What best describes the structure of capillaries?

The structure of capillaries is best described by being one cell thick. This unique feature is crucial because it allows for efficient exchange of gases, nutrients, and waste products between the blood and surrounding tissues. The thinness of the capillary walls facilitates diffusion, enabling oxygen and carbon dioxide to move freely across the membrane. Additionally, a single layer of endothelial cells in capillaries minimizes the distance that substances must travel, enhancing their ability to supply tissues and remove waste effectively.

In contrast, thick elastic walls and muscular walls are characteristic of arteries and veins, which need to withstand higher pressures and regulate blood flow, respectively. A large lumen is also more typical of veins, which accommodate a larger volume of blood, rather than capillaries, where the focus is on exchange rather than volume. Thus, the one-cell-thick structure is what makes capillaries distinct and specialized for their function in the circulatory system.