The urinary function is regulated mainly through control of the kidneys on various levels. It can be affected by factors outside the kidneys, particularly circulatory factors. In fact, the amount of urine formed by the kidneys depends heavily on the amount of blood filtered. When an organism's blood volume decreases, less urine is produced and vice versa.
The nervous system also plays a role, both by influencing renal function and by affecting urination, i.e., the bladder. Many nerves innervate the kidneys and affect the renal blood vessels. They can rapidly decrease the rate of renal perfusion, resulting in a decreased volume of urine produced.
Finally, hormones have the greatest regulatory effect. Many hormones are involved in controlling the elimination of water and ions. Most of them, however, have an effect only in pathological situations. The most important of these hormones is vasopressin, also called antidiuretic hormone. Vasopressin is secreted by the pituitary gland (hypophysis) located at the base of the brain. This hormone acts on the ends of the nephrons, namely on the end of the convoluted portion of the distal tubule and on the collecting tubule. Secretion is triggered by increased osmotic pressure in the blood., i.e., a decrease in the amount of water relative to the other molecules in the blood; or by a decrease in arterial pressure. Other stimuli may have an effect as well. Stress, a decrease in ambient temperature, or physical exercise can trigger secretion of antidiuretic hormone.
Vasopressin is captured by receptors on the cells of the collecting tubule's surface. Increased reabsorption of water in the nephron results. This mechanism allows the animal to conserve some of the water contained in its cells.
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