Renin-Angiotensin-Aldosterone System

Posted by e-Medical PPT
The  renin-angiotensin-aldosterone system (RAAS) is a hormone system that regulates blood pressure and water balance.When blood volume is low, juxtaglomerular cells in the kidneys secrete renin. Renin stimulates the production of angiotensin I, which is then converted to angiotensin II. It is believed that angiotensin I may have some minor activity, but angiotensin II is the major bio-active product. Angiotensin II has a variety of effects on the body:
    * Throughout the body, it is a potent vasoconstrictor of arterioles.
    * In the kidneys, it constricts glomerular arterioles (The Rheese-McKinney mechanism),angiotensin II constricts efferent arterioles, which forces blood to build up in the glomerulus, increasing glomerular pressure. The glomerular filtration rate (GFR) is thus maintained, and blood filtration can continue despite lowered overall kidney blood flow.
    * Angiotensin II stimulates Na+/H+ exchangers located on the apical membranes of cells in the proximal tubule and thick ascending limb of the loop of Henle in addition to Na+ channels in the collecting ducts. This will ultimately lead to increased sodium reabsorption
    * Angiotensin II stimulates the hypertrophy of renal tubule cells, leading to further sodium reabsorption.
    * In the adrenal cortex, it acts to cause the release of aldosterone. Aldosterone acts on the tubules (e.g., the distal convoluted tubules and the cortical collecting ducts) in the kidneys, causing them to reabsorb more sodium and water from the urine. This increases blood volume and, therefore, increases blood pressure.
    * Release of anti-diuretic hormone.ADH also acts on the central nervous system to increase an individual's appetite for salt, and to stimulate the sensation of thirst.
If the renin-angiotensin-aldosterone system is too active, blood pressure will be too high. There are many drugs that interrupt different steps in this system to lower blood pressure. These drugs are one of the main ways to control high blood pressure (hypertension), heart failure, kidney failure, and harmful effects of diabetes.


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