<P> The system can be activated when there is a loss of blood volume or a drop in blood pressure (such as in hemorrhage or dehydration). This loss of pressure is interpreted by baroreceptors in the carotid sinus . It can also be activated by a decrease in the filtrate sodium chloride (NaCl) concentration or a decreased filtrate flow rate that will stimulate the macula densa to signal the juxtaglomerular cells to release renin . </P> <Ol> <Li> 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 . </Li> <Li> Renin cleaves a decapeptide from angiotensinogen, a globular protein . The decapeptide is known as angiotensin I . </Li> <Li> Angiotensin I is then converted to an octapeptide, angiotensin II by angiotensin - converting enzyme (ACE), which is thought to be found mainly in endothelial cells of the capillaries throughout the body, within the lungs and the epithelial cells of the kidneys . One study in 1992 found ACE in all blood vessel endothelial cells . </Li> <Li> 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 . </Li> </Ol> <Li> 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 . </Li> <Li> Renin cleaves a decapeptide from angiotensinogen, a globular protein . The decapeptide is known as angiotensin I . </Li>

In the renin-angiotensin system which protein is the active vasoconstrictor