How Does the Kidney Control Water Balance?
Learn how ADH controls kidney water reabsorption, why the loop of Henle matters, and how negative feedback keeps blood water potential within limits in A Level Biology.
The kidney controls water balance by changing how much water is reabsorbed from the filtrate into the blood. This is osmoregulation: keeping the water potential of the blood within limits. The hormone ADH helps control how much water leaves the body in urine.
When the blood has too little water
The blood’s water potential becomes more negative. Receptors in the hypothalamus detect this change, and the posterior pituitary releases more ADH into the blood. ADH makes the distal convoluted tubule and collecting ducts more permeable to water by causing more aquaporins (water-channel proteins) to be inserted into their cell membranes.
Water then moves by osmosis from the filtrate into the surrounding tissue and returns to the blood. The kidney produces a smaller volume of more concentrated urine.
Why water moves out of the filtrate
The loop of Henle helps create a low water potential in the kidney medulla, the inner region through which the collecting ducts pass. This provides a water-potential gradient, so water can leave the collecting ducts by osmosis when ADH makes them permeable. Earlier in the nephron, the proximal convoluted tubule reabsorbs useful substances, including glucose, as well as water.
When the blood has too much water
The blood’s water potential becomes less negative. Less ADH is released, so fewer aquaporins are present in the membranes of the distal convoluted tubule and collecting ducts. Less water is reabsorbed there, producing a larger volume of dilute urine. As blood water potential returns towards its usual level, ADH release adjusts again. This is negative feedback.
Exam mistake to avoid: ADH does not add water to the blood. It changes the permeability of parts of the nephron, allowing more or less water to be reabsorbed.
