The loop of Henle explained for CCEA A2 Biology

Start with the purpose

The loop of Henle exists to make the medulla salty. That is the whole point. A very negative water potential in the medullary tissue is what allows water to leave the collecting duct by osmosis later on, producing concentrated urine and conserving body water.

If your answer never mentions the water potential of the medulla, it is describing the machinery without stating what the machinery is for — and CCEA questions almost always want the purpose.

The mechanism, in exam order

• Descending limb: permeable to water, impermeable to ions. Water leaves by osmosis into the medulla, so filtrate becomes more concentrated towards the hairpin.
• Ascending limb: impermeable to water. Sodium and chloride ions are actively transported out into the medullary tissue, so filtrate becomes more dilute as it rises.
• Because ions are pumped out of the ascending limb into the tissue that the descending limb passes through, each small difference is multiplied along the length of the loop. This is the counter-current multiplier.
• Result: a water potential gradient down the medulla, most negative at the tip of the loop.
• Collecting duct: passes back down through that gradient, so water leaves by osmosis along its whole length whenever the duct is permeable.

Where ADH fits

ADH does not move water. It changes how permeable the collecting duct is to water. Osmoreceptors in the hypothalamus detect a fall in blood water potential; the posterior pituitary releases ADH; ADH causes aquaporins to be inserted into the collecting duct membrane; more water is reabsorbed by osmosis and a smaller volume of more concentrated urine is produced.

That is negative feedback: the reabsorbed water raises blood water potential, which reduces ADH release.

A very common lost mark: writing that ADH 'makes the kidney absorb more water' without naming the collecting duct, aquaporins or osmosis.

Long loops and adaptation questions

CCEA frequently asks you to apply the loop to an unfamiliar animal — a desert rodent, for example. The reasoning is always the same: a longer loop of Henle passes deeper into the medulla, so a steeper and more extensive water potential gradient can be maintained, so more water is reabsorbed from the collecting duct and more concentrated urine is produced. That conserves water in a habitat where drinking water is scarce.

Answer that pattern in one linked chain — longer loop, steeper gradient, more reabsorption, more concentrated urine, water conserved — and you collect the marks in order.

Data questions on filtrate composition

You will often be given a table of concentrations in the glomerular filtrate and in urine, and asked to explain a difference. Two principles handle almost every version: proteins and blood cells are absent from filtrate because they are too large to pass the basement membrane during ultrafiltration; glucose is absent from urine because it is entirely reabsorbed by active transport and co-transport in the proximal convoluted tubule.

Where glucose does appear in urine, the answer is that the transport proteins are saturated because the blood glucose concentration exceeded the renal threshold — apply it, do not simply recall it.