The Renin-Angiotensin System play an important role in regulating blood volume and systemic vascular resistance which together influence cardiac output and arterial pressure. There are two important components to this system 1) Renin 2)Angiotensin. Renin which is released primarily by the kidney, stimulates the formation of angiotensin in blood and tissues. Renin is a proteolytic enzyme which is released into the circulation by the kidneys when :
- Sympathetic nerve activation
- Renal artery hypotension
- Decreased sodium delivery to the distal tubules of kidneys.
Although the sodium ion (Na+) is the major electrolyte in blood, Na+ is continuously being lost from the body in urine and sweat. Specialized cells in the walls of certain blood vessels within the kidney can act as osmoreceptors and continuously monitor the Na+ concentration of the blood. If a drop in Na+ concentration and hence blood osmolarity is noted, these cells release a substance, renin, which acting as an enzyme, is able to split a plasma protein into a smaller peptide. This peptide is then acted on by yet another enzyme to release a still smaller peptide, angiotensin II, a hormone. Angiotensin II then stimulates certain cells of the adrenal cortex to release aldosterone, another hormone. This hormone then acts on certain collecting tubules of the kidney to cause resorption of Na+ from the urine. Thus a complex set of hormonal actions are brought into play to maintain the proper levels of blood Na+.
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