Renin-Angiotensin-Aldosterone Made Simple

The body has many systems that work together in order to maintain homeostasis. The renin-angiotensin-aldosterone pathway is no different. It has 3 functions: (1) to maintain a proper blood pressure/blood flow, (2) to maintain the right concentration of sodium (Na+) in the blood, and finally, (3) to maintain the right amount of water in the blood. To make sure that these 3 things stay at proper levels, several hormones and several organs work together to accomplish this task. Now to break the pathway down!

RENIN-ANGIOTENSIN-ALDOSTERONE PATHWAY

This process starts out in the kidneys, but becomes very systemic (all over body) throughout the pathway. It does return to the kidneys at several instances. 

1. Low blood pressure/blood flow is sensed by the Juxtaglomerular apparatus in the kidney (which are cells next to the glomerulus). This is because a decrease in Na+ will reduce the amount of water in the blood, thus the blood will have a lower pressure. This follows the principle of osmosis, which states that water will diffuse to areas that have highly concentrated solutes.

2.  In response to this, the glomerulus  (the bed of capillaries that is wrapped up by Bownan's capsule and is next to the proximal convoluted tubule) releases a hormone known as renin into the blood stream.

3. Renin then moves to the liver, where is converts an inactive peptide (protein) angiotensinogen to an active angiotensin I.

4. Angiotensin I then travels to the lungs where an enzyme known as the Angiotensin Converting Enzyme (ACE), converts Angiotensin I to Angiotensin II. One effect Angiotensin II has on the body is in its ability to constrict blood vessel, thus increasing blood pressure. Another function of it is to stimulate the adrenal glands on top of the kidneys to produce the hormone Aldosterone.

5. Aldosterone stimulates the reabsorption of sodium (Na+) in the distal convoluted tubules. Increasing sodium reabsorption means that water and chloride (Cl-) will follow, thus increasing blood volume.

6. An increase in blood volume may also trigger the release of a hormone known as Atrial Natriuretic Hormone, which inhibits the release of Aldosterone, keeping the body's water and sodium levels at the homeostatic levels.This last step is known as a negative feedback loop.

And that is the renin-angiotensin-aldosterone pathway. If one can grasp the concept of osmosis and how water follows highly concentrated solutes, one will be able to understand blood pressure and other methods of osmoregulation (water balancing) in the kidneys.