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The renin pathway is a human biological system that controls certain aspects of the circulation, such as blood pressure. This pathway involves a cascade of reactions that are initiated by the kidneys as a response to low blood pressure. Also known as the renin-angiotensin-aldosterone system (RAS), the renin pathway can be manipulated with medication to reduce blood pressure in some people with hypertension.
This pathway begins in the kidneys as a response to low renal blood flow, which is a sign of low blood pressure. When low renal blood flow is detected, the kidneys make an enzyme called renin and secrete it into the bloodstream. In the blood, renin reacts with a protein called angiotensinogen to create angiotensin I. An enzyme called angiotensin converting enzyme (ACE) converts some of the angiotensin I into angiotensin II. Both of these new proteins are vasoconstrictors, which cause blood vessels to constrict, and increase blood pressure in the kidneys.
The primary way in which the renin pathway controls blood pressure is by regulating blood volume. The volume of blood in the body is affected by the amount of sodium and water which is ingested and excreted. Excretion of sodium and water is regulated by the kidneys, which release more of these substances into the urine when greater amounts are ingested.
Along with regulating blood volume, the renin pathway has another important function: controlling systemic vascular resistance. This is a measure of the amount of blood flow resistance exerted by the body's network of arteries and vessels. Systemic vascular resistance is influenced by a number of things, including renin activity and factors such as artery and vessel blockage. For example, if an artery is partially blocked by plaque deposits, greater pressure is required to push blood through the system.
Blood volume and systemic vascular resistance in turn control cardiac output and arterial pressure. Cardiac output is a measurement based on the amount of blood ejected by the heart when it beats, and the number of times the heart beats per minute. Arterial pressure is the pressure generated when blood is pumped out of the left ventricle of the heart. Together, all of these factors regulate blood pressure.
Ultimately, the renin pathway controls blood pressure by directly regulating blood volume and systemic vascular resistance, which in turn influence cardiac output and arterial pressure. These effects are initiated by the kidneys, and mediated by ACE. This means that medications which prevent the effects of ACE can help some people with high blood pressure, and in fact, this is the basis of a class of hypertension drugs called ACE inhibitors. These medications help reduce blood pressure by preventing ACE from converting angiotensin I into angiotensin II.
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