The water content of the blood is controlled by a hormone called ADH (anti-diuretic hormone). Different amounts of ADH are released into the bloodstream according to the concentration of the blood plasma.
The diagrams show what happens when there is either too little or too much water in the blood.
|Plasma||Problem||ADH release||Effect of ADH||Effect on urine|
|High concentration||Too little water||Increases||More water reabsorbed by nephrons||More concentrated|
|Low concentration||Too much water||Decreases||Less water reabsorbed by nephrons||More dilute|
This type of control is an example of negative feedback mechanism. It keeps the concentration of the blood plasma approximately constant.