IGCSE Biology / Chemical coordination
Controlling the water content of the blood
Edexcel International GCSE Biology (4BI1), chapter 7: Chemical coordination, specification point 2.76B, 2.78B, 2.95B. Free lesson.
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ADH helps keep the water content of the blood steady by changing how permeable the kidney collecting ducts are to water. This affects how much water is reabsorbed and the volume and concentration of urine.
How the water-balance loop works
The body uses negative feedback to keep blood water content near normal. Receptors in the hypothalamus detect a change. The pituitary gland then releases ADH, a hormone, into the blood. ADH changes the permeability of the kidney’s collecting ducts, which controls how much water is reabsorbed. ADH does not move water itself. It changes how easily water can pass through the collecting duct. Water then moves back into the blood by osmosis.
When blood water content falls
Sweating or not drinking enough lowers the water content of the blood, so the blood becomes more concentrated. The body responds by conserving water. As water returns to the blood, its water content rises towards normal and less ADH is released. The urine is a small volume and concentrated because more water has been reabsorbed.
When blood water content rises
After drinking a lot of water, the blood becomes more dilute than normal. The response reverses the same loop: less water is reabsorbed, so more remains in the kidney tubules and leaves the body in urine. The urine is a large volume and dilute. As water leaves in the urine, blood water content falls towards normal.
How to describe a low-water response
For a situation such as a long run on a hot day, link the cause to the urine produced. Sweating lowers blood water content, which triggers the hypothalamus and pituitary response. This leads to more water being reabsorbed and less water being lost in urine. Low blood water: more ADH, more water reabsorbed, small volume of concentrated urine. High blood water: less ADH, less water reabsorbed, large volume of dilute urine.