Excretory System & Homeostasis

Q1. (3 marks)

• Functional unit: nephron (1)
• Renal corpuscle = glomerulus (1) + Bowman's (renal) capsule (1) Why: the renal corpuscle is the filtering head where blood plasma is forced out under pressure.

Q2. (4 marks) (a) Ultrafiltration: filtration of blood under high pressure in the glomerulus, forcing water and small solutes through the capillary wall into Bowman's capsule while retaining cells and large proteins (2). (b) Selective reabsorption: the process by which useful substances (glucose, amino acids, ions, water) are reabsorbed from the filtrate back into the blood, mainly in the proximal convoluted tubule (2).

Q3. (3 marks)

• Filtration → glomerulus / Bowman's capsule (1)
• Reabsorption → mainly proximal convoluted tubule (also loop of Henle & collecting duct) (1)
• Secretion → distal convoluted tubule (and tubules) (1)

Q4. (3 marks)

• Glucose is filtered at the glomerulus (1) but is completely reabsorbed in the proximal convoluted tubule (1) by active transport / co-transport, so none remains in urine (1). Why: transporters normally have enough capacity to reclaim all filtered glucose.

Q5. (6 marks — ½ per correct cell, rounded to whole marks; award 1 mark per fully correct row)

Why: less water available on land favours less toxic, more concentrated wastes.

Q6. (5 marks)

• ADH is released from the posterior pituitary gland (1) (produced by hypothalamus).
• Released when blood water potential falls / body is dehydrated (1).
• It increases the permeability of the collecting duct (and DCT) to water (1).
• More water is reabsorbed back into the blood (1).
• Result: small volume of concentrated urine; blood water potential rises to normal (negative feedback) (1).

Q7. (2 marks) — any two:

• Increases reabsorption of sodium (Na⁺) ions in the DCT/collecting duct (1)
• Increases secretion/excretion of potassium (K⁺) (1)
• (Indirectly increases water reabsorption / raises blood volume & pressure)

Q8. (3 marks) — any three:

• Vasodilation of skin arterioles → more blood to surface → radiate heat (1)
• Sweating → evaporation removes heat (1)
• Reduced metabolic heat / behavioural (rest, seek shade) (1)
• Flattening of hair (erector muscles relax) (1)

Q9. (4 marks) — any four:

• Deamination of excess amino acids → urea formation (1)
• Regulation of blood glucose (glycogenesis/glycogenolysis) (1)
• Detoxification of toxins/alcohol/drugs (1)
• Heat production contributing to thermoregulation (1)
• Storage of glycogen, iron, vitamins / breakdown of old red blood cells (1)

Q10. (4 marks) (a) ADH secretion increases (1). (b) Collecting duct becomes more permeable to water (1) → more water reabsorbed → urine volume decreases (1) → urine becomes more concentrated (1).

[
  {"claim": "Nephron count sanity: filtered glucose fully reabsorbed leaves 0 in urine (mass balance)", "code": "filtered=180; reabsorbed=180; urine=filtered-reabsorbed; result = (urine==0)"},
  {"claim": "Aldosterone: Na reabsorbed up and K excreted up are two distinct correct effects", "code": "effects={'Na_reabsorption':'up','K_excretion':'up'}; result = (effects['Na_reabsorption']=='up' and effects['K_excretion']=='up')"},
  {"claim": "Dehydration -> ADH increases -> urine volume decreases (inverse relation)", "code": "adh='increase'; urine_volume = 'decrease' if adh=='increase' else 'increase'; result = (urine_volume=='decrease')"},
  {"claim": "Toxicity ranking: ammonia > urea > uric acid", "code": "tox={'ammonia':3,'urea':2,'uric_acid':1}; result = (tox['ammonia']>tox['urea']>tox['uric_acid'])"}
]