Hormonal Control of Osmoregulatory Functions

 In mammals and birds, the amount of water excreted in the urine, and thus the concentration of the urine, varies according to the changing needs of the body. Acting through the mechanisms described next, the kidneys excrete a hypertonic urine when the body needs to conserve water. If an animal drinks excess water, the kidneys excrete a hypotonic urine.
 As a result, the volume of blood, the blood pressure, and the osmolarity of blood plasma are maintained relatively constant by the kidneys, no matter how much water you drink. The kidneys also regulate the plasma Kt and Nat concentrations and blood pH within very narrow limits. These homeostatic functions of the kidneys are coordinated primarily by hormones

Antidiuretic hormone causes water to be conserved

 Antidiuretic hormone (ADH) is produced by the hypothalamus and secreted by the posterior pituitary gland. The primary stimulus for ADH secretion is an increase in the osmolarity of the blood plasma. When a person is dehydrated or eats salty food, the osmolarity of plasma increases. Osmoreceptors in the hypothalamus respond to the elevated blood osmolarity by sending increasing action potentials to the integration center (also in the hypothalamus). This, in turn, triggers a sensation of thirst and an increase in the secretion of ADH.


 ADH causes the walls of the distal convoluted tubules and collecting ducts in the kidney to become more permeable to water. Water channels called aquaporins are contained within the membranes of intracellular vesicles in the epithelium of the distal convoluted tubules and collecting ducts; ADH stimulates the fusion of the vesicle membrane with the plasma membrane, similar to the process of exocytosis. The aquaporins are now in place and allow water to flow out of the tubules and ducts in response to the hypertonic condition of the renal medulla. This water is reabsorbed into the bloodstream. When secretion of ADH is reduced, the plasma membrane pinches in to form new vesicles that contain aquaporins, so that the plasma membrane becomes less permeable to water. More water is then excreted in urine.

 Under conditions of maximal ADH secretion, a person excretes only 600 mL of highly concentrated urine per day. A person who lacks ADH due to pituitary damage has the disorder known as diabetes insipidus and constantly excretes a large volume of dilute urine. Such a person is in danger of becoming severely dehydrated and succumbing to dangerously low blood pressure.
 

Aldosterone and 'atrial natriuretic hormone control sodium ion concentration

 Sodium ions are the major solute in the blood plasma. When the blood concentration of Ne falls, therefore, the blood osmolarity also falls. This drop in osmolarity inhibits ADH secretion, causing more water to remain in the collecting duct for excretion in the urine. As a result, the blood volume and blood pressure decrease.


 A decrease in extracellular Ne also causes more water to be drawn into cells by osmosis, partially offsetting the drop in plasma osmolarity, but further decreasing blood volume and blood pressure. If Ne deprivation is severe, the blood volume may fall so low that blood pressure is insufficient to sustain life. For this reason, salt is necessary for life. Many animals have a "salt hunger" and actively seek salt, such as deer do at "salt licks."


A drop in blood Na+ concentration is normally compensated for by the kidneys under the influence of the hormone aldosterone, which is secreted by the adrenal cortex. Aldosterone stimulates the distal convoluted tubules and collecting ducts to reabsorb Na+ decreasing the excretion of Na+ in the urine. Indeed, under conditions of maximal aldosterone secretion, Ne may be completely absent from the urine. The reabsorption of Ne is followed by reabsorption of C1- and by water, so aldosterone has the net effect of promoting the retention of both salt and water. It thereby helps to maintain blood volume osmolarity, and pressure.

 The secretion of aldosterone in response to a decreased blood level of Na+ is indirect. Because a fall in blood Na+ is accompanied by decreased blood volume, the flow of blood past a group of cells called the juxtaglomerular apparatus is reduced.