American Board of Internal Medicine (ABIM) Certification Practice Exam

What happens to urine sodium concentration with volume depletion?

1. It generally increases

2. It is unchanged

3. It becomes very high

4. It is generally low

The correct answer is: It is generally low

In the context of volume depletion, the body initiates several compensatory mechanisms to conserve water and sodium. When there is a reduction in blood volume, the kidneys respond by decreasing the excretion of sodium in an attempt to retain as much fluid as possible. This leads to a low urine sodium concentration. In a state of volume depletion, factors such as increased secretion of aldosterone and activation of the renin-angiotensin-aldosterone system (RAAS) work together to promote sodium reabsorption in the renal tubules. The kidneys will also concentrate the urine in terms of sodium as a means of retaining electrolytes and fluid. As a result, urine sodium concentration is generally low, reflecting the body's effort to conserve sodium in response to decreased volume status. This physiological response is crucial for maintaining blood pressure and the overall fluid balance in the body under conditions of dehydration or significant volume loss.