Read the following scenario to answer the following questions. Blood flowing away from the heart is under tremendous pressure, able to travel to just about any portion of the body. However, blood pressure drops considerably as it moves away from the heart. When blood finally reaches capillaries, the blood pressure drops to nearly zero. Therefore, when a person is standing, blood returning from the feet is not moved back towards the heart by contractions of the heart. Blood is propelled back to the heart by .

Blood is propelled back to the heart by:

a. Negative diastolic pressure

b. skeletal muscle pump

Explanation:

Unlike blood flow from the heart which is primarily driven by he contraction of the heart, blood flow from the veins back into the heart occurs by the following mechanism.

1. Negative diastolic pressure: for blood to flow back into the heart from the extremities such as the feet, the pressure in the veins must be greater that the pressure in the atrial compartment of the heart, and this is achieved by the diastolic (relaxation) phase of the heartbeat cycle. This relaxation reduces the pressure in the atrial chamber significantly hence causing a 'suction' effect dragging blood back into the heart.

2. Skeletal muscle pump: pressure in veins are increased by the contraction of their surrounding skeletal muscles. This helps to counter-act the force of gravity, increasing pressure hence moving blood upwards back into the heart. This is also in tandem with one way valves in the vessels that prevent back-flow of blood in the opposite direction to the skeletal muscle contraction.

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