How does increased aldosterone secretion increase blood pressure?

Answer 1

Your body can be divided up into a number of compartments, the first two being the intracellular and extracellular compartments. The two compartments contain fluid referred to as intracellular fluid (ICF) and extracellular fluid (ECF). By convention, blood is considered part of the extracellular compartment, so it contributes to ECF. One of the major determinants of blood pressure is the volume of blood in the blood vessels. All other things being equal, if you raise blood volume, you raise blood pressure; conversely, if you lower blood volume, you lower blood pressure.

Since the blood is in the extracellular compartment, the blood volume is a component of the extracellular fluid. Therefore if you raise ECF, you raise blood volume; as we saw above, raising blood volume raises blood pressure. How aldosterone contributes to blood pressure is via the regulation of ECF volume.

Sodium (Na) is the major cation in the ECF. Sodium is very important for the nervous system and other parts of the body; consequently, the body works very hard to keep the blood concentration of sodium within narrow limits. If suddenly the amount of sodium in the blood were increased, your brain would trigger the sensation of thirst; you would drink water and increase the ECF volume to dilute the sodium and bring it back within the normal range. So by adding extra sodium to the blood, you increase ECF volume. And what happens to blood pressure? It increases, because ECF is a major determinant of blood pressure.

What does this have to do with aldosterone?

Aldosterone is a steroid hormone that's synthesized by the adrenal gland. It acts on the kidneys to increase sodium reabsorption. Having more aldosterone around means that more sodium ultimately winds up in the blood, activating thirst mechanisms that cause you to drink water, which increases ECF volume and therefore increases blood pressure.

You might be interested to know that the body really does use aldosterone to regulate blood pressure. When blood pressure drops, the kidneys sense a subtle change in blood flow and start secreting an enzyme called renin. Renin works through a pathway called the renin-angiotensin-aldosterone system (RAAS; see link at left) to ultimately increase aldosterone secretion in order to return blood pressure to normal.