Which of the following blood vessels have lowest blood velocity with correct reason for low blood velocity?
- A. Arteries: Due to thick walls
- B. Capillaries: due to highest overall cross-sectional area
- C. Veins: due to blood flow against the gravity
- D. Capillaries: Due to two-way blood flow in capillaries
Explanation
Although each capillary is tiny, there are so many in parallel that their combined cross sectional area is far greater than that of the aorta, and since the same volume must pass every second, the velocity falls to about a millimetre per second. That slow flow is exactly what exchange requires, giving time for diffusion. The fourth option names the right vessel but an invented reason, since capillary flow is one way.