Urine Regulation: Aldosterone

Tutoring Biology 12, you realize that with most organ systems, the hormonal control is the most difficult to retain.  The Biology tutor continues about urine regulation with this discussion of aldosterone.

Of course, urine is produced by the kidneys.  If you missed it, you can read how a kidney works in my post here.

In my previous post I talked about ADH and how the body uses it to regulate urine volume. There is another hormone – called aldosterone – that the body uses to control how much water is reclaimed from the filtrate. (Recall that the filtrate is the mix of water, ions, and small molecules first removed from the blood by the kidneys.)

Aldosterone is released by the adrenal cortex. However, the adrenal cortex needs to be informed to do so by the presence of renin in the blood. Renin is secreted by the cells of the juxtaglomerular apparatus, which are adjacent to the glomerulus and sense the blood pressure within. Specifically, when the cells of the juxtaglomerular apparatus sense that the blood pressure is too low, they respond by secreting renin into the bloodstream.

The renin circulates through the body to the adrenal cortex. Detecting the renin, the adrenal cortex responds by secreting aldosterone.

Aldosterone targets the cells of the distal convoluted tubule, telling them to let go of more K+ (K+ means potassium ions), but reclaim more Na+ (sodium ions) in compensation. The effect is that more water is reabsorbed from the filtrate, increasing blood volume and decreasing urine volume.

Unlike ADH, aldosterone does not result in blood dilution, since more ions are reclaimed alongside the extra water that is reabsorbed. Someone might ask, “If aldosterone increases the reclamation of sodium ions, how does that mean increased water reabsorption?” The answer is that sodium ions have a powerful pull on water – more powerful than potassium ions. So if you reabsorb sodium ions instead of potassium ions, more water will be drawn back into the blood as well.

Ultimately, the kidneys release renin – which leads to the release of aldosterone – in order to defend their own function.  If blood pressure is too low, the kidneys cannot filter the blood properly.  By increasing water reabsorption and therefore blood volume, aldosterone helps maintain the necessary blood pressure for proper filtration.

Source: Biology 12, Module 4: Human Biology 2. Open School BC, 2007.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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