What change in blood osmolarity does aldosterone cause?

Aldosterone primarily functions to regulate sodium and water balance in the body. It is produced by the adrenal cortex and acts mainly on the kidneys, specifically on the distal convoluted tubules and collecting ducts. When aldosterone is released, it promotes the reabsorption of sodium (Na+) back into the bloodstream. As sodium is reabsorbed, water follows passively due to osmotic forces, effectively increasing blood volume.

While one might think that reabsorbing sodium would increase osmolarity, the key point is that the reabsorption of water occurs simultaneously. Since both sodium and water are being reabsorbed, the overall osmolarity of the blood remains stable. Thus, the body does not experience an increase in osmolarity despite the fact that sodium concentration in the blood rises.

This regulatory mechanism is crucial for maintaining homeostasis in the body's fluids, particularly in situations where blood pressure and blood volume need to be controlled. It is this balancing act—the simultaneous action of sodium reabsorption and water conservation—that leads to no change in osmolarity overall, which underscores why the correct response is that aldosterone causes no change in osmolarity.