Kaplan Nursing Entrance Exam Practice 2026 – Comprehensive Prep

Red blood cells must pass through capillaries in a single file due to the very small diameter of these blood vessels. Capillaries are the smallest blood vessels in the circulatory system, and their narrow lumens are approximately the same size as a red blood cell. This structure allows for the efficient exchange of oxygen, carbon dioxide, nutrients, and waste products between the blood and surrounding tissues.

The single-file movement is particularly important because it maximizes the surface area exposure of red blood cells to the tissues, facilitating effective gas exchange. As red blood cells pass through the capillaries, they can closely interact with the endothelial cells lining the capillary walls, allowing for the release of oxygen and the uptake of carbon dioxide.

In contrast, arteries and veins have larger diameters to accommodate the flow of blood and do not require red blood cells to pass through in a single file. Lymph vessels serve different functions in the immune system and do not transport red blood cells, so they also do not concern the context of this question.