Aldosterone is secreted by the adrenal gland, which is a small structure on top of each kidney. This gland consists of two parts – the cortex and the medulla. Steroid hormones with a variety of functions are secreted by the cortex, while the medulla secrets adrenalin.
Aldosterone is a steroid hormone from the cortex of the adrenal gland and its primary function is to increase the re-absorption of sodium ions (Na+) or potassium ions (K+) in the loop of Henle and distal tubules of the nephron. For example, if Na+ ions are in lower than normal concentration in the blood, less sodium is excreted as these ions are moved from the nephron into the surrounding capillaries by active transport and water also moves as a result of osmosis. This increases the blood volume and so maintains blood pressure, as well as the sodium ion levels of the body fluids.
Maintenance of blood pressure is essential to the efficient transport of materials around the body and in the functioning of many organs, including the kidneys themselves. Blood pressure determines the filtration rate from the glomerulus into Bowman’s capsule of the nephrons of the kidney.
Aldosterone secretion is also controlled by a feedback system, but this system is complicated by a number of influences on the action of the adrenal cortex, including the secretion of hormones from the pituitary. The system is much more complex than the regulation of body water involving ADH.
However, in general terms, changed secretion rates of Aldosterone in response to changes in the ionic composition of the blood and/or in blood pressure act to maintain homeostatic control of blood pressure and ionic composition of the body fluids.
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