USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What is one consequence of a volume-depleted state on renal physiology?

1. Increased RPF

2. Decreased filtration fraction

3. Increased peritubular oncotic pressure

4. Increased ECF volume

The correct answer is: Increased peritubular oncotic pressure

In a volume-depleted state, the body activates various compensatory mechanisms to conserve volume and maintain blood pressure. One significant consequence of this dehydration or volume loss is the alteration of oncotic pressures, particularly in the peritubular capillaries. When the body is in a volume-depleted state, there tends to be an increase in plasma protein concentration due to the loss of free water. This elevated protein concentration leads to an increase in oncotic pressure within the peritubular capillaries. Since oncotic pressure attracts water, the increased peritubular oncotic pressure in a dehydrated state promotes the reabsorption of water from the renal tubules back into the bloodstream. Consequently, it enhances the body’s ability to retain fluid and adjust to the hypovolemic condition. Understanding this mechanism is critical, as it reflects how the kidneys respond to changes in systemic volume and maintain homeostasis. In contrast, options related to renal blood flow (like renal plasma flow), filtration fraction, and extracellular fluid volume contradict the physiological adaptations the body makes in response to volume depletion.