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.