What is the compensatory response when vascular volume decreases?

What is the compensatory response when vascular volume decreases?

• A. The heart rate increases to conserve fluid.
• B. The kidneys excrete more water to increase volume loss.
• C. The brain reduces thirst signals.
• D. The RAAS and ADH mechanisms cause the body to retain water.

Answer: (D)

When vascular (effective circulating) volume falls, the body rapidly works to conserve water and sodium to restore volume and pressure. The main players are the renin-angiotensin-aldosterone system (RAAS) and antidiuretic hormone (ADH). Low pressure or perfusion signals the kidneys to release renin, starting a cascade that produces angiotensin II, which constricts vessels and stimulates aldosterone release. Aldosterone then promotes sodium (and thus water) reabsorption in the distal nephron, increasing blood volume. ADH is released in response to low BP or higher plasma osmolality and acts on the collecting ducts to reabsorb more water, further boosting volume. Together, these mechanisms retain water and sodium to compensate for the loss. Other options don’t fit because increasing heart rate is a perfusion-focused reflex that helps keep circulation going but doesn’t specifically conserve fluid. Excreting more water would worsen the volume deficit. Reducing thirst signals would oppose the body’s natural drive to drink and restore volume, whereas thirst is typically stimulated in dehydration to help correct the deficit.

When vascular (effective circulating) volume falls, the body rapidly works to conserve water and sodium to restore volume and pressure. The main players are the renin-angiotensin-aldosterone system (RAAS) and antidiuretic hormone (ADH). Low pressure or perfusion signals the kidneys to release renin, starting a cascade that produces angiotensin II, which constricts vessels and stimulates aldosterone release. Aldosterone then promotes sodium (and thus water) reabsorption in the distal nephron, increasing blood volume. ADH is released in response to low BP or higher plasma osmolality and acts on the collecting ducts to reabsorb more water, further boosting volume. Together, these mechanisms retain water and sodium to compensate for the loss.

Other options don’t fit because increasing heart rate is a perfusion-focused reflex that helps keep circulation going but doesn’t specifically conserve fluid. Excreting more water would worsen the volume deficit. Reducing thirst signals would oppose the body’s natural drive to drink and restore volume, whereas thirst is typically stimulated in dehydration to help correct the deficit.