Exchange of materials between blood and tissues occurs only in capillaries because their walls are
Correct answer: A. one cell thick, formed of a single layer of endothelium
- A. one cell thick, formed of a single layer of endothelium
- B. made of smooth muscle that contracts rhythmically
- C. lined with valves at close intervals
- D. thick and elastic to withstand pressure
Explanation
A capillary wall is a single layer of flattened endothelial cells, giving a diffusion distance of well under a micrometre, and the enormous total surface area and slow flow complete the design. Arteries and veins have thick walls of muscle and elastic tissue that make exchange impossible, so they are transport vessels only. The narrowness of a capillary also forces red cells through in single file.
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About Blood Vessels
Blood vessels include arteries, arterioles, capillaries, venules and veins, whose wall thickness, elasticity, lumen size and valves suit their roles in circulation. Questions compare pressure and blood flow in each vessel and distinguish pulmonary vessels, which carry blood to and from the lungs, from systemic vessels.
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