Veins are one of three types of blood vessels found in the body and the venae cavae are the largest veins in the body. They are responsible for bringing blood to the heart and are a very important part of the cardiovascular system. There are two of these veins, the superior and inferior vena cava, and they supply blood directly to the heart.
Like the other blood vessels in the body, veins are hollow tubes through which blood flows back to the heart. Veins are actually made of similar materials to arteries. They have three layers which are an inner layer of fat cells, an elastic middle layer of smooth muscle tissue, and an outer layer of connective tissue. However, contrary to arteries, the layers of veins are thinner.
Blood flows into the veins from the capillaries and the oxygen on the hemoglobin of blood cells has already been exchanged with carbon dioxide. Therefore the blood in the veins is de-oxygenated or oxygen poor. The blood is also at a much lower pressure than arteries and the veins don’t need to be as thick or strong. The veins are also not as stiff as the arteries and when blood is not in them, they will collapse. All of the veins in the body travel on a parallel course to the arteries and they all will eventually lead to the venae cavae.
Superior vena cava
The superior vena cava (SVC) is a very wide vein but is also very short. It begins at the aortic arch and runs to the right atrium of the heart where it deposits de-oxygenated blood. The brachiocephalic veins run right into the SVC and the azygos vein also joins it right before it ends at the right atrium. There is also not a valve between the venae cavae and the right atrium.
Inferior vena cava
The inferior vena cava (IVC) is much longer than SVC and has many more tributaries. The vein runs parallel to the abdominal aorta all the way to the heart and also carries de-oxygenated blood. It really forms in the lower abdomen from the joining of the common lilac veins but also has the following veins flowing into it before it reaches the right atrium: renal vein, the testicular vein, hepatic vein, lumbar vein, and the suprarenal vein.
Unlike arteries, some veins actually have valves in them. This feature is extremely important in many veins and especially in those that supply the inferior vena cava. The reason for these valves actually has to do with gravity. The superior vena cava is helped by gravity as the blood will flow downwards to the right atrium; however this is not the same for the inferior vena cava. Blood going to the inferior vena cava must travel upward the legs against the force of gravity. To make sure that the blood will reach the heart, there are vein valves in the legs that prevent blood from flowing back down the legs. This ensures that the blood will get to the inferior vena cava despite gravity.