Why are capillary walls only one cell thick?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

Capillary walls are only one cell thick to facilitate the efficient exchange of substances between the blood and surrounding tissues. This extremely thin structure acts as a semipermeable membrane, allowing for the rapid diffusion of essential molecules—such as oxygen, nutrients, water, and waste products—driven by differences in concentration and pressure.12

Because these walls consist of a single layer of endothelial cells surrounded by a thin basement membrane, they provide a minimal barrier for the movement of gases and other substances. This anatomical design is critical for maintaining the metabolic needs of the body's organs and tissues, as it enables the necessary materials to pass directly through the vessel wall or through minute pores.12