Kidneys & Blood Pressure

Applied Science: BTEC Level 3 National Extended Diploma, topic 4.7.3

The kidneys also have a role in maintaining blood pressure. The renin-angiotensin-aldosterone system (RAAS) largely controls this.

Another hormone responsible for maintaining electrolyte concentrations in extracellular fluids is aldosterone.
Aldosterone is a steroid hormone that is produced by the adrenal cortex.
Aldosterone maintains proper water balance by enhancing Na+ reabsorption and K+ secretion from the extracellular fluid of the cells in kidney tubules.
This contrasts ADH, which promotes the reabsorption of water to maintain proper water balance

Aldosterone release is stimulated by:
Decrease in blood sodium levels.
Decrease in blood volume.
Decrease in blood pressure.
Increase in blood potassium levels.
It also prevents the loss of Na⁺ from sweat, saliva, and gastric juice.
The reabsorption of Na⁺ also results in the osmotic reabsorption of water, which alters blood volume and blood pressure.

Aldosterone production can be stimulated by low blood pressure, which triggers a sequence of chemical release.
When blood pressure drops, the renin-angiotensin-aldosterone system (RAAS) is activated.
Cells in the juxtaglomerular apparatus detect this and release renin.
The juxtaglomerular apparatus is next to the glomerulus & regulates the functions of the nephrons of the kidney.
Renin, an enzyme, circulates in the blood and reacts with a plasma protein produced by the liver called angiotensinogen.

When angiotensinogen is cleaved by renin, it produces angiotensin I.
Angiotensin I is then converted into angiotensin II in the lungs.
Angiotensin II functions as a hormone and then causes the release of the hormone aldosterone by the adrenal cortex.
This results in increased Na⁺ reabsorption, water retention, and an increase in blood pressure.
Angiotensin II also causes an increase in ADH and increased thirst, both of which help to raise blood pressure.

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