USA Biology Olympiad (USABO) Practice Exam

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What compound in the blood does antidiuretic hormone help to conserve in the kidneys?

Glucose
Sodium
Potassium
Water

The correct answer is: Water

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance in the body. When the body is dehydrated or when blood volume is low, ADH is released from the posterior pituitary gland. This hormone acts primarily on the kidneys, promoting the reabsorption of water from the filtrate back into the bloodstream in the collecting ducts. By doing so, it helps to concentrate urine and minimize water loss, effectively conserving water in the body. In contrast, glucose, sodium, and potassium are regulated by other mechanisms within the body. While sodium regulation does involve hormones such as aldosterone, it is not the primary target of ADH. Similarly, potassium levels are managed through different hormones and processes, including renal excretion and the action of the mineralocorticoid hormones. Therefore, it is the conservation of water that is the primary function of antidiuretic hormone in the kidneys.