Anatomy and Physiology II Practice Test

Which of the following fluid compartments varies the least in volume?

1. Plasma

2. Interstitial fluid

3. Intracellular fluid

4. All vary equally

The correct answer is: Plasma

The plasma fluid compartment varies the least in volume compared to the other fluid compartments of the body. Plasma is the liquid component of blood and is maintained at a relatively stable volume through several regulatory mechanisms, such as hormonal control and the action of the kidneys. The body closely regulates plasma volume through processes like the balance of electrolytes, osmotic pressure, and water intake and loss, which helps maintain homeostasis. In contrast, interstitial fluid and intracellular fluid volumes can fluctuate more significantly due to changes in cellular metabolism, hydration status, and movement of water between compartments based on osmotic gradients. The interstitial fluid, for example, can vary as fluid shifts in response to tissue hydration needs or from conditions such as edema. Similarly, the intracellular fluid volume can change in response to cellular activity, such as during stress or metabolic changes, making it less stable than plasma volume. Thus, the plasma compartment is recognized for its more consistent volume, primarily due to the body's homeostatic mechanisms working to keep it stable. This understanding of fluid compartments is essential in anatomy and physiology, particularly when studying systems impacted by fluid balance and distribution.