrosome reaction)
4. The sperm nucleus fuses with the ovum nucleus — fertilisation produces a zygote
5. The zygote divides by mitosis as it travels along the oviduct (forms a ball of cells called a morula, then a blastocyst)
6. The blastocyst implants into the thickened endometrium (~7 days after fertilisation)
7. The placenta forms, providing nutrients and gas exchange between mother and foetus

Prevention: Use of condoms; vaccination (HPV); monogamy; regular testing.

A complex machine: The human body is like a city — the heart is the pump, lungs are air filters, kidneys are water treatment plants, and the brain is city hall. Each organ system keeps the body functioning.

Why it matters: Understanding physiology helps you make informed health decisions and explains how lifestyle choices affect your body.

The key insight: Homeostasis — the body’s ability to maintain internal stability — is the foundation of health. When homeostasis fails, disease occurs.

1. Confusing excretion and egestion: Egestion (removal of faeces) is not excretion. Faeces never entered the body’s metabolic processes. CO$_2$ from respiration and urea from deamination are excretory products.

2. Stating bile digests fat: Bile emulsifies fat (physical breakdown into droplets). It does NOT chemically digest fat. Lipase digests fat.

3. Mixing up ventilation pressures: During inhalation, lung pressure is LOWER than atmospheric pressure. During exhalation, lung pressure is HIGHER. The volume-pressure relationship is inverse (Boyle’s law).

4. Saying veins carry deoxygenated blood: The pulmonary vein carries oxygenated blood from the lungs to the heart. Always specify pulmonary vs systemic.

5. Confusing osmoregulation direction: High ADH means concentrated urine (less water excreted), not dilute urine. ADH promotes water reabsorption.

6. Confusing antagonistic hormones: Insulin LOWERS blood glucose. Glucagon RAISES blood glucose. Students often swap them.

7. Mixing up sympathetic and parasympathetic: Sympathetic = fight or flight (pupil dilation, heart rate up). Parasympathetic = rest and digest (pupil constriction, heart rate down).

8. Forgetting that sensory neurons carry impulses TO the CNS: The direction of impulse travel matters. Sensory: receptor to CNS. Motor: CNS to effector.

9. Writing that the loop of Henle reabsorbs water in both limbs: Only the descending limb is permeable to water. The ascending limb actively transports Na$^+$ and Cl$^-$ and is impermeable to water.

10. Confusing the roles of FSH and LH: FSH stimulates follicle development. LH triggers ovulation and maintains the corpus luteum. Do not swap them.

Problem 1: A student sets up an experiment to investigate the effect of pH on the activity of salivary amylase. Test tubes containing starch solution and amylase are incubated at different pH values. After 10 minutes, the presence of starch is tested with iodine solution.

(a) At which pH is amylase most active? Explain.

(b) Explain why starch remains at pH 3.

(c) If the temperature were raised to 80 degrees C at pH 7, what would happen to the results? Why?

If you get this wrong, revise: Digestive System — The Mouth; Enzymes

Problem 2: Explain how the structure of an alveolus is adapted for efficient gas exchange. Refer to at least three structural features in your answer.

If you get this wrong, revise: Gas Exchange — Gas Exchange in the Alveoli

Problem 3: A person produces 180 dm$^3$ of glomerular filtrate per day but only excretes 1.5 dm$^3$ of urine per day.

(a) Calculate the percentage of filtrate that is reabsorbed.

(b) Which part of the nephron is primarily responsible for this reabsorption? Explain the mechanism.

If you get this wrong, revise: Excretion — Ultrafiltration; Selective Reabsorption

Problem 4: A patient with blood group B requires a blood transfusion. Explain why blood group O can be used but blood group A cannot.

If you get this wrong, revise: Transport in Humans — Blood Groups

Problem 5: Describe the roles of FSH, oestrogen, LH, and progesterone in the menstrual cycle.

If you get this wrong, revise: Human Reproduction — The Menstrual Cycle

Problem 6: A myelinated neuron has nodes of Ranvier spaced 1.5 mm apart. The action potential travels at 120 m/s in myelinated regions. An equivalent unmyelinated neuron conducts at 0.5 m/s. Compare the time for an impulse to travel 30 mm in each neuron.

If you get this wrong, revise: Nervous Coordination — Nerve Impulses

Problem 7: Explain why the oxygen dissociation curve shifts to the right during exercise. What is the significance of this shift?

If you get this wrong, revise: Transport in Humans — The Oxygen Dissociation Curve

Problem 8: A person with Type 1 diabetes has a fasting blood glucose level of 280 mg/100 cm$^3$. The renal threshold for glucose is approximately 180 mg/100 cm$^3$.

(a) Explain why glucose is present in the urine.

(b) Explain why this person feels thirsty and produces large volumes of urine.

(c) How does insulin injection help to correct this condition?

If you get this wrong, revise: Hormonal Coordination — Blood Glucose Regulation; Diabetes

Problem 9: Explain why the left ventricle has a thicker muscular wall than the right ventricle, and explain the role of valves in the heart.

If you get this wrong, revise: Transport in Humans — Structure of the Heart; Cardiac Cycle

Problem 10: Explain how a nerve impulse is transmitted across a synapse. Why is transmission in one direction only?

If you get this wrong, revise: Nervous Coordination — Synapses

The large intestine is approximately 1.5 metres long. Its primary functions are water reabsorption and faeces formation.

Key processes:

1. Water reabsorption: Approximately 90% of remaining water is reabsorbed by osmosis
2. Mineral absorption: $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ are actively absorbed
3. Vitamin synthesis: Commensal bacteria synthesise vitamins K and B, which are absorbed
4. Mucus secretion: Goblet cells secrete mucus to lubricate faeces
5. Faeces composition: Undigested fibre, bacteria, water, bile pigments (stercobilin)

Bile is NOT an enzyme. It emulsifies fats physically (increases surface area) but does NOT chemically digest fats. Lipase chemically digests fats.

• Human Reproduction and Homeostasis — Homeostasis extends the organ system physiology covered here to reproductive and endocrine regulation.
• Plant Biology — Comparing animal and plant transport systems highlights adaptations to different environments.
• Genetics — Enzyme and protein synthesis underlying physiological processes are encoded by genes.
• Organic Chemistry — Metabolism of carbohydrates, lipids, and proteins involves organic reaction mechanisms.