Renin-angiotensin system

Image:Renin-angiotensin-aldosterone system.pngThe renin-angiotensin system (RAS) or the renin-angiotensin-aldosterone system (RAAS) is a hormone system that helps regulate long-term blood pressureand bloodvolume in the body.

The system can be activated when there is a loss of bloodvolume or a drop in blood pressure(such as in a hemorrhage).

If the perfusion of the juxtaglomerular apparatusin the kidneysdecreases, then the juxtaglomerular cells release the enzymatichormone renin. Renin cleaves an inactive peptidecalled angiotensinogen, converting it into angiotensin I. Angiotensin I is then converted to angiotensin II by angiotensin-converting enzyme(ACE), which is found mainly in lungcapillaries. Angiotensin I may have some minor activity, but angiotensin II is more potent.

Angiotensin II has a variety of effects on the body:

• It is a potent vasoconstrictorthroughout the body.
• In the kidneys, it constricts glomerulararterioles, having a greater effect on efferent arterioles than afferent. As with most other capillary beds in the body, the constriction of afferent arterioles increases the arterioler resistance, raising systemicarterial blood pressureand decreasing the blood flow. However, the kidneys must continue to filter enough blood despite this drop in blood flow, necessitating mechanisms to keep glomerular blood pressure up. To do this, Angiotensin II constricts efferent arterioles, which forces blood to buildup 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.
• It also acts on the adrenal cortexcausing the release of aldosterone. Aldosterone acts on the tubules (i.e. the distal convoluted tubulesand the corticalcollecting ducts) in the kidneys, causing them to reabsorb more sodiumand water from the urine. Potassium is secreted into the tubule in exchange for the sodium, which is reabsorbed. Aldosterone also acts on the central nervous systemto increase a person's appetite for salt, and to make them feel thirsty.

These effects directly act to increase the amount of fluid in the blood, making up for a loss in volume, and to increase blood pressure.

The renin-angiotensin system is often manipulated clinically to treat high blood pressure. Inhibitors of angiotensin-converting enzymeare often used to reduce the formation of the more potent angiotensin II. Alternatively, angiotensin receptor blockers(ARBs) can be used to prevent angiotensin II from acting on angiotensin receptors.

Interestingly, ACE cleaves a number of other peptides, and in this capacity is an important regulator of the kinin-kallikrein system.de:Renin-Angiotensin-Aldosteron-System fr:Système rénine-angiotensine-aldostérone sv:RAAS

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It uses material from the http://en.wikipedia.org/wiki/Renin-angiotensin+system Wikipedia article Renin-angiotensin system.