Explain how the kidney and ADH regulate water balance through filtration, reabsorption and negative feedback

What this dot point is asking

You need to explain the kidney's role in water balance, outline filtration and reabsorption in the nephron, and explain how ADH adjusts water reabsorption as a negative feedback loop.

What osmoregulation is

Osmoregulation is the control of the water and solute (salt) balance of the blood and body fluids. If the blood becomes too concentrated or too dilute, water moves into or out of cells by osmosis, damaging them. The kidneys maintain this balance while also removing nitrogenous waste (urea).

The nephron and how the kidney works

Each kidney contains about a million tiny tubes called nephrons, where blood is processed in two key steps:

1. Filtration. Blood enters a knot of capillaries (the glomerulus) under high pressure. Small molecules - water, glucose, salts and urea - are forced out into the nephron, forming filtrate. Large molecules such as proteins and blood cells stay in the blood.
2. Selective reabsorption. As filtrate passes along the nephron, useful substances are reabsorbed back into the blood: all the glucose, much of the salt, and a regulated amount of water. Urea and excess water and salts remain to form urine.

The amount of water reabsorbed is adjustable, which is how the body fine-tunes water balance.

Where reabsorption happens

Tracing the filtrate through the nephron helps explain how the kidney recovers most of what it filters. In the proximal convoluted tubule, all of the glucose, most of the salts and a large fraction of the water are reabsorbed (much of it by active transport followed by osmosis). The loop of Henle sets up a concentration gradient in the surrounding tissue that allows water to be drawn out of the filtrate. Finally, the distal tubule and collecting duct carry out the fine, hormone-controlled adjustment of water reabsorption. The vast majority of the filtered water is recovered; only a small, regulated volume leaves as urine, which is why osmoregulation is mostly about adjusting the last few percent of water reabsorption.

ADH and the feedback loop

The hormone ADH (antidiuretic hormone), released by the pituitary gland under the control of the hypothalamus, controls how much water is reabsorbed from the nephron.

The hypothalamus detects the water concentration of the blood (its solute concentration):

• Blood too concentrated (dehydrated): the hypothalamus triggers more ADH release. The nephron walls become more permeable to water, so more water is reabsorbed into the blood. Urine becomes small in volume and concentrated. Blood water returns toward the set point.
• Blood too dilute (over-hydrated): less ADH is released. Less water is reabsorbed, so a large volume of dilute urine is produced, removing the excess water.

How ADH changes permeability

A common higher-mark question asks for the cellular mechanism. ADH binds to receptors on the cells lining the collecting duct, which causes water channel proteins called aquaporins to be inserted into the membrane facing the filtrate. With more aquaporins present, water moves out of the filtrate by osmosis far more readily, so more is reabsorbed and the urine becomes concentrated. When ADH levels fall, the aquaporins are removed, the wall becomes less permeable, and more water stays in the filtrate to form dilute urine. This is why ADH is described as adjusting permeability rather than pumping water directly.