For this discussion post I will be talking about arteries versus veins. The main function of the arteries is to carry blood away from the heart. This can be further broken down into two categories that include systemic arteries and pulmonary arteries. Systemic arteries carry oxygen-rich blood from the left ventricle to the body tissues. Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs. Blood is pumped from the ventricles into large elastic arteries that continue to branch into smaller and smaller arteries until the branching results in arteries called arterioles. The arterioles play a key role in regulating blood flow into the tissue capillaries where exchange of nutrients and waste occur. The main function of the veins is to carry blood from the body back to the heart. This again can be broken down into two categories, systemic and pulmonary. In the pulmonary circuit, the pulmonary veins transport blood from the lungs to the left atrium of the heart. This blood has a high oxygen content because it has just been oxygenated in the lungs. Systemic veins transport blood from the body tissue to the right atrium of the heart. This blood has a reduced oxygen content because the oxygen has been used for metabolic activities in the tissue cells. The walls of veins are thinner than those of arteries, which relates to the fact that blood in the veins has less pressure than in the arteries. Because the walls of the veins are thinner than arteries, veins can hold more blood and almost 70 percent of the total blood volume is in the veins at any given time