AEMCA Practice Exam

Which substance is primarily responsible for increasing blood volume by promoting water retention?

1. Renin

2. ADH

3. Atrial natriuretic peptide

4. Angiotensin II

The correct answer is: ADH

The substance that plays a primary role in increasing blood volume by promoting water retention is antidiuretic hormone (ADH), also known as vasopressin. ADH is released by the posterior pituitary gland in response to high plasma osmolarity or low blood volume. Once released into the bloodstream, ADH acts primarily on the kidneys, specifically on the collecting ducts, increasing their permeability to water. This action allows more water to be reabsorbed back into the bloodstream, thus increasing blood volume and ultimately aiding in the regulation of blood pressure and fluid balance in the body. In contrast, renin is an enzyme that starts the renin-angiotensin-aldosterone system (RAAS) but does not directly promote water retention. Atrial natriuretic peptide plays a role in reducing blood volume by promoting sodium excretion, which leads to loss of water. Angiotensin II, while it does contribute to raising blood pressure and stimulating aldosterone production (which increases sodium and water reabsorption), is not as directly involved in promoting water retention as ADH. Therefore, ADH is the most accurate answer when considering the direct mechanism of increasing blood volume through water retention.