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The secretion of antidiuretic hormone (ADH), also called vasopressin, occurs in the pituitary gland of the brain and keeps water in the bloodstream. The main factors affecting its release are a reduction in the amount of water in the plasma and an increase in the number of solid molecules, like sugar and salt, in the bloodstream. ADH works partly by causing the kidneys to reabsorb water rather than to excrete it as urine. When this process is impaired, diabetes insipidus results.
Vasopressin is a peptide hormone that regulates the reabsorption of water and salts in humans and other mammals. In the kidneys, it controls changes in the permeability of the tubules in order to prevent the excretion of salts and glucose. The manufacture and secretion of antidiuretic hormone occurs in the posterior pituitary gland of the brain, where it is one of only two hormones released, the other being oxytocin. ADH is secreted whenever the body becomes hypovolemic from significant fluid loss.
When a decrease in blood plasma osmolarity, a marker of water loss, is detected by receptors in the blood vessels, they send signals to the brainstem's medulla. Here nerves relay the information to the hypothalamus, which controls pituitary secretion of hormones. Receptors for angiotensin are located on the hypothalamus, so this hormone also can increase vasopressin secretion. Blood pressure drops that get detected by arterial stretch receptors cause ADH release too. The brain centers that respond to blood pressure and osmolarity are the ones modulating vomiting and nausea, both of which increase vasopressin release in a complex feedback loop.
The prevention of water excretion in urine is the main function of vasopressin. This is why ingesting diuretics causes compensatory secretion of antidiuretic hormone. When activated by vasopressin, the collecting ducts of the kidneys open water channels that reabsorb water, which otherwise gets expelled as urine. If ADH does not reach these channels and bind to the receptors that control them, more water is lost. Alcohol consumption can reduce the effects of the hormone and cause water loss and dehydration, since ethanol binds to ADH receptors in the collecting ducts, blocking the hormone's actions.
Diabetes insipidus results from problems with the secretion of antidiuretic hormone. Central diabetes is caused when the hypothalamus, injured by trauma or pressured by a tumor, stops producing the chemicals that tell the pituitary to make ADH. This can usually be treated by giving the patient supplemental vasopressin. Diabetes insipidus also results when the receptors in the collecting ducts of the kidneys do not respond to antidiuretic hormone, and the water channels remain closed. Patients with diabetes insipidus produce great quantities of urine as a result of their condition, and experience increasing thirst as a result.
So a lack of ADH or lack of reaction to ADH is what causes diabetics to feel so thirsty and urinate so much.