The Renin-Angiotensin-Aldosterone (RAA) Pathway

The First Stages Person is dehydrated, has a sodium ion deficiency or is hemorrhaging, any of these stimuli will initiate the pathway All of these conditions cause a decrease in blood volume This leads to decreased blood pressure and juxtaglomerular cells in the kidneys are stimulated to release the enzyme RENIN This in turn increases blood levels of renin

Second Stages Renin will convert angiotensinogen (plasma protein made by the liver) into angiotensin I. Blood levels of Angiotensin I rise as a result. As blood flows another enzyme (Angiotensin- converting enzyme) (ACE), converts Angiotensin I into Angiotensin II. Blood levels of Angiotensin II now rise. Angiotensin II stimulates the adrenal cortex to secrete aldosterone

Third Phase In the kidneys, increased aldosterone levels cause an increase in the reabsorbing of sodium ions and water Retain more, urinate less of both Aldosterone also stimulates the kidneys to increase secretion of potassium and hydrogen ions into urine Increased water retention increases the blood volume This in turn increases blood pressure and brings it back to normal

Third Phase Continued In addition to increased blood volume, Angiotensin II also stimulates vasoconstriction of arterioles which increases blood pressure as well Under normal circumstances this should return blood pressure back to normal.