Capillaries are primarily responsible for which of the following?

• A. Storage of blood
• B. Filtration of plasma into tissue fluid
• C. Exchange of nutrients and gases
• D. Formation of clots

Answer: (C)

Capillaries are specialized for exchange between blood and tissues. Their walls are only one cell thick, which makes the diffusion distance very short. This thin barrier, along with a huge overall surface area and the slow flow of blood through the network, allows nutrients and gases to move readily between the blood and surrounding tissue fluid. Oxygen and nutrients diffuse from arterial blood into cells, while carbon dioxide and other wastes diffuse from cells back into the blood for removal. This exchange is the primary purpose of the capillary bed across most tissues. They aren’t designed for storing blood, and they’re not the main site of clot formation, though clots can form if injury triggers coagulation in a vessel.