Osmoregulation

Osmoregulation is the control of the water potential of body fluids. For a mammal, this means keeping the concentration of solutes in the blood plasma (and therefore the tissue fluid) within narrow limits despite variable water and salt intake, variable losses in sweat and breath, and the constant excretion of urea by the kidneys. The mammalian kidney carries out osmoregulation with two beautiful molecular devices: the countercurrent multiplier in the loop of Henle, and the antidiuretic hormone (ADH) system acting on the collecting duct. This lesson examines both, matching OCR A-Level Biology A specification module 5.1.2(h).

Key Definitions:

• Osmoregulation — the control of water potential of body fluids by regulating the amount of water and solutes in the body.
• Countercurrent multiplier — the arrangement of the loop of Henle that generates a high solute concentration in the medulla.
• ADH (antidiuretic hormone) — vasopressin, released by the posterior pituitary; increases water permeability of the collecting duct.
• Aquaporin — a water channel protein inserted into the collecting duct membrane under the action of ADH.

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Why Osmoregulation Matters

A human body is about 60 % water, most of which is inside cells. The water potential of the tissue fluid determines whether cells shrivel (crenate), remain in equilibrium, or swell and burst (lyse). Even small changes in the solute concentration of the plasma disturb these balances.

Two examples illustrate the challenge:

• A hot day after strenuous exercise. Losses of ~2 L of sweat raise blood solute concentration sharply. Without osmoregulation, cells would dehydrate.
• Drinking 2 L of water in one go. Blood solute concentration falls sharply. Without osmoregulation, cells would swell and in severe cases neurones in the brain could rupture (water intoxication).

The kidney handles both situations by adjusting how much water it returns to the blood, producing either very dilute or very concentrated urine as required.

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The Loop of Henle: Setting Up the Medullary Gradient

To concentrate urine, the kidney needs a region through which the collecting duct can pass that has an extremely high solute concentration. Water will then move out of the collecting duct by osmosis. The loop of Henle creates that high-solute region — the medulla — using a countercurrent multiplier mechanism.

The Descending Limb

• The descending limb has a thin epithelium that is permeable to water but not to ions.
• As filtrate descends through the increasingly salty medulla, water leaves the tubule by osmosis and enters the surrounding vasa recta.
• The filtrate inside the tubule therefore becomes progressively more concentrated as it reaches the bottom of the loop.

The Ascending Limb

• The ascending limb is impermeable to water but actively transports Na⁺ and Cl⁻ out of the tubule into the medullary tissue. In the thick ascending limb, a Na⁺/K⁺/2Cl⁻ cotransporter on the apical membrane drives Na⁺ into the cells, from where it is pumped out into the interstitium by Na⁺/K⁺ pumps.
• Because water cannot follow, the filtrate becomes increasingly dilute as it ascends.
• The salt pumped out of the ascending limb adds to the high solute concentration in the medullary tissue — the concentration is highest deep in the medulla and falls towards the cortex.

The Countercurrent Multiplier Effect

The descending and ascending limbs flow in opposite directions, which multiplies the osmotic gradient:

• Salt pumped out of the ascending limb builds up in the medulla.
• The descending limb, which is permeable to water, loses water to this salty medullary tissue, making the filtrate inside it even more concentrated by the time it reaches the hairpin.
• More concentrated filtrate entering the ascending limb means more salt to be pumped out, maintaining the gradient.
• The overall effect is that the concentration at the bottom of the loop is far higher (up to ~1200 mOsm kg⁻¹ in humans, even higher in desert mammals) than could be achieved by a single-pass arrangement.

flowchart TB
    A[Filtrate enters descending limb<br/>300 mOsm] --> B[Water leaves<br/>filtrate concentrates]
    B --> C[Hairpin<br/>~1200 mOsm]
    C --> D[Ascending limb]
    D --> E[Na+, Cl- pumped out<br/>no water follows]
    E --> F[Filtrate dilutes to ~100 mOsm]
    E -.high salt in medulla.-> G[Medulla gradient established]
    G -.draws water from.-> B

The Vasa Recta

Blood in the vasa recta (the capillaries around the loop of Henle) also runs in a countercurrent pattern. This prevents the capillaries from washing away the salt gradient: blood descending into the medulla gains salt and loses water, then loses salt and gains water as it ascends back to the cortex. The net effect is that the medullary gradient is maintained, and water reabsorbed from the collecting duct is carried away by the blood.

Species Differences

Mammals living in dry environments have unusually long loops of Henle, reaching deep into the medulla. Kangaroo rats, for example, can produce urine up to five times more concentrated than human urine, allowing them to survive without drinking — they obtain all their water from metabolic water and food. The length of the loop of Henle correlates directly with the maximum urine concentration an animal can achieve.

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ADH and the Collecting Duct