What hormone stimulates Na+ and K+ secretion to increase water reabsorption?

The hormone that stimulates sodium (Na+) reabsorption and potassium (K+) secretion to increase water reabsorption is aldosterone. Aldosterone is produced by the adrenal cortex and plays a critical role in regulating electrolyte balance and maintaining blood pressure.

When aldosterone is released, it acts primarily on the distal tubules and collecting ducts of the nephron in the kidneys. It promotes the reabsorption of sodium ions back into the bloodstream, which also leads to the reabsorption of water due to the osmotic gradient created by the increased sodium concentration. Simultaneously, aldosterone facilitates the secretion of potassium ions into the tubular fluid, which helps maintain electrolyte balance.

This action is important because, as sodium is reabsorbed, it draws water along with it, thus increasing blood volume and, subsequently, blood pressure. The overall effect of aldosterone is to enhance water retention, making it crucial in the body’s fluid regulation mechanisms.

In contrast, cortisol, while also a steroid hormone, primarily influences metabolism and has some effects on the immune system rather than directly regulating sodium and potassium in the same manner as aldosterone. Adrenaline is involved in the 'fight or flight' response and does not primarily function to regulate water reabsorption.