AP BIOLOGY

Detailed explanation-1: -In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the skeletal muscle pump (Figure 6), helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart.

Detailed explanation-2: -Most veins carry deoxygenated blood from bodily tissue back to the heart, where it receives fresh oxygen. Veins must work against gravity to return blood the heart. The muscles of the leg help that process, and valves within the veins usually keep the moving blood from flowing back down toward the feet.

Detailed explanation-3: -Every time we stand up, gravity pulls blood into parts of the body that are below the heart. A simple experiment you can do to prove this is to let your hand hang down for a short time then examine the veins in the back of your hand. These veins will get bigger because they are full of blood.

Detailed explanation-4: -Blood primarily moves in the veins by the rhythmic movement of smooth muscle in the vessel wall and by the action of the skeletal muscle as the body moves. Because most veins must move blood against the pull of gravity, blood is prevented from flowing backward in the veins by one-way valves.

Detailed explanation-5: -The arteries (red) carry oxygen and nutrients away from your heart, to your body’s tissues. The veins (blue) take oxygen-poor blood back to the heart. Arteries begin with the aorta, the large artery leaving the heart. They carry oxygen-rich blood away from the heart to all of the body’s tissues.