Biological Systems MCAT Practice Exam

Aldosterone is a steroid hormone produced by the adrenal cortex and plays a crucial role in regulating sodium reabsorption in the kidneys. It stimulates the nephron's distal convoluted tubule and collecting duct to reabsorb sodium ions from the filtrate back into the bloodstream. In doing so, it helps to maintain blood pressure and fluid balance within the body.

The action of aldosterone is linked to its role in the renin-angiotensin-aldosterone system (RAAS), where it is released in response to low blood pressure or low sodium levels. By increasing sodium reabsorption, aldosterone also indirectly increases water reabsorption, leading to an increase in blood volume and pressure.

In contrast, while angiotensin I is part of the precursor to angiotensin II, which eventually stimulates aldosterone release, it does not have a direct effect on sodium reabsorption itself. Cortisol, while it can influence some aspects of kidney function, primarily affects glucose metabolism and has different systemic roles. Antidiuretic hormone (ADH) focuses on water reabsorption rather than sodium, acting mainly on the collecting ducts of the kidneys to promote water retention.

Thus, aldosterone stands out as the hormone specifically responsible for increasing