Renin Angiotensin Aldosterone System
The renin angiotensin system (RAS) or the renin angiotensin aldosterone system (RAAS) is a hormone system that regulates blood pressure and water (fluid) balance.
When blood volume is low, juxtaglomerular cells in the kidneys secrete renin directly into circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I. Angiotensin I is subsequently converted to angiotensin II by the enzyme angiotensin converting enzyme found in the lungs. Angiotensin II is a potent vaso-active peptide that causes blood vessels to constrict, resulting in increased blood pressure. Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood. This increases the volume of fluid in the body, which also increases blood pressure.
If the renin–angiotensin–aldosterone system is abnormally 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.
Activation of Renin Angiotensin Aldosterone System:
The system can be activated when there is a loss of blood volume or a drop in blood pressure (such as in hemorrhage). This loss of pressure is interpreted by baroreceptors in the carotid sinus. In alternative fashion, a decrease in the filtrate NaCl concentration and/or decreased filtrate flow rate will stimulate the macula densa to signal the juxtaglomerular cells to release renin.
- If the perfusion of the juxtaglomerular apparatus in the kidney’s macula densa decreases, then the juxtaglomerular cells (granular cells, modified pericytes in the glomerular capillary) release the enzyme renin.
- Renin cleaves a zymogen, called angiotensinogen, converting it into angiotensin I.
- Angiotensin I is then converted to angiotensin II by angiotensin-converting enzyme (ACE), which is thought to be found mainly in lung capillaries. One study in 1992 found ACE in all blood vessel endothelial cells.
- Angiotensin II is the major bioactive product of the renin-angiotensin system, binding to receptors on intraglomerular mesangial cells, causing these cells to contract along with the blood vessels surrounding them and causing the release of aldosterone from the zona glomerulosa in the adrenal cortex. Angiotensin II acts as an endocrine, autocrine/paracrine, and intracrine hormone.
Renin Angiotensin Aldosterone System effects on Cardiovascular:
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, having a greater effect on efferent arterioles than afferent, raising systemic arterial blood pressure and decreasing the blood flow 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 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. In exchange for the reabsorbing of sodium to blood, potassium is secreted into the tubules, becomes part of urine and is excreted.
- Release of anti-diuretic hormone (ADH), also called vasopressin – ADH is made in the hypothalamus and released from the posterior pituitary gland. As its name suggests, it also exhibits vaso-constrictive properties, but its main course of action is to stimulate reabsorption of water in the kidneys. ADH also acts on the central nervous system to increase an individual’s appetite for salt, and to stimulate the sensation of thirst.
These effects directly act in concert to increase blood pressure.
Clinical significance of Renin Angiotensin Aldosterone System :
- Inhibitors of angiotensin-converting enzyme (ACE inhibitors) are often used to reduce the formation of the more potent angiotensin II. Captopril is an example of an ACE inhibitor. ACE cleaves a number of other peptides, and in this capacity is an important regulator of the kinin–kallikrein system, as such blocking ACE can lead to side effects.
- Angiotensin receptor blockers (ARBs) can be used to prevent angiotensin II from acting on angiotensin receptors.
- Direct renin inhibitors can also be used for hypertension. The drugs that inhibit renin are aliskiren and the investigational remikiren.
- Vaccines against angiotensin II, for example CYT006-AngQb, have been investigated
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