HSC Exam Practice

Sample response. Osmoregulation is the homeostatic control of blood osmolarity (the concentration of solutes in the blood) through the regulation of water and solute balance, primarily carried out by the kidneys.

Marking notes. 1 mark for identifying osmoregulation as homeostatic control of blood osmolarity/solute concentration; 1 mark for identifying the kidneys as the primary effector organ. Accept “water and solute balance” rather than “osmolarity” if the concept is clear.

Sample response. ADH acts on the collecting duct of the nephron [1]. When blood osmolarity rises above the set point, ADH is released from the posterior pituitary and travels to the kidneys, where it causes aquaporin water channel proteins to be inserted into the apical membrane of the collecting duct cells [1]. Aquaporins increase the permeability of the collecting duct membrane to water, so water moves by osmosis from the filtrate (which is less concentrated than the surrounding medullary tissue) into the blood, producing concentrated urine and restoring blood osmolarity [1].

Marking notes. 1 mark — collecting duct correctly identified; 1 mark — aquaporin channels inserted into the collecting duct membrane; 1 mark — water reabsorbed by osmosis through the aquaporin channels, producing concentrated urine.

Sample response. ADH is released in response to a rise in blood osmolarity (detected by osmoreceptors in the hypothalamus) — typically caused by dehydration or a high salt intake [1]. Aldosterone is released in response to a fall in blood pressure / blood volume (detected by juxtaglomerular cells in the kidney via the RAAS cascade) — typically caused by blood loss, prolonged dehydration, or low sodium intake [1].

Marking notes. 1 mark per stimulus correctly identified and paired with the correct hormone. Must correctly distinguish osmolarity from pressure/volume.

Sample response. Renin is an enzyme released by juxtaglomerular cells in the kidney when blood pressure falls. It catalyses the conversion of angiotensinogen (a plasma protein produced by the liver) to angiotensin I, which is then converted by ACE (angiotensin-converting enzyme, produced in the lungs) to angiotensin II — the active hormone that stimulates the adrenal cortex to secrete aldosterone. Renin is therefore the initial enzymatic trigger of the RAAS cascade.

Marking notes. 1 mark for identifying renin as released by juxtaglomerular cells in response to low blood pressure and that it converts angiotensinogen to angiotensin I; 1 mark for identifying that this initiates the cascade that ultimately produces angiotensin II to stimulate aldosterone secretion (accept abbreviated cascade).

Sample response. The student’s statement is only partly correct because aldosterone does not directly cause water reabsorption [1]. Aldosterone’s direct action is on the distal convoluted tubule (DCT), where it increases Na&sup+; reabsorption by upregulating sodium transport channels and Na&sup+;/K&sup+;-ATPase pumps [1]. Water then follows the reabsorbed Na&sup+; passively by osmosis — as Na&sup+; is moved from the filtrate into the blood, the blood becomes more concentrated (higher osmolarity) than the filtrate, and water moves down this concentration gradient into the blood. Water reabsorption is therefore an osmotic consequence of Na&sup+; reabsorption, not a direct action of aldosterone [1].

Marking notes. 1 mark for identifying that aldosterone’s direct action is Na&sup+; reabsorption in the DCT (not water reabsorption directly); 1 mark for explaining the mechanism (Na&sup+; transported from filtrate to blood); 1 mark for explaining that water follows passively by osmosis as a consequence.

Sample response. Neural coordination uses electrical impulses along nerve fibres; hormonal coordination uses chemical hormones transported in the bloodstream [1]. Neural signals act within milliseconds to seconds; hormonal signals take seconds to minutes because hormones must travel in the blood to reach their target [1]. Neural effects are brief — they end when the impulse stops; hormonal effects are more sustained because the hormone persists in the blood until degraded, and the effector response continues while hormone is present [1].

Marking notes. 1 mark per criterion correctly compared (signal type, speed, duration). Must address both neural and hormonal for each criterion to receive the mark.

Sample response (a). Urine Na&sup+; increased from 38 to 112 mmol/L (approximately threefold) and urine volume increased from 1450 to 2280 mL/day after spironolactone treatment [1 — describes both changes with values]. This is accounted for by the aldosterone pathway: spironolactone blocks aldosterone receptors in the DCT, so even though aldosterone is present in the blood, it cannot exert its normal effect of increasing Na&sup+; reabsorption [1 — mechanism of drug action]. With less Na&sup+; reabsorption, more Na&sup+; remains in the filtrate and is excreted in the urine (hence higher urine Na&sup+;) [1 — explains urine Na&sup+; rise]. With less Na&sup+; reabsorbed into the blood, there is less osmotic gradient to draw water by osmosis from the filtrate into the blood; more water therefore passes through and is excreted, increasing urine volume [1 — explains urine volume rise via osmosis]. Correspondingly, blood Na&sup+; falls slightly (144 → 139 mmol/L) and blood pressure falls (152 → 134 mmHg) because less Na&sup+; and therefore less water is retained in the blood, reducing blood volume [1 — coherent with blood changes].

Sample response (b). Blood pressure fell as a result of spironolactone treatment (152 → 134 mmHg). The RAAS negative feedback loop normally suppresses aldosterone when blood pressure returns to normal: high blood pressure → renin release falls → angiotensin II falls → aldosterone falls. However, spironolactone blocks aldosterone receptors at the DCT — it does not reduce the amount of aldosterone in the blood [1 — identifies that spironolactone blocks receptor, not hormone production]. Because aldosterone’s effect on blood pressure is blocked, blood pressure remains below normal from the RAAS system’s perspective, so the juxtaglomerular cells continue to detect low pressure and continue releasing renin, driving continued angiotensin II production and continued stimulation of the adrenal cortex to release aldosterone [1 — RAAS continues because the effector response is blocked]. Plasma aldosterone therefore rises above pre-treatment levels as the body compensates for the apparent failure to restore blood pressure, even though the additional aldosterone cannot exert its normal effect because its receptors are blocked [1 total for (b): 2 marks possible, accept 2 marks for complete explanation with RAAS loop logic].

Marking notes part (a). 1 mark — describes both urine Na&sup+; and urine volume increases with at least one value cited; 1 mark — identifies that spironolactone blocks aldosterone receptors in the DCT, preventing Na&sup+; reabsorption; 1 mark — explains higher urine Na&sup+; (less Na&sup+; reabsorbed → more excreted); 1 mark — explains higher urine volume via osmosis (less Na&sup+; gradient → less water reabsorption by osmosis); 1 mark — links blood Na&sup+; fall and blood pressure fall to reduced water retention in blood. Marking notes part (b). 1 mark — correctly identifies that spironolactone blocks the receptor at the DCT (not aldosterone production); 1 mark — explains that the RAAS continues to drive aldosterone production because the effector response is blocked and blood pressure remains low from the feedback loop’s “perspective”.

Sample response. In normal kidney function, ADH and aldosterone together regulate both blood osmolarity and blood volume. ADH acts on the collecting duct: when blood osmolarity rises above ~295 mOsm/kg, osmoreceptors in the hypothalamus signal the posterior pituitary to release ADH, which inserts aquaporin channels into the collecting duct membrane, increasing water reabsorption and producing concentrated urine. Aldosterone acts on the DCT: when blood pressure falls, the RAAS cascade releases aldosterone from the adrenal cortex, causing Na&sup+; reabsorption in the DCT, with water following by osmosis, raising blood volume and pressure back toward normal. Both responses are governed by negative feedback: corrective responses reduce the signal that triggered them.

When nephrons no longer respond to either hormone, both effector responses fail. Without ADH response: collecting duct permeability to water cannot increase regardless of blood osmolarity; large volumes of dilute urine are produced continuously; blood osmolarity rises progressively as water is lost without compensation. Without aldosterone response: Na&sup+; cannot be reabsorbed in the DCT; Na&sup+; is lost in urine; the osmotic gradient that drives water retention from the filtrate is lost; blood volume and blood pressure fall progressively. Both feedback loops are broken at the effector step: stimuli are detected and hormones are released, but the kidney cannot respond. Blood osmolarity and blood pressure both deviate from their set points and cannot be self-corrected, representing a complete failure of two homeostatic systems simultaneously.

Renal dialysis partially compensates by performing the concentration function the nephron can no longer carry out. The patient’s blood is passed across a semi-permeable membrane into a dialysis fluid of controlled composition. By adjusting the osmolarity and ionic composition of the dialysis fluid, the technician can control how much water and Na&sup+; move out of the blood by osmosis and diffusion. Excess water and accumulated solutes (urea, K&sup+;, waste) are removed to restore blood composition toward normal — the machine acts as an artificial effector replacing the kidney’s role.

However, dialysis has significant limitations as a homeostatic substitute. First, it is not continuous: dialysis is typically performed three times per week for 4–5 hours, so between sessions blood composition deviates from normal (accumulation of waste products and fluid). Second, dialysis does not regulate blood pressure dynamically — it cannot respond to second-by-second changes in blood volume as the RAAS pathway does. Third, dialysis cannot regulate the ADH-mediated responses that occur on the timescale of minutes to hours. The patient must restrict fluid and dietary Na&sup+; intake between sessions to compensate for the absence of continuous kidney function. Dialysis is therefore a partial, intermittent effector, not a true homeostatic replacement — the body cannot maintain osmolarity or pressure within normal tolerance ranges continuously, only periodically.

Marking notes.

• 1 mark — Correctly describes the normal role of ADH: osmoreceptors detect high osmolarity; posterior pituitary releases ADH; collecting duct aquaporins inserted; water reabsorbed; concentrated urine; negative feedback.
• 1 mark — Correctly describes the normal role of aldosterone: RAAS; juxtaglomerular cells detect low pressure; renin → angiotensin II → adrenal cortex → aldosterone; DCT Na&sup+; reabsorption; water follows by osmosis; blood volume/pressure restored; negative feedback.
• 1 mark — Analyses consequences of ADH pathway failure: large-volume dilute urine; rising blood osmolarity; feedback loop broken at effector step.
• 1 mark — Analyses consequences of aldosterone pathway failure: Na&sup+; lost in urine; blood volume and pressure fall; both feedback loops broken simultaneously.
• 1 mark — Explains how dialysis compensates: semi-permeable membrane; controlled dialysis fluid composition; removes excess water, Na&sup+; and waste by osmosis/diffusion; acts as artificial effector.
• 1 mark — Evaluates dialysis limitation 1: intermittent (not continuous); composition deviates between sessions.
• 1 mark — Evaluates dialysis limitation 2 or 3: cannot dynamically regulate blood pressure in real-time; cannot respond on the ADH timescale of minutes to hours; or requires strict dietary restriction to compensate for lost kidney function between sessions.
• 1 mark — Overall evaluative judgement stated explicitly: dialysis is a partial, intermittent substitute that cannot replicate the continuous, real-time homeostatic regulation that the kidney provides via ADH and aldosterone pathways. Uses precise lesson terminology throughout.