Blood collected from local capillary networks returns to the heart through progressively larger veins that ultimately drain into the two largest veins, the venae cavae (singular, vena cava). The venae cavae drain blood from the head and body into the heart. There is little pressure available to push the blood forward in the veins because pressure has been lost to the resistance of the arterioles and capillaries. Consequently, veins are thin walled and have little smooth muscle or elastic connective tissue. Because of the low pressure, blood tends to accumulate within the veins: As much as 80% of your total blood volume resides in the venous side of your circulation at any one time. When you stand or sit for a long time, blood may pool within the veins of your limbs.

Veins located in the limbs and in the body below the heart have one-way valves that help prevent blood from pooling owing to gravity (see Fig. 39.17a). However, the most important mechanism of returning blood to the heart is the voluntary muscle contractions that occur during walking and exercise, which exert pressure on the veins. Above the level of the heart, the return of blood to the heart through the veins is assisted by gravity.