In its absence, urine output can increase more than ten times its normal output, from 1-2 L of urine per day to about 20 L of urine per day. At its maximum concentration, the kidneys can excrete as little as 400-500 mL of urine per day. Regulation of Secretion and Actions of ADH Dehydration or loss of blood due to hemorrhage, diarrhea or vomiting stimulates osmoreceptors, neurons that monitor blood osmotic pressure, in the hypothalamus. When these cells receive the excitatory input from the osmoreceptors, nerve impulses are generated that cause vesicles filled with ADH to exocytose from the axon terminals of the posterior pituitary. The kidneys respond to the elevation in the concentration of ADH by retaining more water, thus causing a decrease in urine output.
Sweat glands decrease their activity, lowering the rate of water loss by perspiration.
The smooth muscles contract, decreasing the diameter of the lumen of the blood vessels, which increases resistance and in turn blood pressure. Overhydration or a higher than normal water concentration or blood volume inhibits the osmoreceptors. This inhibition reduces or stops ADH secretion. With a decrease in ADH, the kidneys retain less water, which causes an increase in urine volume output. Sweat glands increase their activity.