Biological Systems MCAT Practice Exam 2025 – The Comprehensive All-in-One Guide for Exam Success!

The hormone released by the posterior pituitary to increase water reabsorption in the kidneys is antidiuretic hormone (ADH), also known as vasopressin. ADH plays a crucial role in regulating the body's water balance. When the body is dehydrated or when blood osmolarity is high, the posterior pituitary releases ADH into the bloodstream.

ADH acts primarily on the kidneys, specifically targeting the collecting ducts. It increases their permeability to water, allowing more water to be reabsorbed back into the bloodstream rather than being excreted in urine. This process reduces urine output and helps concentrate the urine, effectively conserving water in the body.

The other hormones mentioned play different roles in the body's regulation of fluids and electrolytes. For instance, renin is part of the renin-angiotensin-aldosterone system but does not act directly to increase water reabsorption. Aldosterone, while it promotes sodium retention in the kidneys, indirectly influences water reabsorption because water follows sodium osmotically. Angiotensin II is a potent vasoconstrictor and stimulates thirst and aldosterone release, but it does not act on the kidneys in the same direct manner as ADH. Thus, ADH