What is the role of antidiuretic hormone (ADH) in fluid balance?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating fluid balance in the body by increasing the permeability of the kidneys to water. When ADH is released from the posterior pituitary gland, it acts primarily on the collecting ducts of the kidneys.

By binding to specific receptors in the cells of the collecting ducts, ADH promotes the insertion of water channels called aquaporins into the cell membranes. This action increases the kidneys' ability to reabsorb water back into the bloodstream, leading to concentrated urine and reduced urine output. Consequently, this mechanism helps maintain blood volume and blood pressure, especially during dehydration or low blood volume states.

In contrast, options regarding increases in urine production or decreased kidney permeability to water would not accurately describe the function of ADH, misrepresenting its role as supportive of fluid retention rather than expulsion. Additionally, while the regulation of blood glucose levels is critical for maintaining overall homeostasis, it is not a function of ADH; instead, hormones like insulin and glucagon primarily handle blood glucose regulation.