The Kidney and Osmoregulation

Spec mapping: AQA 7402 Section 3.6.4 — control of blood water potential, with the mammalian kidney as the model excretory and osmoregulatory organ (refer to the official AQA specification document for exact wording).

The mammalian kidney is the most intricate effector organ in the homeostatic repertoire. It must simultaneously excrete nitrogenous waste, conserve essential solutes, regulate plasma ionic composition and defend blood water potential — all while operating across the wide range of fluid intakes that everyday life imposes. The functional architecture is built around a million parallel processing units (nephrons) that share a common four-stage logic: bulk filtration, selective reabsorption, secretion of regulated solutes, and concentration of urine through the medullary countercurrent. Each stage is exquisitely tuned to the physical and chemical properties of the substances it handles, and the integration with hormonal regulation (ADH, aldosterone, atrial natriuretic peptide) creates closed-loop control over plasma osmolality and blood pressure. This lesson develops the gross anatomy, the nephron's cellular machinery, the four functional stages, and the hormonal layer — and culminates in the 25-mark synoptic essay typical of AQA Paper 3.

Key Definition: Osmoregulation is the homeostatic control of the water potential of the blood. Excretion is the removal of metabolic waste products (principally urea, CO₂ and excess ions); osmoregulation and excretion are functionally combined in the mammalian kidney.

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Gross Anatomy of the Kidney

Each kidney is a bean-shaped organ ~10–12 cm long, weighing ~150 g in an adult human. It is encased in a tough fibrous capsule and embedded in protective perirenal fat. Three concentric regions are visible in longitudinal section.

flowchart TB
  RA[Renal artery] --> Cortex
  Cortex[Cortex<br/>glomeruli, Bowman's capsules,<br/>PCT, DCT] --> Medulla
  Medulla[Medulla<br/>loops of Henle,<br/>collecting ducts] --> Pelvis
  Pelvis[Renal pelvis<br/>urine collection] --> Ureter
  Ureter[Ureter → bladder]

1. Cortex — outer rim, granular in appearance. Contains glomeruli, Bowman's capsules, the proximal convoluted tubule (PCT) and the distal convoluted tubule (DCT). All ultrafiltration and most reabsorption happen here.
2. Medulla — inner zone, striated in appearance because of the parallel arrangement of tubules. Contains the loops of Henle and the collecting ducts. The medulla is organised into renal pyramids whose apices (papillae) drain into the pelvis.
3. Pelvis — funnel-shaped cavity at the hilum. Collects urine emerging from the papillae and channels it into the ureter.

Blood supply. The renal artery delivers ~20–25% of cardiac output to the kidneys. After branching through interlobar, arcuate and interlobular arteries, each afferent arteriole supplies one glomerulus; an efferent arteriole drains it, then breaks into the peritubular capillary network (around cortical nephrons) or the vasa recta (around juxtamedullary nephrons in the medulla). The vasa recta is critical because its hairpin loop preserves the medullary concentration gradient (see countercurrent exchange below).

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Microanatomy of the Nephron

The nephron is the kidney's functional unit. Each consists of five compartments arranged in series.

1. Bowman's Capsule (renal capsule)

A cup-shaped epithelial sac in the cortex enveloping the glomerulus — a dense tuft of capillaries fed by the afferent arteriole. The inner (visceral) layer of Bowman's capsule is composed of specialised cells called podocytes — literally "foot cells". Podocytes wrap around the glomerular capillaries with finger-like processes (pedicels) that interdigitate, leaving narrow gaps (~30 nm) called filtration slits spanned by a diaphragm of nephrin protein. The geometry presents a vast, fine-pored filtration surface.

2. Proximal Convoluted Tubule (PCT)

A heavily coiled tube ~14 mm long in the cortex, lined by tall cuboidal epithelial cells exquisitely adapted for bulk reabsorption:

• Apical microvilli — brush border with ~5,000 microvilli per cell, multiplying surface area for absorption.
• Basal infoldings — extensive folding of the basolateral membrane housing dense Na⁺/K⁺-ATPase pumps.
• Mitochondria — packed (the cell is one of the most mitochondria-rich in the body) to fuel active transport.
• Co-transporter proteins — sodium-glucose linked transporters (SGLT2), sodium-amino acid co-transporters, and many specific transport proteins on the apical membrane.
• Tight junctions between adjacent PCT cells that are partially leaky to small ions but not to proteins.

3. Loop of Henle

A hairpin descent into the medulla and return to the cortex, in two functionally distinct limbs.

• Descending limb — thin epithelium, freely permeable to water (rich in aquaporin-1), impermeable to NaCl. Water leaves passively as the filtrate descends through the hypertonic medulla.
• Ascending limb — thick epithelium in its upper section, impermeable to water, with apical Na⁺-K⁺-2Cl⁻ co-transporters (NKCC2) that actively pump NaCl out into the medullary interstitium. The thin ascending segment shows passive NaCl efflux.

The two limbs operate in opposite directions — countercurrent flow — generating the medullary osmotic gradient (see below).

4. Distal Convoluted Tubule (DCT)

A shorter, coiled tube in the cortex. The DCT fine-tunes filtrate composition under hormonal control. Aldosterone acts here (and on the early collecting duct) to upregulate apical Na⁺ channels and basolateral Na⁺/K⁺-ATPase, increasing Na⁺ reabsorption with K⁺ secretion in exchange. The DCT and connecting tubule house the principal cells (water/Na⁺ handling) and intercalated cells (acid–base regulation).

5. Collecting Duct

Multiple nephrons drain into a shared collecting duct that traverses the medulla from cortex to papilla. The collecting-duct epithelium contains principal cells whose apical water permeability is set by ADH — the final point of regulated water reabsorption that determines urine concentration.

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Stage 1: Ultrafiltration

Filtration occurs in the Bowman's capsule under hydrostatic pressure.

How high pressure is generated

The afferent arteriole has a wider lumen than the efferent arteriole. Blood entering the glomerulus encounters a higher resistance on exit than on entry, raising hydrostatic pressure within the glomerular capillaries to ~55 mmHg — substantially higher than in capillaries elsewhere in the body. This hydrostatic pressure drives plasma water and small solutes outward across the filtration barrier into Bowman's space.

Net filtration pressure (NFP) is:

NFP = hydrostatic pressure in capillary − (osmotic pressure of plasma proteins + hydrostatic pressure in Bowman's capsule)

Typical values: NFP ≈ 55 − (30 + 15) = 10 mmHg. Despite this small net pressure, the surface area for filtration is enormous (~1 m² across both kidneys), producing a glomerular filtration rate (GFR) of ~120 cm³ min⁻¹, or ~180 dm³ day⁻¹ — over 99% of which must be reabsorbed downstream.

The filtration barrier (three layers)

1. Capillary endothelium — large fenestrations (~70 nm) freely admit plasma but retain blood cells and platelets.
2. Basement membrane — a glycoprotein–collagen mesh that excludes molecules with M_r above ~69,000 (the molecular mass of albumin). Negatively-charged glycoproteins also repel anionic plasma proteins, providing electrostatic as well as size selectivity.
3. Podocyte filtration slits — spanned by nephrin diaphragms, refining selectivity further.

Composition of glomerular filtrate

Filtered into Bowman's space	Retained in blood
Water	Erythrocytes, leukocytes, platelets
Glucose	Plasma proteins (albumin, globulins)
Amino acids	Large peptides (insulin is filtered then reabsorbed)
Urea, uric acid, creatinine	Lipids (bound to albumin)
Na⁺, K⁺, Cl⁻, HCO₃⁻, Ca²⁺, phosphate
Small peptides and hormones

Exam Tip: Glucose or protein in urine is clinically significant. Glucosuria indicates that filtered glucose has exceeded the PCT reabsorptive capacity (classically diabetes mellitus, lesson 2). Proteinuria indicates damage to the basement membrane (nephrotic syndrome, hypertensive damage, diabetic nephropathy).

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Stage 2: Selective Reabsorption (the PCT)

Approximately 85% of the glomerular filtrate is reabsorbed in the PCT. Reabsorption is selective — glucose, amino acids and most solutes are recovered; urea is largely retained for excretion.

The driving force: the Na⁺/K⁺-ATPase pump

The basolateral Na⁺/K⁺-ATPase pumps 3 Na⁺ out for every 2 K⁺ in, hydrolysing one ATP. This creates a low intracellular [Na⁺] — the primary active transport step that drives almost every other reabsorption process in the PCT (and in many other epithelia). The remaining transports are secondary active — they exploit the Na⁺ gradient.

Specific reabsorption

• Glucose — 100% reabsorbed at normal plasma concentrations. Apical SGLT2 co-transports glucose with Na⁺ down the Na⁺ gradient; basolateral GLUT2 then exports glucose by facilitated diffusion into the peritubular blood. When plasma glucose exceeds ~10 mmol dm⁻³ the SGLT2 capacity saturates (transport maximum, T_m) and glucose appears in urine — the basis of glucosuria in untreated diabetes.
• Amino acids — reabsorbed by Na⁺-coupled co-transporters specific to amino-acid families (acidic, basic, neutral).
• Bicarbonate — reabsorbed via apical Na⁺/H⁺ exchange and luminal carbonic anhydrase, conserving plasma buffer capacity.
• Na⁺ — pumped out basally, enters apically in exchange or co-transport; net effect: substantial Na⁺ reabsorption.
• Water — follows osmotically through aquaporin-1 channels and paracellularly through tight junctions, because solute removal lowers the water potential of the peritubular blood relative to the filtrate.
• Urea — some passive reabsorption follows water; net effect is to concentrate urea slightly while still permitting most to pass on to excretion.

Adaptations of the PCT cell

• Microvilli — high surface area for absorption.
• Many mitochondria — ATP supply for Na⁺/K⁺-ATPase.
• Basolateral infoldings — high surface area for pump insertion.
• Diverse apical co-transporters — substrate specificity.
• Tight junctions — permit controlled paracellular flow.

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Stage 3: The Loop of Henle and the Countercurrent Multiplier

The loop of Henle generates the medullary osmotic gradient that allows the collecting duct to concentrate urine. Without the loop, urine could not be more concentrated than plasma; with it, mammalian urine can reach ~1200 mOsm kg⁻¹ (human) or much higher in desert species.

The mechanism

1. The ascending limb actively pumps Na⁺, K⁺ and Cl⁻ (via NKCC2) out of the filtrate into the medullary interstitium. The ascending limb is water-impermeable, so water cannot follow.
2. The medullary interstitium becomes hyperosmotic — its water potential falls (becomes more negative).
3. The descending limb is freely water-permeable but impermeable to NaCl. Water leaves the descending limb by osmosis into the hyperosmotic interstitium.
4. As the filtrate descends, it loses water and becomes progressively more concentrated; at the bottom of the loop it reaches the same osmolality as the surrounding medulla.
5. As the now-concentrated filtrate enters the ascending limb, NaCl is pumped out, diluting the filtrate; the filtrate emerges at the top of the loop hypoosmotic to plasma.
6. Because the two limbs run in opposite directions ("countercurrent"), each level of the medulla is continuously enriched by ions arriving from above — the gradient is multiplied along the length of the loop.

Significance

• Longer loops of Henle → steeper gradient → more concentrated urine. The kangaroo rat can produce urine ~5000 mOsm kg⁻¹, allowing it to survive on metabolic water alone in deserts.
• Humans have a mixed population: ~85% cortical nephrons (short loops) and ~15% juxtamedullary nephrons (long loops). The juxtamedullary group does the heavy lifting in urine concentration.
• The vasa recta runs parallel to the loop of Henle as a hairpin countercurrent exchanger — it supplies the medulla with O₂ while preserving the osmotic gradient, because water and solutes redistribute symmetrically across the hairpin.

Exam Tip: "Countercurrent multiplier" (loop of Henle generates the gradient) and "countercurrent exchanger" (vasa recta preserves it) are distinct concepts. Examiners reward students who can articulate both.

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Stage 4: ADH, Aquaporins and the Final Concentrating Step

The final urine concentration is set by antidiuretic hormone (ADH), also called vasopressin.

When blood water potential falls (dehydration)

1. Osmoreceptors in the hypothalamus shrink osmotically (water leaves as their water potential exceeds the now-low blood water potential).
2. Shrinkage triggers release of stored ADH from the posterior pituitary into blood.
3. ADH binds V2 receptors on principal cells of the collecting duct.
4. G-protein–cAMP–PKA cascade (lesson 4) causes vesicles containing aquaporin-2 to fuse with the apical membrane — exocytosis of pre-formed water channels.
5. Apical water permeability rises sharply. Water flows from the collecting-duct lumen down its water-potential gradient into the hypertonic medulla and then into the vasa recta.
6. Urine emerging at the papilla is small in volume and concentrated.

When blood water potential rises (over-hydration)

1. Osmoreceptors swell.
2. ADH secretion is suppressed.
3. Aquaporin-2 is internalised by endocytosis from the apical membrane.
4. Collecting-duct water permeability falls; water cannot leave; urine emerges large in volume and dilute.

The whole loop is negative feedback: the corrective response reverses the original deviation; receptors continuously sample blood water potential.

Exam Tip: ADH does not pump water — it controls the permeability of the collecting duct by inserting aquaporins. Water then moves passively by osmosis. The energy for water movement comes from the medullary gradient established by the loop of Henle.

Aldosterone and atrial natriuretic peptide

Two additional hormones fine-tune sodium and water balance:

• Aldosterone — secreted by the adrenal cortex in response to falling blood pressure (via the renin–angiotensin system, paraphrasing Goldblatt's discovery of renin's pressor role — A-Level extension) or rising plasma K⁺. Acts on DCT/collecting duct principal cells to upregulate apical Na⁺ channels and basolateral Na⁺/K⁺-ATPase. Net effect: Na⁺ retention (water follows), K⁺ secretion. A* candidates should note: aldosterone primarily controls Na⁺ (and hence blood pressure), distinct from ADH which controls water (and hence osmolality). Mis-treating the two as equivalent is a common A* discriminator.
• Atrial natriuretic peptide (ANP) — secreted by the cardiac atria in response to volume overload. Opposes aldosterone; promotes Na⁺ excretion and reduces blood pressure.

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Common Errors and Mark-Loss Patterns

Many candidates lose marks on this topic by:

• Saying ADH "moves water" — it does not; it inserts aquaporins and water moves by osmosis.
• Confusing the loop of Henle's two limbs — descending = water permeable, ion impermeable; ascending = ion permeable (active pumping), water impermeable. Examiners deduct marks for inversion.
• Mis-stating filtration: the basement membrane sieves by size (~69,000 M_r) and charge, not by "active selection".
• Using "osmotic pressure" and "water potential" interchangeably with opposite signs. Pure water has osmotic pressure 0 and water potential 0; a solution has positive osmotic pressure and negative water potential. Examiners reward sign-precise candidates.
• Confusing ADH and aldosterone: ADH controls water reabsorption (aquaporins); aldosterone controls Na⁺ reabsorption (Na⁺ channels and pumps).
• Forgetting the vasa recta. Asked "how is the medullary gradient maintained?", many candidates describe the loop of Henle but omit the parallel countercurrent exchange in the vasa recta.

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A-Level-Depth Misconceptions