What is the primary function of aldosterone in the kidneys?

Aldosterone is a steroid hormone produced by the adrenal glands, and its primary function in the kidneys is to increase the reabsorption of sodium and water, which in turn helps to maintain blood pressure and fluid balance in the body. Specifically, aldosterone acts on the cells of the distal convoluted tubule and collecting ducts of the nephron, where it promotes the insertion of sodium channels and sodium-potassium pumps in the renal tubular cells. As sodium is reabsorbed back into the bloodstream, water follows osmotically, leading to increased fluid retention.

The overall effect of aldosterone is to enhance the body's ability to hold onto sodium and thereby water, which raises blood volume and blood pressure. This is crucial in situations where blood pressure is low or when the body is dehydrated.

The other options do not reflect the primary action of aldosterone in the kidneys. For example, decreasing water and salt reabsorption would lead to increased urine production and decreased blood volume, which is contrary to the main role of aldosterone. Additionally, stimulating the production of urine conflicts with aldosterone's role in promoting fluid retention. Lastly, while aldosterone has effects on potassium by promoting its excretion in urine, inhibiting reabsorption is not its primary