What is the function of capillaries?

• A. To allow transfer of gases, nutrients, fluids, electrolytes, and wastes
• B. To transport hormones
• C. To produce red blood cells
• D. To join arteries and veins directly

Answer: (A)

Capillaries are the site of exchange between blood and body tissues. Their walls are extremely thin—just one endothelial cell layer—so substances can move readily between the blood and surrounding cells. This is how oxygen and carbon dioxide, nutrients like glucose and amino acids, electrolytes, and fluids are transferred, and how wastes are removed from tissues for elimination. That direct exchange is why the best description of capillary function is the transfer of gases, nutrients, fluids, electrolytes, and wastes. Hormones travel through the bloodstream, but the capillary walls’ main role is to enable this tissue-level exchange. Production of red blood cells occurs in bone marrow, and capillaries connect arterioles to venules rather than joining arteries and veins directly.