Explain how the kidneys maintain fluid balance.

Question

What will be an ideal response?

Answer 1

In response to increased osmotic pressure from inadequate water intake, the pituitary gland releases antidiuretic hormone (ADH). ADH signals the kidneys to retain water, thus reducing urine output (Fig. 14-8). At the same time, falling blood pressure initiates another sequence of events-highly sensitive pressure receptors in the kidneys trigger the release of the enzyme renin (Fig. 14-9). Renin, in turn, activates angiotensinogen (a circulating blood protein made in the liver), forming angiotensin I. In the lungs, angiotensin I is converted to angiotensin II, which, among other effects, causes the constriction of blood vessels and release of the hormone aldosterone by the adrenal glands. Aldosterone signals the kidneys to retain more sodium and chloride, and therefore more water. (Remember, water always follows electrolytes.) Thus, low blood pressure, through this roundabout sequence, causes the kidneys to increase water conservation in the body. There is, however, a limit to how concentrated urine can become. Eventually, if sufficient fluid is not consumed, dehydration and ill effects ensue.

When water intake exceeds that needed to excrete waste via the urine and replace insensible losses, the kidneys are able to decrease their reabsorption of water. With this, urine volume increases and the urine becomes more dilute. In rare cases, urinary excretion cannot match high water intake, and water intoxication occurs.