Renin-angiotensin 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 pressure is low, the kidneys secrete renin. Renin stimulates the production of angiotensin. Angiotensin causes blood vessels to constrict resulting in increased blood pressure. Angiotensin also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the tubules of the kidneys to retain sodium and water. This increases the volume of fluid in the body, which also increases blood pressure.

If the renin-angiotensin-aldosterone system is too active, blood pressure will be too high. There are many drugs which 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. [cite web |url=http://www.merck.com/mmhe/sec03/ch022/ch022a.html |title=High Blood Pressure: Heart and Blood Vessel Disorders |format= |work=Merck Manual Home Edition |accessdate=] [cite journal
last = Solomon
first = Scott D
coauthors = Anavekar, Nagesh
title = A Brief Overview of Inhibition of the Renin-Angiotensin System: Emphasis on Blockade of the Angiotensin II Type-1 Receptor
journal = Medscape Cardiology
volume = 9
issue = 2
publisher =
location =
year = 2005
url =http://www.medscape.com/viewarticle/503909
doi =
id =
accessdate = ]

Activation

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

# If the perfusion of the juxtaglomerular apparatus in the kidneys decreases, then the juxtaglomerular cells release the enzyme renin.
# Renin cleaves an inactive peptide called "angiotensinogen", converting it into "angiotensin I".
# Angiotensin I is then converted to "angiotensin II" by angiotensin-converting enzyme (ACE) [cite journal |author=Paul M, Poyan Mehr A, Kreutz R |title=Physiology of local renin-angiotensin systems |journal=Physiol. Rev. |volume=86 |issue=3 |pages=747–803 |year=2006 |month=July |pmid=16816138 |doi=10.1152/physrev.00036.2005 |url=http://physrev.physiology.org/cgi/content/full/86/3/747] which is found mainly in lung capillaries.
# Angiotensin II is the major bioactive product of the renin-angiotensin system. Angiotensin II acts as an endocrine, autocrine/paracrine, and intracrine hormone.
# Patil Jaspal et al. has shown local synthesis of Angiotensin II in neurons of sympathetic ganglia. [cite journal |author=Patil J, Heiniger E, Schaffner T, Mühlemann O, Imboden H |title=Angiotensinergic neurons in sympathetic coeliac ganglia innervating rat and human mesenteric resistance blood vessels |journal=Regul. Pept. |volume=147 |issue=1-3 |pages=82–7 |year=2008 |month=April |pmid=18308407 |doi=10.1016/j.regpep.2008.01.006 |url=]

Effects

:"Further reading: Angiotensin#Effects and Aldosterone#Function"

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. As with most other capillary beds in the body, the constriction of afferent arterioles increases the arteriolar resistance, raising systemic arterial blood pressure and 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 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.
*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. Potassium is secreted into the tubules in exchange for the sodium, which is excreted. Aldosterone also acts on the central nervous system to increase an individual's appetite for salt, and to stimulate the sensation of thirst.
*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.

These effects directly act in concert to increase blood pressure.

Clinical significance

The renin-angiotensin system is often manipulated clinically to treat high blood pressure.
* 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.
* Alternatively, angiotensin receptor blockers (ARBs) can be used to prevent angiotensin II from acting on angiotensin receptors.
* A new drug called Aliskiren released in 2007 acts by directly inhibiting renin receptors.

Other uses of ACE

Interestingly, ACE cleaves a number of other peptides, and in this capacity is an important regulator of the kinin-kallikrein system.

Fetal renin-angiotensin system

In the fetus, the renin-angiotensin system is predominantly a sodium-losing system, as angiotensin II has little or no effect on aldosterone levels. Renin levels are high in the fetus, while angiotensin II levels are significantly lower — this is due to the limited pulmonary blood flow, preventing ACE (found predominantly in the pulmonary circulation) from having its maximum effect.

References

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External links

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