Concept preview

What you will be able to do

1. 01Explain osmoregulation and why controlling body water is essential.
2. 02Describe how ADH adjusts water reabsorption in the collecting duct.
3. 03Describe how aldosterone controls sodium and water balance.
4. 04Explain the kidney's wider roles in blood pressure, blood volume, pH and removing nitrogenous waste.

Your cells live in a bath of watery fluid, and they are exquisitely fussy about how salty that bath is. If the fluid around a cell becomes too concentrated, water is drawn out of the cell by osmosis and it shrivels; if the fluid becomes too dilute, water floods in and the cell swells and may burst. Because you are constantly gaining water by drinking and losing it in urine, sweat and breath, the balance is always being disturbed. The active control of the body's water and salt content to keep this internal fluid steady is called osmoregulation, and it is one of the most important jobs the kidneys perform. Osmoregulation is a classic case of homeostasis: sensors detect a change, a control centre responds, and the response pushes the body back toward its ideal state. Getting it right is not optional — even modest swings in body-fluid concentration disturb the brain, the heart and every cell at once.