Structure of Blood Vessels:
Which of the following mechanisms help blood fight gravity so that, for instance, it can return from blood vessels in your feet back into your heart? (there may be more than one answer)
Select one or more:
a. breathing allows the pressure in your chest to decrease and allow blood to flow up towards your heart
b. as you walk, the skeletal muscles in your legs contract and squeeze the veins; since veins have valves, blood flows up and doesn't backflow back down
c. the force of blood flowing out of the heart and into the arteries pushes all of the blood along, causing blood to return to the heart
d. precapillary sphincters open to allow blood to flow back towards the heart
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