What is the approximate diameter of capillaries?

The diameter of capillaries is approximately 4 microns, making the first choice the most accurate. Capillaries are the smallest blood vessels in the body, and their narrow diameter is essential for their function. The small diameter allows red blood cells to pass through in single file, facilitating efficient gas exchange. Their walls, composed of a single layer of endothelial cells, enable nutrients, gases, and waste products to quickly diffuse between the blood and surrounding tissues.

The other choices reflect dimensions that are larger than that of capillaries and do not accurately represent their structure. Understanding the proper diameter of capillaries is crucial for appreciating their role in the circulatory system and in maintaining homeostasis by regulating the movement of substances at the cellular level.