Excretory System & Homeostasis

Question 1 (12 marks)

(a) [3]

• Glucose is completely reabsorbed (1) by selective/active reabsorption (1) in the proximal convoluted tubule (PCT) (1).
• Why: all filtered glucose is normally recovered via active co-transport with Na⁺ before the filtrate reaches the loop; carriers are not saturated in a healthy person.

(b) [3]

• ~0.0 protein shows the glomerular/filtration barrier is impermeable to large plasma proteins — proteins are too large to pass the basement membrane/filtration slits (1).
• A reading of 0.8 would mean proteins ARE passing into the filtrate (1) → indicates damage to the filtration barrier/glomerulus (e.g. glomerulonephritis / proteinuria) (1).

(c) [3]

• Water is reabsorbed along the nephron (PCT, descending loop, collecting duct) (1), so the volume of filtrate falls (1).
• Urea is reabsorbed much less/slowly, so as water leaves, urea becomes progressively more concentrated — a passive concentration effect, not secretion (1).

(d) [3]

• Hormone: aldosterone (1) — controls Na⁺ reabsorption in the DCT/collecting duct.
• Higher final urine Na⁺ occurs when aldosterone is low (1), triggered by high blood Na⁺ / high blood pressure/volume (or low renin) so less Na⁺ is reabsorbed and more is excreted (1).
• (Accept: high salt intake → suppressed aldosterone → more Na⁺ lost.)

Question 2 (10 marks)

(a) [5] Sequence (1 each):

1. Osmoreceptors in the hypothalamus detect raised plasma osmolarity.
2. Stimulate the posterior pituitary to release ADH (antidiuretic hormone/vasopressin).
3. ADH travels in blood to the kidney.
4. Acts on cells of the collecting duct (and DCT) increasing insertion of aquaporins → membranes more permeable to water.
5. More water reabsorbed → small volume of concentrated urine; plasma osmolarity returns toward normal.

(b) [2]

• Urine low in volume — more water reabsorbed under high ADH (1).
• Urine more concentrated / higher osmolarity / darker — solutes in less water (1).

(c) [3]

• Rapid pure water dilutes the blood → plasma osmolarity falls below normal / hyponatraemia (1).
• Osmoreceptors detect low osmolarity → ADH secretion is inhibited (1).
• Collecting ducts become less permeable → large volume of dilute urine excreted, restoring osmolarity (1). (water intoxication risk noted → credit.)

Question 3 (10 marks)

(a) [6] (2 each = correct waste + valid justification)

• X freshwater fish → ammonia. Very toxic but highly soluble; abundant surrounding water dilutes/removes it easily, so minimal water conservation needed (2).
• Y kangaroo rat → urea. Mammal; urea is less toxic than ammonia and needs less water to excrete — suits water-scarce desert while allowing safe blood levels (2).
• Z bird → uric acid. Nearly insoluble, low toxicity, excreted as paste with minimal water — ideal for dry cliff / conserving water (2).

(b) [2]

• Kangaroo rat has very long loops of Henle (1), creating a steeper medullary osmotic gradient allowing far greater water reabsorption → hyperconcentrated urine (1).

(c) [2]

• Uric acid is insoluble/non-toxic (1), so it can be stored safely inside the closed shelled egg without dissolving in and poisoning the embryo (which cannot excrete waste externally) (1).

Question 4 (10 marks)

(a) [3]

• Liver normally converts toxic ammonia (from amino acid deamination) into urea (1) via the ornithine/urea cycle (1).
• Failure means ammonia is not converted → ammonia accumulates in blood (toxic, esp. to brain) (1).

(b) [4]

• Liver stores glucose as glycogen and releases it when blood glucose falls (1).
• Glycogenolysis — breakdown of glycogen to glucose (1) — impaired.
• Gluconeogenesis — synthesis of glucose from non-carbohydrate sources (amino acids/lactate) (1) — impaired.
• Without these buffering processes, glucose falls between meals → hypoglycaemia/instability (1).

(c) [3]

• Liver metabolism generates substantial heat; failure reduces this internal heat production, contributing to unstable/falling temperature (1) + reasoning that homeostatic heat balance is disrupted (1).
• One other mechanism relied on: shivering thermogenesis (muscle contraction generating heat) OR vasoconstriction of skin arterioles to reduce heat loss (1). (Accept either.)

Question 5 (8 marks)

(a) [3]

• Sweat cools the body by evaporation, which absorbs latent heat from the skin (1).
• Humid air is already near-saturated with water vapour (1), so sweat cannot evaporate efficiently — it drips instead → little heat removed (1).

(b) [3]

• Dry skin despite heat suggests sweating has stopped/failed (1) — a sign of heat stroke / breakdown of thermoregulation (1).
• Dangerous because the main cooling route is lost, so core temperature (40.5 °C) rises unchecked → risk of protein denaturation/organ damage (1).

(c) [2]

• Vasodilation of skin arterioles (1) — increases blood flow near skin surface so more heat is lost by radiation/convection (1).
• (Accept behavioural: seeking shade / removing clothing / reducing activity, with explanation.)

[
  {"claim":"Kangaroo rat urine ~4x human concentration: if human max ~1200 mOsm/L then rat ~4800 mOsm/L, and rat value exceeds seawater (~1000 mOsm/L)","code":"human=1200; rat=4*human; seawater=1000; result = (rat==4800) and (rat>seawater)"},
  {"claim":"Urea concentration rises from 1.0 (filtrate) to 65 (urine), a >60-fold concentration due to water reabsorption","code":"filtrate=1.0; urine=65; fold=urine/filtrate; result = fold>60"},
  {"claim":"Runner A core temp (40.5C) exceeds Runner B (38.0C) by more than 2 degrees, consistent with thermoregulatory failure","code":"A=40.5; B=38.0; result = (A-B)>2"},
  {"claim":"Glucose fully reabsorbed: filtrate 1.0 to 0.0 means 100% recovery","code":"filtrate=1.0; loop=0.0; recovered=(filtrate-loop)/filtrate*100; result = recovered==100"}
]