|Circulatory System, Source: LadyofHats, Wikimedia|
The arteries carry blood away from the heart. They start out large and muscular (the aorta) and branch out into medium muscular arteries and smaller elastic arterioles. The veins start at the capillaries, carrying blood to the heart. They are not muscular, allowing blood to flow based on pressure and skeletal muscle squeezing. They end as the vena cava, emptying into the heart.
Arteries carry oxygenated blood because blood flows between the heart and lungs, where hemoglobin binds oxygen. And veins carry deoxygenated blood, because the oxygen is transferred to the tissues in the capillaries (exchanged for carbon dioxide which dissolves in the blood and is released in the lungs out to the world). The exception to this is the pulmonary system.
The pulmonary circulation is the opposite of the general circulation. The pulmonary artery carries deoxygenated blood because the blood just returned to the heart via the veins, it goes to the lungs for oxygen. The pulmonary vein carries that newly oxygenated blood to the heart to be pumped through the arteries.
What happens when the red blood cells are not working properly? Read on tomorrow.