ADH

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.

Too little water in blood. Hypothalmus detects water level. Pituitary gland releases ADH.  More water reabsorbed by kidneys. Less water lost in urine.  Blood water level returns to normal.Too much water in blood.  Hypothalmus detects water level.  Pituitary gland releases less ADH. Less water reabsorbed by kidneys. More water lost in urine.  Blood water level returns to normal.
PlasmaProblemADH releaseEffect of ADHEffect on urine
High concentrationToo little waterIncreasesMore water reabsorbed by nephronsMore concentrated
Low concentrationToo much waterDecreasesLess water reabsorbed by nephronsMore dilute

This type of control is an example of negative feedback mechanism. It keeps the concentration of the blood plasma approximately constant.

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