Valves are present in veins but not in arteries because
Correct answer: B. blood in veins is at low pressure and would otherwise flow backwards
- A. veins carry deoxygenated blood
- B. blood in veins is at low pressure and would otherwise flow backwards
- C. arteries have thinner walls
- D. veins lie closer to the skin
Explanation
By the time blood reaches the veins the pressure created by the heart has largely been lost in the capillaries, so flow depends on the squeezing of skeletal muscles, and the pocket valves ensure that this squeezing pushes blood only towards the heart. Arteries need no valves because the pressure from ventricular systole is high enough to keep blood moving in one direction. Failure of these valves produces varicose veins.
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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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