The venae cavae are two major veins found in all vertebrates that breathe air. Like all veins, the function of the vena cava is to transfer blood that has been deoxygenated from the body back into the heart. These veins are essential components of the circulatory system, and each one is responsible for returning the blood from half of the body. Blood from the upper half travels through the superior vena cava, while blood from the lower half runs through the inferior vena cava.
Other major veins feed into each vena cava, and reveal which portions of the body they are responsible for. The function of the vena cava can be seen from their tributary veins. The superior vena cava, located just above the heart, is formed from the junction of the left and right brachiocephalic veins. These veins return blood from the head, neck, and arms, as well as the upper spine and chest. Another vein, the azygos, collects blood from the chest wall and lungs, and empties into the superior vena cava, just above the heart.
The function of the vena cava that collects blood from the lower body determines its different structure. The inferior vena cava begins near the small of the back, where the iliac veins join. The iliac veins return blood which has been deoxygenated back from the legs. Many smaller tributaries feed into it as it runs near the backbone, crosses the diaphragm, and connects to the heart. These tributaries feed blood from the genitals, abdomen, kidneys, and liver.
Ultimately, the vena cava's function is to ensure the proper operation of the circulatory system. By returning blood that has been depleted of its oxygen to the heart's right atrium, the heart can then pump this blood to the lungs. In the lungs, the blood receives oxygen, which is vital for survival, and returns it to the heart. The heart can then pump the oxygenated blood throughout the body. These important veins help to return this blood for re-use after the body has utilized it.
To assist in the function of the vena cava, contractions from the heart time the delivery of blood and supply pressure. There are no valves that separate the venae cavae from the right atrium. Instead, contractions of the heart are relayed through other veins and muscles. These contractions provide pressure necessary to push deoxygenated blood to the heart. This process is crucial to ensuring continuous blood flow back to the heart.