Dehydration increases the plasma concentration of all the following hormones EXCEPT
## **Core Concept**
Dehydration triggers a cascade of physiological responses to conserve water and maintain blood pressure. This involves the release of various hormones that play critical roles in fluid balance and electrolyte homeostasis. The primary hormones involved in the response to dehydration include antidiuretic hormone (ADH), also known as vasopressin, aldosterone, and atrial natriuretic peptide (ANP), among others.
## **Why the Correct Answer is Right**
Dehydration leads to decreased blood volume (hypovolemia) and increased osmolality of the blood. In response:
- **ADH (Antidiuretic Hormone)** is released by the posterior pituitary gland. It acts on the kidneys to increase water reabsorption, concentrating the urine and reducing water loss. Therefore, its plasma concentration increases during dehydration.
- **Aldosterone** is released by the adrenal cortex. It promotes sodium retention in the kidneys, which leads to water retention and helps to restore blood volume. Its levels also increase in response to dehydration.
- **Atrial Natriuretic Peptide (ANP)** is released by the atria of the heart in response to increased stretch (which is decreased in dehydration). ANP promotes sodium excretion and vasodilation. Therefore, its plasma concentration **decreases** during dehydration.
## **Why Each Wrong Option is Incorrect**
- **Option A:** ADH levels increase during dehydration to promote water conservation.
- **Option B:** Aldosterone levels increase to promote sodium and water retention.
- **Option D:** Other hormones like renin also increase during dehydration as part of the renin-angiotensin-aldosterone system (RAAS) to help retain fluid.
## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that in a dehydrated state, the body tries to conserve water and electrolytes. This is reflected in the hormonal changes, with increases in ADH and aldosterone. A notable exception is ANP, which decreases to allow for fluid conservation.
## **Correct Answer:** . Atrial Natriuretic Peptide (ANP)