Biology 9700/43 — May/June 2022

Background Concept

A mammalian kidney has three main regions when cut in longitudinal section: an outer cortex, a deeper medulla (divided into pyramids in humans), and a central pelvis that collects urine into the ureter. The nephron — the functional unit of the kidney — is not confined to a single region. Its different segments lie in different parts of the kidney, and this positioning is critical to the way the kidney concentrates urine.

The nephron consists of:

• A renal capsule (Bowman's capsule) and glomerulus — in the cortex.
• A proximal convoluted tubule (PCT) — in the cortex.
• A loop of Henle — descends into the medulla (descending limb), loops at the tip (hairpin), and ascends back to the cortex (ascending limb).
• A distal convoluted tubule (DCT) — in the cortex.
• A collecting duct — passes from the cortex down through the medulla to the papilla, where it opens into the pelvis.

The longer the loop of Henle (and the longer the collecting duct it feeds into), the deeper it can penetrate the medulla and the steeper the osmotic gradient it can maintain, allowing more water to be reabsorbed and more concentrated urine to be produced. This is why desert mammals (e.g. the kangaroo rat) have very long loops of Henle, and why the relative medullary thickness (RMT) correlates with urine-concentrating ability.

Understanding the Question

The stem describes water-potential regulation in blood and introduces the medulla of the kidney. Part (a)(i) simply asks the candidate to name the parts of the nephron that lie in the medulla. Two structures are expected.

Approach

Recall the layout of a nephron and identify which segments pass into the medulla. The two key structures that dip into the medulla are the loop of Henle and the collecting duct.

Step-by-Step Reasoning

• The loop of Henle has a descending limb that plunges down into the medulla, a hairpin turn at the bottom (the tip of the loop), and an ascending limb that climbs back to the cortex. The medullary portion is what is being credited.
• The collecting duct receives filtrate from many nephrons and runs from the cortex, through the medulla, to the renal pelvis. It is the final site at which water can be reabsorbed under the control of ADH, and it is in the medulla where this controlled reabsorption occurs.
• The Bowman's capsule, PCT, and DCT are not in the medulla — they are in the cortex — so they are not credited here.

Key Takeaways

• The medulla contains the loop of Henle and the collecting duct.
• These are the segments that interact with the cortico-medullary osmotic gradient.

Common Mistakes

• Writing "nephron" on its own — too vague, does not name a specific part.
• Listing the proximal convoluted tubule or Bowman's capsule — these lie in the cortex, not the medulla.
• Writing "distal convoluted tubule" — this is in the cortex, not the medulla.

Things to Be Careful About

• The question asks for parts of the nephron. The glomerulus is not a named part of the nephron in this specification, and it sits in the cortex anyway.
• The loop of Henle is the standard CIE term; "loop of the nephron" is too vague and would not normally be credited.

Background Concept

Osmoregulation in mammals is achieved largely by adjusting how much water the kidney reabsorbs from the forming urine, and this adjustment is controlled by hormones. The two main hormones involved in mammalian water/salt balance are:

• Antidiuretic hormone (ADH) — also called vasopressin. It is released from the posterior pituitary gland (made in the hypothalamus) and acts on the collecting duct, increasing its permeability to water so more water is reabsorbed back into the blood and the urine becomes more concentrated.
• Aldosterone — from the adrenal cortex, acts on the distal tubule and collecting duct to reabsorb Na⁺ (which indirectly increases water reabsorption). Often classed as a salt-balance hormone.

Other hormones that touch water balance (e.g. atrial natriuretic peptide, ANP) exist but are not the primary osmoregulatory hormone at A-level.

Understanding the Question

The question gives 1 mark for the name of a hormone involved in osmoregulation. There is one mark available so only one clear, correctly spelled answer is needed.

Approach

The classic answer at CIE is ADH (or the accepted synonym vasopressin). State it and stop.

Step-by-Step Reasoning

• ADH is released when blood water potential falls (e.g. after sweating or in dehydration), detected by osmoreceptors in the hypothalamus.
• It travels in the blood to the collecting duct, where it causes aquaporin-2 channels to be inserted into the apical membrane of the collecting-duct cells.
• Water then leaves the collecting duct down the osmotic gradient set up by the loop of Henle, producing small volumes of concentrated urine.
• ADH is therefore the hormone most directly involved in osmoregulation.

Key Takeaways

• ADH (vasopressin) is the principal osmoregulatory hormone in mammals.
• It acts on the collecting duct, increasing water reabsorption.

Common Mistakes

• Writing "ADH" only with no expansion — acceptable, but a candidate is safer writing the full name at least once.
• Writing "insulin" — this regulates blood glucose, not water balance.
• Writing "glucagon" — this also regulates blood glucose, not water balance.
• Writing "aldosterone" — this is acceptable biologically but is not the first answer CIE examiners expect for "a hormone involved in osmoregulation"; if used it should be paired with a brief note that it acts on the distal tubule / collecting duct.

Things to Be Careful About

• Spelling: "antidiuretic" is often mis-spelled as "antidiabetic". The question is about water balance, not blood-sugar control.
• "Vasopressin" is the only accepted alternative at CIE; "ADH" alone is fine; "anti-diuretic hormone" (with a hyphen) is also acceptable.

Background Concept

The mammalian kidney concentrates urine by using a counter-current multiplier in the loop of Henle. The thick ascending limb actively pumps out Na⁺ and Cl⁻ into the medullary interstitium but is impermeable to water, so the interstitium becomes increasingly salty towards the tip of the papilla. The descending limb is permeable to water but not to salts, so water leaves the filtrate as it descends, concentrating it. The net effect is a steep osmotic gradient from cortex (low solute, ~300 mOsm) to medulla (very high, up to ~1200 mOsm in humans, and considerably higher in desert mammals).

When ADH is present, the collecting duct (which also runs through the medulla) becomes permeable to water, so water flows out of the filtrate down this gradient and back into the blood. The longer the loop of Henle, the deeper the gradient, and the more water that can be reabsorbed. This is why the relative medullary thickness (RMT) is a useful proxy for a species' urine-concentrating ability: a thicker medulla means longer loops and more capacity to build a steep gradient, and hence to produce concentrated urine.

The ecological corollary is that mammals in dry habitats (desert species) tend to have higher RMTs, longer loops of Henle, and the ability to produce very concentrated (small-volume) urine, conserving water. Mammals in aquatic habitats (e.g. beavers) have low RMTs and produce dilute, large-volume urine because they have plenty of water.

Understanding the Question

The stem provides Table 1.1 giving RMT and urine-concentration values for four mammals from different habitats. The question has two halves:

1. Describe the relationship between RMT and urine concentration.
2. Explain the difference between the beaver (RMT 1.4, urine 0.90 a.u.) and the kangaroo rat (RMT 8.6, urine 10.50 a.u.).

The mark scheme requires (i) a description of the overall trend, (ii) two paired figures from the table, and (iii) an explanation involving the habitat, the length of the loop of Henle / collecting duct, water reabsorption, and the concentration of the urine produced. "ORA" (or reverse argument) is allowed, so a candidate could equally explain the beaver's data in the same framework.

Approach

1. State the general relationship in one short sentence — there is a positive correlation between RMT and urine concentration.
2. Quote paired data for beaver and kangaroo rat (both numbers, with no need for units because the table header gives the arbitrary units).
3. For the explanation, build a chain: habitat → loop of Henle / collecting duct length → amount of water reabsorbed → urine concentration / volume. This is exactly the chain the mark scheme rewards.

Step-by-Step Reasoning

Point 1 — the relationship:
Reading across the rows of Table 1.1, RMT rises from 1.4 (beaver) to 2.8 (warthog) to 3.2 (human) to 8.6 (kangaroo rat), and urine concentration rises in the same order (0.90 → 2.35 → 2.50 → 10.50 a.u.). The relationship is positive: as one variable increases, the other increases. The mark scheme accepts "positive correlation" or "as the RMT increases the concentration of urine increases".

Point 2 — paired figures:
The beaver has an RMT of 1.4 and produces urine of concentration 0.90 a.u.; the kangaroo rat has an RMT of 8.6 and produces urine of concentration 10.50 a.u. These are the two extreme data points and clearly illustrate the trend. (A candidate could also use warthog and human, but the mark scheme's worked example uses beaver and kangaroo rat.)

Points 3–6 — the biological explanation:

• The kangaroo rat lives in the desert where very little drinking water is available; the beaver lives in rivers and lakes where water is abundant. This is the ecological driving force.
• In the kangaroo rat the loop of Henle (and collecting duct) is much longer than in the beaver, so it plunges deeper into the medulla and the medulla itself is thicker — hence the higher RMT.
• A longer loop of Henle establishes a steeper osmotic gradient in the medulla, and the long collecting duct travels through this steep gradient, so more water is reabsorbed from the filtrate back into the blood.
• As a result, the kangaroo rat produces a much smaller volume of much more concentrated urine, conserving water. The beaver, with a short loop and thin medulla, cannot generate a steep gradient and so cannot reabsorb as much water; it produces large volumes of dilute urine — but it does not need to conserve water because it lives in it.

ORA (or reverse argument) for the beaver is explicitly allowed: the beaver lives where water is plentiful, has a short loop of Henle, reabsorbs less water, and produces dilute urine in large volume.

Key Takeaways

• Relative medullary thickness (RMT) is a good proxy for urine-concentrating ability.
• A long loop of Henle + long collecting duct = steep medullary osmotic gradient = high water reabsorption = small volume of concentrated urine.
• Desert mammals are adapted to conserve water by having long loops and concentrated urine; aquatic mammals (e.g. beaver) are not — they have short loops and dilute urine.

Common Mistakes

• Vague description such as "they are related" or "they are connected" — too imprecise; the mark scheme wants "as one increases the other increases".
• Quoting only one number from the table — the mark scheme requires two pairs of figures.
• Saying "the kangaroo rat produces more urine" — the volume is actually smaller; it is the concentration that is higher.
• Saying the kangaroo rat "retains more water" without saying from where (i.e. from the filtrate in the collecting duct) — the mark scheme explicitly rejects "retaining more water".
• Saying the kangaroo rat "drinks less water" — the question is about the kidney's ability to make concentrated urine, not about drinking behaviour.
• Explaining only the habitat and not the loop-of-Henle mechanism.

Things to Be Careful About

• The data are quoted as paired figures; do not mix the beaver's RMT with the kangaroo rat's urine concentration.
• "More reabsorption of water occurs" is credited; "more water is retained" is not credited (it is on the reject list).
• The mark scheme says "ignore desert" on its own — the examiner wants the underlying reason ("little water available"), not a one-word habitat label.