cross the cell membranes. Meanwhile, carbondioxide and other wastes leave the cell, diffuse through the interstitial fluid, cross thecapillary walls, and enter the blood. In this way, the blood delivers nutrients and removeswastes without leaving the capillary tube.After delivering oxygen to tissues and absorbing wastes, the deoxygenated bloodin the capillaries then starts the return trip to the heart. The capillaries merge to form tinyveins, called venules. These veins in turn join together to form progressively larger veins.Ultimately, the veins converge into two large veins: the inferior vena cava, bringing bloodfrom the lower half of the body; and the superior vena cava, bringing blood from the upperhalf. Both of these two large veins join at the right atrium of the heart.Because the pressure is dissipated in the arterioles and capillaries, blood in veinsflows back to the heart at very low pressure, often running uphill when a person isstanding. Flow against gravity is made possible by the one-way valves, located severalcentimeters apart, in the veins. When surrounding muscles contract, for example in the calfor arm, the muscles squeeze blood back toward the heart. If the one-way valves workproperly, blood travels only toward the heart and cannot lapse backward. The precapillarysphincter controls whether blood enters a capillary....
