Homeostasis

Principles of Homeostasis

What is Homeostasis?

Definition. Homeostasis is the maintenance of a constant internal environment despite changes in the external environment. It is essential for the proper functioning of enzymes and metabolic processes, which require specific conditions of temperature, pH, water potential, and glucose concentration.

Key concept: Homeostasis involves detecting a change from the set point, initiating a corrective response, and returning the condition to its normal range. This is achieved through negative feedback mechanisms.

Negative Feedback

Negative feedback is the most common mechanism of homeostatic control. When a physiological variable deviates from its set point, the body initiates a response that reverses the deviation, bringing the variable back to normal.

General pattern of negative feedback:

1. Stimulus: A change in the internal environment (e.g., rise in blood glucose)
2. Receptor: Detects the change and sends signals to a coordinator
3. Coordinator: Processes the information and sends signals to an effector
4. Effector: Carries out a response that reverses the change
5. Return to normal: The variable returns to the set point; the receptor detects this and reduces its output (feedback)

Stimulus --> Receptor --> Coordinator --> Effector --> Response
                                              |
        <--- Reduced output (feedback) -------+

Examples of negative feedback in the body:

Physiological Variable	Set Point (Approximate)	Receptor	Coordinator	Effector	Response
Blood glucose	90 mg/100 cm$^3$	Pancreas ($\beta$/$\alpha$ cells)	Pancreas (hormonal)	Liver, muscles	Glycogenesis/glycogenolysis
Body temperature	37 degrees C	Hypothalamus (thermoreceptors)	Hypothalamus	Skin, blood vessels	Sweating, vasodilation/shivering, vasoconstriction
Blood water potential	Normal osmolarity	Hypothalamus (osmoreceptors)	Hypothalamus --> posterior pituitary	Kidneys (collecting duct)	ADH release; water reabsorption
Blood pH	7.35-7.45	Chemoreceptors (aorta, carotid, medulla)	Medulla (breathing centre)	Lungs, kidneys	Changes in breathing rate; bicarbonate excretion

Positive Feedback

Positive feedback amplifies a change from the set point, pushing the variable further from normal. Unlike negative feedback, which restores balance, positive feedback accelerates a process until it is completed.

Examples of positive feedback:

Process	Mechanism
Ovulation	Rising oestrogen levels stimulate LH release (positive feedback); the LH surge triggers ovulation. After ovulation, the positive feedback loop ends.
Blood clotting	When a vessel is damaged, clotting factors are activated in a cascade; each step activates more factors, amplifying the response until a clot forms.
Childbirth (parturition)	The baby's head pushes against the cervix, stimulating stretch receptors. Impulses cause the posterior pituitary to release oxytocin, which causes stronger uterine contractions. Stronger contractions push the baby further against the cervix, stimulating more oxytocin release. This cycle continues until birth.

warning

A common DSE pitfall is to assume all feedback loops are negative. The DSE specification requires knowledge of positive feedback examples (ovulation and childbirth are the most commonly tested). Remember: negative feedback maintains homeostasis; positive feedback amplifies change and does NOT maintain a constant internal environment.

Components of a Homeostatic Control System

Component	Role	Example
Receptor	Detects changes in the internal environment (stimuli)	Thermoreceptors in the hypothalamus and skin detect temperature changes
Coordinator	Processes information from receptors and sends signals to effectors	Hypothalamus processes temperature information and sends nerve impulses to effectors
Effector	Carries out the response that restores the variable to its set point	Sweat glands secrete sweat; blood vessels dilate or constrict; muscles shiver
Set point	The normal value or range of the physiological variable	Body temperature: 37 degrees C; blood glucose: approximately 90 mg/100 cm$^3$
Feedback	The output of the effector is monitored by the receptor, allowing the system to self-regulate	As body temperature falls back to normal, thermoreceptors reduce their firing rate

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Blood Glucose Regulation

Normal Blood Glucose Homeostasis

Blood glucose concentration is maintained at approximately 90 mg/100 cm$^3$ (range: approximately 70-110 mg/100 cm$^3$) by the antagonistic actions of two pancreatic hormones: insulin and glucagon.

After a meal (blood glucose rises):

1. Blood glucose concentration increases as glucose is absorbed from the small intestine
2. $\beta$ cells in the islets of Langerhans detect the elevated glucose
3. $\beta$ cells secrete more insulin into the blood
4. Insulin binds to receptors on target cells (liver, muscle, adipose tissue), triggering:
   • Increased glucose uptake by cells (via GLUT4 transporter proteins, which are translocated to the cell membrane in response to insulin)
   • Increased rate of glycolysis (glucose respiration within cells)
   • Increased glycogenesis (conversion of glucose to glycogen for storage in liver and muscle)
   • Decreased glycogenolysis (inhibition of glycogen breakdown)
   • Increased lipogenesis (conversion of glucose to fat for long-term storage)
   • Decreased gluconeogenesis (inhibition of new glucose synthesis)
5. Blood glucose concentration decreases back to the set point
6. As blood glucose falls, $\beta$ cells detect this and reduce insulin secretion (negative feedback)

Between meals or during exercise (blood glucose falls):

1. Blood glucose concentration decreases as cells consume glucose
2. $\alpha$ cells in the islets of Langerhans detect the low glucose
3. $\alpha$ cells secrete more glucagon into the blood
4. Glucagon binds to receptors on liver cells (hepatocytes), triggering:
   • Increased glycogenolysis (breakdown of glycogen into glucose in the liver)
   • Increased gluconeogenesis (conversion of amino acids and glycerol to glucose in the liver)
   • Increased lipolysis (breakdown of fat into fatty acids and glycerol)
5. Blood glucose concentration increases back to the set point
6. As blood glucose rises, $\alpha$ cells reduce glucagon secretion (negative feedback)

The Role of the Liver in Blood Glucose Regulation

The liver is the central organ in blood glucose homeostasis. It acts as a "glucose buffer," removing glucose from the blood when levels are high and releasing glucose when levels are low.

Process	Trigger	Description	Direction
Glycogenesis	High blood glucose	Conversion of glucose to glycogen for storage	Stores glucose
Glycogenolysis	Low blood glucose	Breakdown of glycogen to glucose-6-phosphate, then to free glucose (liver has glucose-6-phosphatase)	Releases glucose
Gluconeogenesis	Low blood glucose	Synthesis of glucose from non-carbohydrate sources (lactate, amino acids, glycerol)	Produces new glucose

Critical distinction -- liver vs muscle:

• The liver can both store glycogen AND release free glucose into the blood because hepatocytes possess the enzyme glucose-6-phosphatase, which removes the phosphate from glucose-6-phosphate, allowing free glucose to diffuse into the blood.
• Muscle cells can store glycogen and break it down for their own use (glycolysis), but they CANNOT release free glucose into the blood because they lack glucose-6-phosphatase. Muscle glycogenolysis produces glucose-6-phosphate, which can only be used within the muscle cell itself.

The Renal Threshold for Glucose

The kidneys normally reabsorb all glucose from the glomerular filtrate in the proximal convoluted tubule (PCT). The maximum rate at which glucose can be reabsorbed is approximately 375 mg/min, corresponding to a blood glucose concentration of approximately 180 mg/100 cm$^3$ (the renal threshold).

• If blood glucose is below 180 mg/100 cm$^3$: all glucose is reabsorbed; no glucose in urine
• If blood glucose exceeds 180 mg/100 cm$^3$: carrier proteins in the PCT become saturated; excess glucose remains in the filtrate and appears in the urine (glycosuria)

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Temperature Regulation (Thermoregulation)

The Hypothalamus as the Thermoregulatory Centre

The hypothalamus contains the body's thermostat. It receives input from two types of thermoreceptor:

1. Central thermoreceptors: Located in the hypothalamus itself, they monitor the temperature of the blood passing through the brain
2. Peripheral thermoreceptors: Located in the skin (dermis), they monitor the external temperature

The hypothalamus compares the input from these receptors with the set point (approximately 37 degrees C) and initiates corrective responses when the temperature deviates.

Response to Heat (Cooling Mechanisms)

When the body temperature rises above the set point:

Response	Mechanism
Vasodilation	Arterioles supplying the skin dilate, increasing blood flow to the skin surface. More heat is lost by radiation from the blood to the environment.
Sweating	Sweat glands secrete sweat onto the skin surface. Water in sweat evaporates, absorbing latent heat of vaporisation from the skin (approximately 2.4 kJ per gram of water). This cools the blood in the skin capillaries.
Increased breathing	Panting or faster breathing increases evaporative heat loss from the lungs and respiratory tract.
Behavioural changes	Removing clothing; seeking shade; drinking cold fluids
Decreased metabolic rate	Cells reduce their rate of respiration, producing less heat as a by-product

Response to Cold (Warming Mechanisms)

When the body temperature falls below the set point:

Response	Mechanism
Vasoconstriction	Arterioles supplying the skin constrict, reducing blood flow to the skin surface. Less heat is lost from the blood to the environment. More blood is diverted to the core organs to maintain their temperature.
Shivering (thermogenesis)	Rapid, involuntary contractions of skeletal muscles. Muscle respiration generates heat as a by-product, warming the body. Shivering can increase heat production by 2-5 times the resting rate.
Piloerection	Hair erector muscles contract, causing body hairs to stand upright. This traps a layer of insulating air next to the skin (more significant in furry animals than in humans).
Increased metabolic rate	Thyroid gland releases more thyroxine, increasing the basal metabolic rate of cells. Cells respire more, producing more heat. Adrenaline also increases metabolic rate.
Behavioural changes	Putting on warm clothing; curling up to reduce surface area; seeking shelter; drinking hot drinks
Brown fat metabolism	Brown adipose tissue (especially in newborns) contains many mitochondria that can uncouple respiration from ATP production, releasing energy directly as heat (non-shivering thermogenesis).

Worked Example: Thermoregulation Calculation

A person produces 150 g of sweat per hour during exercise in a hot environment. The latent heat of vaporisation of water is 2.4 kJ/g. Calculate the rate of heat loss through sweating.

Solution

Heat loss = mass of sweat $\times$ latent heat of vaporisation = $150 \mathrm{ g} \times 2.4 \mathrm{ kJ/g} = 360 \mathrm{ kJ/hour}$

This is a significant contribution to heat loss. The basal metabolic rate produces approximately 300 kJ/hour, so sweating during exercise can remove more heat than the body produces at rest. This explains why adequate hydration is critical during exercise in hot environments -- sweat that is not replaced leads to dehydration and reduced blood volume, impairing thermoregulation.

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Osmoregulation

The Kidneys and Water Balance

The kidneys regulate the water potential of the blood by adjusting the volume and concentration of urine produced. This process is called osmoregulation.

Key principle: If blood water potential is too low (blood is too concentrated), the kidneys produce a small volume of concentrated urine to conserve water. If blood water potential is too high (blood is too dilute), the kidneys produce a large volume of dilute urine to excrete excess water.

ADH and the Regulation of Water Balance

Antidiuretic hormone (ADH / vasopressin) is the key hormone in osmoregulation.

Synthesis and release:

• ADH is produced by neurosecretory cells in the hypothalamus
• It is transported along their axons and stored in the posterior pituitary gland
• It is released into the blood when needed

When blood water potential is low (dehydration):

1. Osmoreceptors in the hypothalamus detect the decreased blood water potential (increased solute concentration)
2. The hypothalamus stimulates the posterior pituitary to release ADH into the blood
3. ADH binds to receptors on the cells of the collecting duct
4. This triggers the insertion of aquaporin (water channel) proteins into the collecting duct cell membrane
5. The collecting duct becomes more permeable to water
6. Water is reabsorbed from the collecting duct by osmosis into the concentrated medullary tissue fluid (maintained by the counter-current multiplier of the loop of Henle)
7. Less water is lost in the urine: urine volume decreases, urine concentration increases
8. Blood water potential increases back to normal (negative feedback)
9. The osmoreceptors reduce their stimulation of the posterior pituitary, decreasing ADH release

When blood water potential is high (over-hydration):

1. Osmoreceptors detect the increased blood water potential (decreased solute concentration)
2. The hypothalamus reduces stimulation of the posterior pituitary
3. Less ADH is released into the blood
4. With less ADH, collecting duct cells remove aquaporin channels from their membranes
5. The collecting duct becomes less permeable to water
6. Less water is reabsorbed: urine volume increases, urine becomes more dilute
7. Excess water is excreted, and blood water potential returns to normal (negative feedback)

The Nephron: Detailed Structure and Function

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

Region	Function
Renal (Bowman's) capsule	Ultrafiltration: high blood pressure in the glomerulus forces small molecules (water, glucose, urea, ions) out of the blood into the capsule
Proximal convoluted tubule (PCT)	Selective reabsorption: reabsorbs ALL glucose, ALL amino acids, approximately 85% of water, and most $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ by active transport and co-transport
Loop of Henle	Creates a concentration gradient in the medulla via the counter-current multiplier. Descending limb: permeable to water (water out). Ascending limb: actively transports $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ out (impermeable to water).
Distal convoluted tubule (DCT)	Fine-tuning of water and ion balance; regulated by aldosterone ($\mathrm{Na}^+$ reabsorption)
Collecting duct	Regulated by ADH: water reabsorption depends on ADH level; permeability controlled by aquaporin channels

The Counter-Current Multiplier

The loop of Henle creates and maintains a concentration gradient in the medulla, with tissue fluid becoming progressively more concentrated from the cortex to the inner medulla (approximately 300 mOsm at the cortex to approximately 1200 mOsm at the inner medulla).

Mechanism:

1. Descending limb: Water passes out by osmosis into the increasingly concentrated medullary tissue fluid. The filtrate becomes more concentrated as it descends.
2. Hairpin turn: At the bottom of the loop, the filtrate is at its most concentrated.
3. Ascending limb (thin segment): $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ diffuse out passively into the medullary tissue fluid.
4. Ascending limb (thick segment): $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ are actively transported OUT of the ascending limb into the medullary tissue fluid. The ascending limb is impermeable to water, so the filtrate becomes increasingly dilute as it ascends.
5. Counter-current flow: Because the descending and ascending limbs flow in opposite directions (counter-current), the active transport of salt from the ascending limb continually adds to the concentration of the medullary tissue fluid adjacent to the descending limb. This amplifies the gradient (multiplier effect).

Purpose of the medullary concentration gradient: The concentrated medullary tissue fluid provides the osmotic gradient that drives water reabsorption from the collecting duct when ADH is present. Without this gradient, the kidneys could not produce concentrated urine.

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Blood pH Regulation

Why Blood pH Must Be Regulated

Blood pH is normally maintained between 7.35 and 7.45. Enzymes and metabolic processes are highly sensitive to pH changes. A blood pH below 7.35 is called acidosis; above 7.45 is called alkalosis. Either extreme can be fatal.

Buffer Systems

A buffer resists changes in pH by minimising the effect of adding acid or alkali.

The bicarbonate buffer system:

$\mathrm{H}^+ + \mathrm{HCO}_3^- \rightleftharpoons \mathrm{H}_2\mathrm{CO}_3 \rightleftharpoons \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}$

• When $\mathrm{H}^+$ ions are added (acid): they combine with $\mathrm{HCO}_3^-$ to form $\mathrm{H}_2\mathrm{CO}_3$, which dissociates into $\mathrm{CO}_2$ and $\mathrm{H}_2\mathrm{O}$. The $\mathrm{CO}_2$ is exhaled by the lungs.
• When $\mathrm{H}^+$ ions are removed (alkali): $\mathrm{H}_2\mathrm{CO}_3$ dissociates to release more $\mathrm{H}^+$, restoring pH.

Haemoglobin as a buffer:

• Haemoglobin can bind $\mathrm{H}^+$ ions, acting as a buffer against acidification.
• In the tissues: $\mathrm{CO}_2$ diffuses into red blood cells and is converted to $\mathrm{HCO}_3^-$ and $\mathrm{H}^+$ by carbonic anhydrase. Haemoglobin binds the $\mathrm{H}^+$, preventing a large pH drop.
• In the lungs: the reaction reverses; $\mathrm{H}^+$ is released, combines with $\mathrm{HCO}_3^-$, and $\mathrm{CO}_2$ is exhaled.

Respiratory Regulation of pH

• Increased $\mathrm{CO}_2$ in the blood produces carbonic acid, lowering pH
• Chemoreceptors detect the decrease in pH and stimulate the ventilation centre in the medulla
• Breathing rate and depth increase, exhaling more $\mathrm{CO}_2$
• Blood pH rises back to normal
• This is a rapid response (seconds to minutes) but can only compensate for changes related to $\mathrm{CO}_2$

Renal Regulation of pH

The kidneys provide a slower but more powerful mechanism for pH regulation (takes hours to days):

• In acidosis (low pH): the kidneys excrete more $\mathrm{H}^+$ ions in the urine and reabsorb more $\mathrm{HCO}_3^-$ into the blood. Ammonia ($\mathrm{NH}_3$) produced by the kidney tubule cells buffers $\mathrm{H}^+$ in the urine, forming ammonium ions ($\mathrm{NH}_4^+$) that are excreted.
• In alkalosis (high pH): the kidneys excrete more $\mathrm{HCO}_3^-$ in the urine and reabsorb fewer $\mathrm{H}^+$ ions, allowing pH to decrease.

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Diabetes Mellitus

Type 1 Diabetes

Feature	Detail
Cause	Autoimmune destruction of $\beta$ cells in the islets of Langerhans
Insulin	NOT produced (absolute insulin deficiency)
Onset	Usually in childhood or adolescence
Treatment	Insulin injections (lifelong); blood glucose monitoring; dietary management
Mechanism	Without insulin, cells cannot take up glucose; blood glucose rises; glucose appears in urine (glycosuria) when it exceeds the renal threshold (approximately 180 mg/100 cm$^3$)

Type 2 Diabetes

Feature	Detail
Cause	Body cells become resistant to insulin (reduced sensitivity of insulin receptors); $\beta$ cells may gradually decline
Insulin	Initially produced (sometimes at higher levels than normal); eventually production may decrease
Onset	Usually in adults (increasingly seen in younger people); strongly linked to obesity, physical inactivity, and diet
Treatment	Lifestyle changes (diet, exercise); oral medication (e.g., metformin, which increases insulin sensitivity and reduces hepatic glucose production); insulin may be needed in advanced cases
Risk factors	Obesity, sedentary lifestyle, family history, high-calorie diet, ageing

Symptoms of Diabetes

Symptom	Mechanism
Hyperglycaemia (high blood glucose)	Insulin deficiency or resistance prevents glucose uptake by cells
Glycosuria (glucose in urine)	Blood glucose exceeds the renal threshold; PCT carrier proteins are saturated
Polyuria (excessive urination)	Excess glucose in the filtrate lowers water potential; less water reabsorbed by osmosis; large volume of urine produced
Polydipsia (excessive thirst)	Loss of water in urine reduces blood volume and increases blood osmolarity, detected by hypothalamic osmoreceptors, stimulating thirst
Weight loss	Body breaks down fat and protein for energy (since glucose cannot enter cells); loss of calories in urine (glucose)
Fatigue	Cells cannot use glucose efficiently for energy
Blurred vision	High blood glucose causes osmotic changes in the lens of the eye
Slow wound healing	High glucose impairs immune function and circulation

Long-Term Complications of Diabetes

Complication	Mechanism
Cardiovascular disease	Damage to blood vessels from chronic hyperglycaemia; increased atherosclerosis; increased risk of heart attack and stroke
Kidney damage (nephropathy)	High glucose damages the glomerular filtration membrane, reducing kidney function; can lead to kidney failure
Nerve damage (neuropathy)	High glucose damages peripheral nerves, causing numbness, tingling, and pain, especially in the extremities
Retinopathy	Damage to blood vessels in the retina; can cause blindness
Foot ulcers and amputation	Reduced sensation (neuropathy) and poor circulation lead to undetected injuries and poor healing

Worked Example: Diabetes and Kidney Function

A patient with uncontrolled Type 1 diabetes has a fasting blood glucose concentration of 320 mg/100 cm$^3$. The renal threshold for glucose is 180 mg/100 cm$^3$. Normal GFR produces 180 dm$^3$ of filtrate per day. The maximum rate of glucose reabsorption in the PCT is 375 mg/min.

(a) Calculate the mass of glucose filtered per day. (b) Calculate the mass of glucose reabsorbed per day. (c) Calculate the mass of glucose excreted in urine per day. (d) Explain why this patient produces large volumes of urine.

Solution

(a) Glucose filtered per day = concentration $\times$ volume = $320 \mathrm{ mg/100 cm}^3 \times 180 \times 10 \mathrm{ dm}^3$

$= 320 \times 10 \mathrm{ mg/dm}^3 \times 180 \mathrm{ dm}^3 = 576\,000 \mathrm{ mg/day} = 576 \mathrm{ g/day}$

(b) Maximum glucose reabsorption = $375 \mathrm{ mg/min} \times 60 \times 24 = 540\,000 \mathrm{ mg/day} = 540 \mathrm{ g/day}$

(c) Glucose excreted = glucose filtered - glucose reabsorbed = $576 - 540 = 36 \mathrm{ g/day}$

(d) The excess glucose in the filtrate (36 g/day that is not reabsorbed) lowers the water potential of the tubule fluid. Less water is reabsorbed from the nephron by osmosis (because the water potential gradient between the tubule fluid and the blood is reduced). This produces a large volume of dilute urine (polyuria). The loss of excess water reduces blood volume and increases blood osmolarity, which stimulates osmoreceptors in the hypothalamus, triggering thirst (polydipsia).

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Common Pitfalls

1. Confusing negative and positive feedback: Negative feedback reverses a change and maintains homeostasis (e.g., blood glucose regulation). Positive feedback amplifies a change and does not maintain homeostasis (e.g., ovulation, childbirth). Do not write that ovulation is controlled by negative feedback.

2. 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 is frequently tested.

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

4. Writing that muscle cells release glucose into the blood: Muscle cells break down glycogen for their own use but CANNOT release free glucose because they lack glucose-6-phosphatase. Only the liver can release glucose into the blood.

5. Stating that both limbs of the loop of Henle are permeable to water: Only the DESCENDING limb is permeable to water. The ASCENDING limb is impermeable to water but actively transports $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ out. This asymmetry is essential for creating the medullary concentration gradient.

6. Writing that high ADH causes dilute urine: The opposite is true. High ADH causes concentrated urine (more water is reabsorbed). Low ADH causes dilute urine (more water is excreted).

7. Confusing the roles of receptors and effectors: Receptors DETECT changes; effectors CARRY OUT responses. Do not write that the skin "detects" cold (skin has thermoreceptors that detect cold, and the skin is also an effector -- blood vessels constrict/dilate, sweat glands secrete).

8. Writing that temperature regulation is controlled by hormones alone: Temperature regulation is primarily controlled by the NERVOUS system (hypothalamus detects changes and sends nerve impulses to effectors). Thyroxine and adrenaline play supporting roles in the cold response but are not the primary mechanism.

9. Confusing acidosis and alkalosis: Acidosis = low blood pH (below 7.35); alkalosis = high blood pH (above 7.45). High $\mathrm{CO}_2$ causes acidosis (respiratory acidosis). Excessive vomiting (loss of stomach acid) can cause alkalosis (metabolic alkalosis).

10. Writing that Type 2 diabetes involves no insulin production: Type 2 diabetes involves insulin RESISTANCE, not absence of insulin. In the early stages, insulin levels may be normal or even elevated. Type 1 diabetes involves NO insulin production.

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Problem Set

Problem 1: Explain the difference between negative and positive feedback. For each type, describe one example of its role in human physiology and explain why it is appropriate for that physiological process.

If you get this wrong, revise: Principles of Homeostasis -- Negative Feedback; Positive Feedback

Solution

Negative feedback reverses a deviation from the set point, restoring the variable to normal. Example: blood glucose regulation. After a meal, blood glucose rises; insulin is released, promoting glucose uptake and storage; blood glucose falls back to normal; insulin secretion decreases. This is appropriate because the body needs to maintain blood glucose within a narrow range for cellular function.

Positive feedback amplifies a deviation, pushing the variable further from the set point. Example: childbirth. Pressure of the baby's head on the cervix triggers oxytocin release; oxytocin causes stronger uterine contractions; stronger contractions push the baby further against the cervix, stimulating more oxytocin release. This is appropriate because the process needs to reach completion (birth) rather than returning to a set point. Once the baby is born, the positive feedback loop ends.

Problem 2: Describe the mechanism by which ADH regulates water balance in the kidneys. Explain why a person in a hot desert environment would have high blood ADH levels.

If you get this wrong, revise: Osmoregulation -- ADH and the Regulation of Water Balance

Solution

ADH is released from the posterior pituitary when blood water potential is low. It binds to receptors on collecting duct cells, triggering the insertion of aquaporin channels into the cell membrane. This makes the collecting duct more permeable to water. Water is reabsorbed from the collecting duct by osmosis into the concentrated medullary tissue fluid (maintained by the loop of Henle counter-current multiplier). The result is a small volume of concentrated urine, conserving water in the body.

In a hot desert, the person loses water through sweating (thermoregulation) and exhalation. This reduces blood volume and increases blood solute concentration (lowers blood water potential). Osmoreceptors in the hypothalamus detect this and stimulate the posterior pituitary to release more ADH. The kidneys produce a small volume of concentrated urine to minimise further water loss, helping the person survive in the arid environment.

Problem 3: Compare Type 1 and Type 2 diabetes in terms of cause, insulin levels, age of onset, treatment, and the mechanism by which blood glucose rises to harmful levels.

If you get this wrong, revise: Diabetes Mellitus -- Type 1 Diabetes; Type 2 Diabetes

Solution

Feature	Type 1 Diabetes	Type 2 Diabetes
Cause	Autoimmune destruction of $\beta$ cells	Insulin resistance (reduced receptor sensitivity); genetic and lifestyle factors
Insulin	Not produced (absolute deficiency)	Initially normal or elevated; may decrease over time (relative deficiency)
Onset	Usually childhood/adolescence	Usually adulthood (but increasingly in younger people)
Treatment	Insulin injections (lifelong)	Diet, exercise, oral medication; insulin in advanced cases
Mechanism	No insulin means cells cannot take up glucose; glucose accumulates in blood	Cells become resistant to insulin; glucose uptake is impaired despite insulin presence

In both types, blood glucose rises because glucose cannot enter cells efficiently. Excess glucose is excreted in the urine (glycosuria), causing water loss (polyuria) and thirst (polydipsia). Long-term complications (cardiovascular disease, kidney damage, nerve damage, retinopathy) result from chronic hyperglycaemia damaging blood vessels and tissues.

Gestational Diabetes

Gestational diabetes mellitus (GDM) is a form of diabetes that develops during pregnancy and usually resolves after childbirth.

Feature	Detail
Cause	Placental hormones (human placental lactogen, progesterone, cortisol) cause insulin resistance in maternal tissues; the pancreas may not produce enough additional insulin to compensate
Onset	Typically during the second or third trimester (24-28 weeks)
Risk factors	Obesity, family history of Type 2 diabetes, previous GDM, maternal age over 35, certain ethnicities
Risks to mother	Increased risk of pre-eclampsia; increased risk of developing Type 2 diabetes later in life
Risks to foetus	Macrosomia (excessive birth weight); hypoglycaemia after birth; respiratory distress syndrome; increased risk of obesity and Type 2 diabetes in adulthood
Management	Dietary control; exercise; blood glucose monitoring; insulin if needed (oral hypoglycaemics are generally avoided in pregnancy)

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Endocrine Disorders

Thyroid Disorders

Hyperthyroidism (overactive thyroid):

Feature	Detail
Cause	Graves' disease (autoimmune); thyroid nodule; excessive TSH
Hormone	Excess thyroxine ($\mathrm{T}_4$) and triiodothyronine ($\mathrm{T}_3$)
Symptoms	Increased BMR; weight loss despite increased appetite; rapid heart rate (tachycardia); anxiety; tremor; heat intolerance; sweating; exophthalmos (bulging eyes in Graves' disease)
Treatment	Anti-thyroid drugs (e.g., carbimazole); radioactive iodine; surgical removal of part or all of the thyroid

Hypothyroidism (underactive thyroid):

Feature	Detail
Cause	Hashimoto's thyroiditis (autoimmune); iodine deficiency; surgical removal; radioactive iodine treatment
Hormone	Insufficient thyroxine ($\mathrm{T}_4$) and triiodothyronine ($\mathrm{T}_3$)
Symptoms	Decreased BMR; weight gain; fatigue; cold intolerance; dry skin; slow heart rate (bradycardia); depression; myxoedema (swelling of skin and tissues)
Treatment	Thyroxine replacement therapy (daily levothyroxine tablets)

Adrenal Disorders

Addison's disease (adrenal insufficiency):

• Destruction of the adrenal cortex (autoimmune or tuberculosis)
• Reduced production of cortisol and aldosterone
• Symptoms: fatigue, weight loss, low blood pressure, hyperpigmentation of skin, salt craving, hypoglycaemia
• Treatment: cortisol replacement (hydrocortisone); fludrocortisone (to replace aldosterone)

Cushing's syndrome:

• Excess cortisol (from prolonged corticosteroid medication or adrenal tumour)
• Symptoms: weight gain (especially abdominal), moon face, buffalo hump, thin skin, easy bruising, high blood pressure, muscle weakness, hyperglycaemia

Worked Example: Endocrine Feedback Disorder

A patient presents with weight gain, fatigue, cold intolerance, and a slow heart rate. Blood tests show elevated TSH and low $\mathrm{T}_4$. Explain the normal negative feedback loop controlling thyroid hormone levels and how it has been disrupted in this patient.

Solution

Normal negative feedback loop:

1. The hypothalamus releases TRH (thyrotropin-releasing hormone), which stimulates the anterior pituitary
2. The anterior pituitary releases TSH (thyroid-stimulating hormone), which stimulates the thyroid gland
3. The thyroid gland produces and releases $\mathrm{T}_4$ (thyroxine) and $\mathrm{T}_3$ (triiodothyronine)
4. $\mathrm{T}_4$ and $\mathrm{T}_3$ exert negative feedback on both the hypothalamus (reducing TRH) and the anterior pituitary (reducing TSH)
5. When $\mathrm{T}_4$/$\mathrm{T}_3$ levels are high, TRH and TSH decrease; when levels are low, TRH and TSH increase

In this patient (hypothyroidism):

The thyroid gland is not producing enough $\mathrm{T}_4$ (low $\mathrm{T}_4$). The reduced negative feedback means the pituitary continues to produce high levels of TSH (elevated TSH) in an attempt to stimulate the thyroid. The thyroid is unable to respond adequately (due to autoimmune destruction in Hashimoto's thyroiditis or other damage). The symptoms are explained by the lack of thyroxine: low BMR causes fatigue and weight gain; low $\mathrm{T}_3$ reduces metabolic heat production, causing cold intolerance; low thyroxine reduces the heart's sensitivity to catecholamines, causing bradycardia.

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Thermoregulation in Non-Mammals

Temperature Regulation in Ectotherms

Ectotherms (reptiles, amphibians, fish, invertebrates) rely primarily on behavioural and physiological mechanisms rather than metabolic heat production to regulate their body temperature.

Mechanism	Description	Example
Basking	Moving to a warm area and exposing the body surface to direct sunlight	Lizards on rocks; snakes in sunlight
Sheltering	Moving to shade, burrows, or crevices during hot periods	Snakes in rock crevices; toads in burrows
Nocturnal activity	Being active at night when temperatures are lower	Geckos; desert rodents
Countercurrent heat exchange	Alternating the direction of blood flow in limbs to either conserve or lose heat depending on need	Dolphin flukes; woodpecker feet
Altering body shape	Flattening the body to increase surface area for heat loss or reducing it to conserve heat	Horned vipers flatten to increase surface area for heat absorption
Adjusting heart rate	Increasing heart rate to deliver more blood to the body surface for heat loss	Lizards in sun increase heart rate
Changing colour	Some reptiles can darken their skin to absorb more heat	Bearded dragons darken when cold

Temperature Regulation in Endotherms

Birds and mammals generate heat internally through metabolism and use insulation and behavioural mechanisms to maintain body temperature.

Mechanism	Description	Example
Shivering thermogenesis	Rapid, involuntary muscle contractions that generate heat as a by-product of respiration	Mammals in cold environments
Non-shivering thermogenesis	Brown adipose tissue (BAT) uncouples respiration from ATP production, releasing energy directly as heat; stimulated by sympathetic nervous system	Newborn human infants; hibernating mammals
Piloerection	Hair erector muscles contract, trapping insulating air next to the skin	Goosebumps in humans; mammals fluffing fur
Vasoconstriction	Reducing blood flow to the skin surface to reduce heat loss	Mammals in cold environments
Insulation (fur/blubber)	Trapped air in fur or fat provides effective insulation against heat loss	Arctic mammals have thick blubber layers
Huddling	Group behaviour to reduce surface area to volume ratio; reduces heat loss per individual	Penguins in the Antarctic; macaques
Nest building	Constructing insulated structures to reduce heat loss	Birds building insulated nests
Countercurrent exchange	Arterial blood flowing to limbs passes close to venous blood returning from limbs, allowing heat exchange between them	Dolphin flukes; wolf paws

Hibernation and Aestivation

Hibernation: A state of reduced metabolic rate and body temperature during winter, allowing animals to survive when food is scarce.

Feature	Description
Body temperature	Drops to near ambient (1-5 degrees C in many mammals); dramatically reduces metabolic rate (to approximately 2-5% of normal)
Heart rate	Drops to approximately 5% of normal
Breathing rate	Very slow and shallow
Fat reserves	Accumulated brown fat provides energy throughout hibernation
Periodic arousal	Animals periodically wake up to urinate, feed briefly, and restore normal body temperature
Animals that hibernate	Bats, hedgehogs, marmots, ground squirrels, dormice, bears (shallow hibernation)

Aestivation: A similar state of reduced metabolic rate during summer, triggered by heat and drought. Common in desert animals (e.g., desert tortoise, African lungfish).

Adaptations of Arctic Animals

Adaptation	Mechanism for Heat Conservation
Thick blubber layer	5-10 cm of subcutaneous fat; very effective insulation (thermal conductivity approximately 0.25 W/m/K)
Dense underfur	Trapped air in dense fur provides insulation; hollow guard hairs in polar bears increase insulation further
Small ears and tail	Reduced surface area to volume ratio minimises heat loss
Counter-current heat exchange	Arterioles in legs are wrapped by veins, allowing heat exchange; warm arterial blood transfers heat to returning venous blood near the body, minimising heat loss at the extremities
Behavioural adaptations	Huddling; building snow dens; reducing activity; sheltering from wind

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Thermoregulation in Non-Mammals

Temperature Regulation in Ectotherms

Ectotherms (reptiles, amphibians, fish, invertebrates) rely primarily on behavioural and physiological mechanisms to regulate their body temperature rather than metabolic heat production.

Mechanism	Description	Example
Basking	Moving to a warm area and exposing the body surface to direct sunlight	Lizards on rocks; snakes in sunlight
Sheltering	Moving to shade, burrows, or crevices during hot periods	Snakes in rock crevices; toads in burrows
Nocturnal activity	Being active at night when temperatures are lower	Geckos; desert rodents
Countercurrent exchange	Alternating the direction of blood flow in limbs to either conserve or lose heat depending on need	Dolphin flukes; woodpecker feet
Altering body shape	Flattening the body to increase surface area for heat loss or reducing it to conserve heat	Horned vipers flatten to absorb more heat
Adjusting heart rate	Increasing heart rate to deliver more blood to the body surface for heat loss	Lizards in sun increase heart rate

Temperature Regulation in Endotherms

Mechanism	Description	Example
Shivering thermogenesis	Rapid, involuntary muscle contractions that generate heat as a by-product of respiration	Mammals in cold environments
Non-shivering thermogenesis	Brown adipose tissue (BAT) uncouples respiration from ATP production, releasing energy directly as heat; stimulated by sympathetic nervous system	Newborn human infants; hibernating mammals
Piloerection	Hair erector muscles contract, trapping insulating air next to the skin	Goosebumps in humans; mammals fluffing fur
Vasoconstriction	Reducing blood flow to the skin surface to reduce heat loss	Mammals in cold environments
Insulation (fur/blubber)	Trapped air in fur or fat provides effective insulation against heat loss	Arctic mammals have thick blubber layers
Huddling	Group behaviour to reduce surface area to volume ratio; reduces heat loss per individual	Penguins in the Antarctic; macaques

Hibernation and Aestivation

Hibernation: A state of reduced metabolic rate and body temperature during winter.

Feature	Description
Body temperature	Drops to near ambient (1-5 degrees C); metabolic rate drops to approximately 2-5% of normal
Heart rate	Drops to approximately 5% of normal
Breathing rate	Very slow and shallow
Fat reserves	Accumulated brown fat provides energy throughout hibernation
Periodic arousal	Animals periodically wake to urinate, feed briefly, and restore normal body temperature
Animals that hibernate	Bats, hedgehogs, marmots, ground squirrels, dormice, bears (shallow hibernation)

Aestivation: Similar state of reduced metabolic rate during summer, triggered by heat and drought. Common in desert animals (e.g., desert tortoise, African lungfish).

Adaptations of Arctic Animals

Adaptation	Mechanism for Heat Conservation
Thick blubber layer	5-10 cm of subcutaneous fat; very effective insulation (thermal conductivity approximately 0.25 W/m/K)
Dense underfur	Trapped air in dense fur provides insulation; hollow guard hairs in polar bears increase insulation further
Small ears and tail	Reduced surface area to volume ratio minimises heat loss
Counter-current exchange	Arterioles in legs wrapped by veins allows heat exchange; warm arterial blood transfers heat to returning venous blood near the body, minimising heat loss at extremities
Behavioural adaptations	Huddling; building snow dens; reducing activity; sheltering from wind

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The Nephron in Detail

Structure of the Nephron

Component	Location in Kidney	Description
Renal (Bowman's) capsule	Cortex	Cup-shaped structure that surrounds the glomerulus; consists of an outer parietal layer (simple squamous epithelium) and an inner visceral layer (podocytes)
Glomerulus	Cortex	Knot of capillaries between the afferent and efferent arterioles; high pressure due to the wide afferent and narrow efferent arteriole; site of ultrafiltration
Proximal convoluted tubule (PCT)	Cortex	Highly coiled tube immediately after the renal capsule; lined with cuboidal epithelial cells with microvilli (brush border) and many mitochondria
Loop of Henle	Medulla (descending limb and thin ascending limb); cortex (thick ascending limb)	Hairpin loop that creates a concentration gradient in the medulla; essential for producing concentrated urine
Distal convoluted tubule (DCT)	Cortex	Coiled tube after the loop of Henle; lined with cuboidal epithelial cells (fewer microvilli than PCT); site of selective reabsorption and secretion
Collecting duct	Passes from cortex through the medulla to the renal pelvis	Receives filtrate from multiple nephrons; permeability to water is controlled by ADH; delivers urine to the renal pelvis
Peritubular capillaries	Surround the PCT and DCT; arise from the efferent arteriole	Reabsorb water, glucose, amino acids, and ions from the tubules back into the blood; secrete waste products into the tubules
Vasa recta	Hairpin-shaped capillaries that run alongside the loop of Henle	Maintain the medullary concentration gradient by counter-current exchange

Ultrafiltration

Ultrafiltration is the process by which blood is filtered in the glomerulus to form the glomerular filtrate (a protein-free, cell-free fluid similar to blood plasma).

Conditions for ultrafiltration:

Condition	Description
High blood pressure	The afferent arteriole is wider than the efferent arteriole, creating high hydrostatic pressure (~55 mmHg) in the glomerulus that forces fluid out of the capillaries
Fenestrated endothelium	The capillary walls of the glomerulus have pores (fenestrations) that allow water and small solutes to pass through but restrict large molecules and cells
Basement membrane	A thick, negatively charged glycoprotein layer between the endothelium and the podocytes; acts as an additional filter; repels large proteins (which are negatively charged)
Podocytes	Specialised epithelial cells of the renal capsule with foot processes (pedicels) that wrap around the capillaries; filtration slits between the foot processes act as a final filter

What is filtered? Water, glucose, amino acids, urea, ions (Na$^+$, K$^+$, Cl$^-$, Ca$^{2+}$, HCO$_3^-$), vitamins -- everything in blood plasma EXCEPT large proteins (e.g., albumin, globulins) and blood cells.

Selective Reabsorption

Substance	PCT Reabsorption (approximate %)	Mechanism
Water	~65%	Osmosis (following the reabsorption of solutes)
Sodium ions (Na$^+$)	~65%	Active transport (Na$^+$/K$^+$-ATPase pump on the basolateral membrane); co-transport with glucose and amino acids on the apical membrane
Glucose	100% (in healthy kidneys)	Secondary active transport (co-transport with Na$^+$ via SGLT transporters on the apical membrane)
Amino acids	100% (in healthy kidneys)	Secondary active transport (co-transport with Na$^+$)
Potassium ions (K$^+$)	~65%	Active transport and diffusion
Calcium ions (Ca$^{2+}$)	~65%	Active transport (regulated by parathyroid hormone)
Bicarbonate (HCO$_3^-$)	~85%	Reabsorbed as CO$_2$ (converted to H$_2$CO$_3$ then CO$_2$ and H$_2$O inside the cell; CO$_2$ diffuses across and is reconverted to HCO$_3^-$ on the other side)
Urea	~50%	Passive diffusion (reabsorbed along with water)

The Countercurrent Multiplier

The loop of Henle acts as a countercurrent multiplier to create a concentration gradient in the medulla:

1. Descending limb: Permeable to water but NOT to ions. As the filtrate descends into the increasingly concentrated medulla, water leaves the filtrate by osmosis; the filtrate becomes more concentrated
2. Thin ascending limb: Permeable to ions (Na$^+$, Cl$^-$) but NOT to water. Na$^+$ and Cl$^-$ diffuse out into the medulla, making the medullary interstitial fluid more concentrated; the filtrate becomes less concentrated
3. Thick ascending limb: Impermeable to water; actively transports Na$^+$, K$^+$, and Cl$^-$ OUT of the filtrate (using Na$^+$/K$^+$-ATPase and the Na$^+$-K$^+$-2Cl$^-$ co-transporter). This further increases the concentration of the medullary interstitial fluid
4. The countercurrent flow (descending and ascending limbs flowing in opposite directions) amplifies the concentration gradient
5. Result: the medullary interstitial fluid at the bottom of the loop is hypertonic (up to 1200 mOsm/L, compared to ~300 mOsm/L in the cortex)
6. This gradient allows the collecting duct to reabsorb water and produce concentrated urine (when ADH is present)

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Skin and Temperature Regulation in Detail

Skin Structure

Layer	Components	Functions
Epidermis	Keratinocytes (produce keratin for waterproofing); melanocytes (produce melanin for UV protection); Langerhans cells (immune defence); Merkel cells (touch receptors)	Waterproof barrier; protection against pathogens and UV radiation; vitamin D synthesis (7-dehydrocholesterol $\rightarrow$ cholecalciferol by UV light)
Dermis	Blood vessels; nerves; hair follicles; sebaceous glands; sweat glands (eccrine and apocrine); sensory receptors (Meissner's corpuscles, Pacinian corpuscles)	Nourishment; sensation; thermoregulation; lubrication of hair and skin; structural support (collagen and elastin fibres)
Hypodermis	Adipose tissue (fat); connective tissue	Insulation; energy storage; cushioning

Thermoregulation Pathways

When body temperature RISES (e.g., during exercise, in hot weather):

Response	Mechanism
Vasodilation	Blood vessels in the dermis dilate (smooth muscle in the vessel walls relaxes); more blood flows near the skin surface; heat is lost by radiation and convection from the skin surface
Sweating	Sweat glands secrete sweat onto the skin surface; water in sweat evaporates, taking latent heat of vaporisation from the skin; this is the most effective cooling mechanism; can lose up to 1-2 litres of sweat per hour during intense exercise
Decreased metabolic rate	Cells reduce their rate of respiration, producing less heat as a by-product
Behavioural responses	Removing clothing; seeking shade; drinking cold fluids; fanning

When body temperature FALLS (e.g., in cold weather):

Response	Mechanism
Vasoconstriction	Blood vessels in the dermis constrict (smooth muscle contracts); less blood flows near the skin surface; less heat is lost from the body surface; blood is diverted to the core to protect vital organs
Shivering	Rapid, involuntary contractions of skeletal muscles; respiration in the muscle cells increases, producing more heat as a by-product
Piloerection	Hair erector muscles contract; hairs stand upright, trapping a layer of still air next to the skin (insulating layer); effective in furry animals but limited effect in humans (goosebumps)
Increased metabolic rate	Cells increase their rate of respiration; thyroid hormones (T3, T4) stimulate basal metabolic rate; brown adipose tissue (in newborns and hibernating mammals) generates heat through non-shivering thermogenesis
Behavioural responses	Adding clothing; seeking shelter; curling up (reducing surface area to volume ratio); huddling

Negative Feedback in Thermoregulation

1. Normal body temperature: approximately 37 degrees C (core temperature)
2. Temperature receptors: Thermoreceptors in the skin (detect external temperature changes) and in the hypothalamus (detect blood temperature changes) send nerve impulses to the thermoregulatory centre in the hypothalamus
3. Thermoregulatory centre: The hypothalamus processes the information and sends nerve impulses to effectors:
   • Sweat glands (via sympathetic nervous system)
   • Blood vessels in the skin (via sympathetic nervous system)
   • Skeletal muscles (via motor neurons)
   • Adrenal medulla (triggers adrenaline release for metabolic effects)
4. Negative feedback: When the corrective response brings body temperature back to normal, the thermoreceptors detect this change and the hypothalamus reduces the corrective signals

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Common Pitfalls

• Ultrafiltration occurs in the glomerulus; selective reabsorption occurs primarily in the proximal convoluted tubule. Students often confuse the site of these two processes
• The loop of Henle creates the concentration gradient in the medulla; the collecting duct USES this gradient to produce concentrated urine (when ADH is present). The loop does not reabsorb water; the collecting duct does
• The descending limb of the loop of Henle is permeable to water but NOT to ions; the ascending limb is permeable to ions but NOT to water. Getting these mixed up is a common error
• ADH increases water reabsorption, NOT decreases it. More ADH = more concentrated urine (less volume); less ADH = more dilute urine (more volume)
• Sweating cools the body through EVAPORATION, not through the production of sweat itself. If sweat drips off the skin without evaporating, it does not cool the body. This is why fans help with cooling (they increase evaporation rate)**

tip

tip Ready to test your understanding of Homeostasis? Review the Human Reproduction and Homeostasis diagnostic test which covers homeostatic topics within the DSE specification.

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

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Kidney Dialysis

When Is Dialysis Needed?

When kidneys fail (renal failure), they can no longer:

• Filter metabolic waste products (urea, creatinine, uric acid)
• Regulate water and ion balance
• Regulate blood pH
• Produce erythropoietin (leading to anaemia)

If untreated, toxic levels of urea accumulate (uraemia), causing nausea, confusion, seizures, and death.

Haemodialysis

Feature	Description
Principle	Blood is pumped out of the body, passed through a dialyser (artificial kidney), and returned. Waste products diffuse from the blood across a partially permeable membrane into the dialysis fluid
Dialysis membrane	Made of cellulose acetate or synthetic polymer; permeable to small molecules (urea, ions, glucose) but not to large molecules (proteins, blood cells)
Dialysis fluid (dialysate)	Contains the same concentration of useful substances (glucose, ions) as normal blood plasma but NO urea -- this creates a concentration gradient for urea to diffuse from blood into dialysate
Flow	Blood flows in one direction through the dialyser; dialysate flows in the opposite direction (counter-current exchange) -- this maintains the concentration gradient along the entire length of the dialyser, maximising diffusion efficiency
Frequency	Typically 3 sessions per week, each lasting 3-5 hours
Access	An arteriovenous fistula (surgical connection between an artery and vein) provides a high-flow blood vessel for repeated needle insertion
Anticoagulant	Heparin is added to prevent blood clotting in the dialyser

Peritoneal Dialysis

Feature	Description
Principle	The peritoneum (lining of the abdominal cavity) acts as the dialysis membrane. Dialysis fluid is introduced into the abdominal cavity through a catheter; waste products diffuse from the blood in the peritoneal capillaries into the dialysis fluid
Exchange	After several hours (dwell time), the dialysis fluid (now containing waste) is drained and replaced with fresh fluid
Frequency	Continuous (CAPD -- continuous ambulatory peritoneal dialysis): 4-5 exchanges per day; automated (APD): machine performs exchanges overnight
Advantages	Can be performed at home; no need for a machine (CAPD); more gentle on the body; preserves remaining kidney function better
Disadvantages	Risk of peritonitis (infection of the abdominal cavity); less efficient than haemodialysis for large patients or those with high urea levels

Haemodialysis vs Peritoneal Dialysis

Aspect	Haemodialysis	Peritoneal Dialysis
Location	Hospital or dialysis centre	Home
Membrane	Artificial (dialyser)	Natural (peritoneum)
Efficiency	Higher; removes waste more quickly	Lower; slower process
Convenience	Requires travel to centre 3x/week	Can be done at home; more independence
Infection risk	Low (sterile procedure)	Higher (peritonitis risk)
Diet restrictions	Strict (fluid, potassium, phosphate)	Less strict
Suitable for	Most patients; acute renal failure	Patients who prefer home treatment; children

Kidney Transplant

• The best treatment for end-stage renal failure
• A healthy kidney from a living donor (usually a relative) or a deceased donor is transplanted into the patient's lower abdomen
• The donor kidney is connected to the recipient's blood vessels and ureter
• Advantages over dialysis: no need for regular dialysis sessions; better quality of life; longer life expectancy; fewer dietary restrictions
• Disadvantages: risk of surgical complications; need for lifelong immunosuppressive drugs (increased risk of infections); risk of rejection; shortage of donor organs
• Success rate: approximately 95% survival at 1 year for kidneys from living donors

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ADH and Water Balance in Detail

Osmoregulation Pathway

1. Stimulus: Blood water potential decreases (blood becomes more concentrated; osmolarity increases above normal ~300 mOsm/L). This can be caused by:

   • Sweating (water loss through skin)
   • Insufficient water intake (dehydration)
   • High salt intake (increases blood osmolarity)
   • Diarrhoea or vomiting (water loss)

2. Detection: Osmoreceptors in the hypothalamus detect the decrease in blood water potential. Osmoreceptors are specialised neurons that shrink when the surrounding fluid becomes more concentrated, triggering nerve impulses.

3. Coordination:

   • The hypothalamus sends nerve impulses to the posterior pituitary gland, stimulating it to release antidiuretic hormone (ADH, vasopressin) into the blood
   • Simultaneously, the hypothalamus stimulates the thirst centre in the brain, creating the sensation of thirst and motivating water-drinking behaviour

4. Effector response (kidneys):

   • ADH binds to receptors on the cells of the collecting ducts and distal convoluted tubules in the nephrons
   • This triggers the insertion of aquaporin water channels into the luminal membrane of these cells
   • Water is now reabsorbed from the filtrate into the blood by osmosis (the medullary interstitial fluid is hypertonic due to the counter-current multiplier in the loop of Henle)
   • More concentrated urine is produced (small volume, high urea concentration)
   • Blood water potential returns to normal

5. Negative feedback: As blood water potential returns to normal, osmoreceptors in the hypothalamus detect the change and reduce ADH secretion. Aquaporin channels are removed from the collecting duct membranes, reducing water reabsorption.

ADH Levels and Urine Output

Condition	Blood Osmolarity	ADH Level	Aquaporin Channels	Water Reabsorption	Urine Volume	Urine Concentration
Dehydrated	High	High	Many inserted	High	Low	High (concentrated)
Normal hydration	Normal	Normal	Moderate	Moderate	Normal	Normal
Over-hydrated	Low	Low	Few	Low	High	Low (dilute)

Diabetes Insipidus

• A condition characterised by the inability to concentrate urine, resulting in the production of large volumes of dilute urine (up to 20 litres per day)
• Central diabetes insipidus: The hypothalamus or posterior pituitary does not produce or release sufficient ADH (caused by head injury, tumour, infection)
• Nephrogenic diabetes insipidus: The kidneys do not respond to ADH -- the collecting duct cells lack functional receptors or aquaporin channels (caused by genetic mutations, certain drugs such as lithium)
• Treatment: desmopressin (synthetic ADH) for central DI; treating the underlying cause for nephrogenic DI

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Blood Glucose Regulation in Detail

The Role of the Pancreas

The pancreas has both exocrine and endocrine functions:

Function	Description
Exocrine	Acinar cells produce digestive enzymes (amylase, lipase, trypsinogen) and bicarbonate, secreted into the duodenum via the pancreatic duct
Endocrine	Islets of Langerhans produce hormones released into the blood: $\alpha$ cells produce glucagon; $\beta$ cells produce insulin

Insulin and Glucagon Actions

Aspect	Insulin ($\beta$ cells)	Glucagon ($\alpha$ cells)
Trigger	Blood glucose concentration rises above normal (~5 mmol/L)	Blood glucose concentration falls below normal (~5 mmol/L)
Effect on blood glucose	Decreases (lowers blood glucose)	Increases (raises blood glucose)
Target organs	Liver, muscle cells, adipose tissue	Liver
Key mechanisms	-- Increases glucose uptake by cells (by stimulating translocation of GLUT4 transporters to the cell membrane in muscle and fat cells)	-- Stimulates glycogenolysis (breakdown of glycogen to glucose) in the liver
	-- Stimulates glycogenesis (conversion of glucose to glycogen) in the liver and muscles	-- Stimulates gluconeogenesis (synthesis of glucose from amino acids and lactate) in the liver
	-- Stimulates lipogenesis (conversion of glucose to fat for storage) in adipose tissue	-- Stimulates lipolysis (breakdown of fat to fatty acids and glycerol) in adipose tissue
	-- Inhibits gluconeogenesis and glycogenolysis in the liver	-- Increases blood glucose
	-- Increases rate of glucose uptake and use by cells (increases respiration rate)

Glycogenesis, Glycogenolysis, and Gluconeogenesis

Process	Meaning	Direction	Stimulated By	Inhibited By
Glycogenesis	Glycogen formation	Glucose $\rightarrow$ Glycogen	Insulin	Glucagon
Glycogenolysis	Glycogen breakdown	Glycogen $\rightarrow$ Glucose	Glucagon	Insulin
Gluconeogenesis	New glucose synthesis	Amino acids/lactate $\rightarrow$ Glucose	Glucagon	Insulin

Adrenaline and Blood Glucose

During the fight-or-flight response, the adrenal medulla releases adrenaline, which:

• Stimulates glycogenolysis in the liver (rapidly increases blood glucose)
• Inhibits insulin secretion and stimulates glucagon secretion
• This ensures that muscles have an adequate supply of glucose for respiration during physical activity or stress
• The effect is short-lived compared to the effects of cortisol from the adrenal cortex

Glycosuria

• Normally, all glucose is reabsorbed from the filtrate in the proximal convoluted tubule (via sodium-glucose co-transporters)
• The renal threshold for glucose is approximately 9-10 mmol/L -- if blood glucose exceeds this level, the transporters become saturated and glucose appears in the urine (glycosuria)
• Glycosuria is a characteristic symptom of diabetes mellitus

Common Pitfalls

• Insulin decreases blood glucose; glucagon increases it. Students often confuse the two -- remember: "Insulin is IN, glucose goes IN to cells"
• Glucagon acts primarily on the liver, NOT on muscle cells. Muscle cells lack the receptor for glucagon. Only the liver can release glucose into the blood
• Insulin and glucagon are antagonistic hormones -- they have opposite effects on blood glucose concentration
• Type 1 diabetes is caused by autoimmune destruction of $\beta$ cells (not $\alpha$ cells); Type 2 is caused by insulin resistance
• Diabetes insipidus is NOT related to blood glucose. It is a disorder of water balance (ADH deficiency or resistance), causing dilute urine. It should not be confused with diabetes mellitus
• The renal threshold is the blood glucose concentration at which glucose first appears in the urine (~9-10 mmol/L), NOT the normal blood glucose level (~5 mmol/L)