What effect does ADH have on the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, primarily affects the kidneys by increasing water reabsorption. It does this by promoting the insertion of aquaporin-2 water channels in the collecting ducts of the nephron, which allow for more water to be reabsorbed back into the bloodstream. This process is crucial for maintaining the body’s fluid balance and regulating blood osmolarity.

When ADH levels rise, the kidneys become more permeable to water, leading to concentrated urine and reduced urine volume, which is particularly important during states of dehydration. This action contrasts with a scenario where ADH is lacking, leading to diluted urine and increased urine output, as seen in conditions such as diabetes insipidus.

In summary, the role of ADH in enhancing water reabsorption is vital for the body’s ability to conserve water and maintain homeostasis, making it the correct response to the question regarding ADH's effect on the kidneys.