What happens to blood after it leaves the heart?

Deoxygenated blood leaves the right side of heart via the Pulmonary Artery (the only artery in the body to carry oxygenated blood) and enters the capillaries surrounding the alveoli of the lungs, here it becomes Oxygenated by diffusion. The newly Oxygenated blood then re-enters the heart on the left side via the Pulmonary vein (the only vein in the body to carry Oxygenated blood) and is pumped out the Aorta to the bodies cells and organs. Capillaries surrounding these exchange materials, oxygen into the cell and Carbon Dioxide out. The blood is now classed as "deoxygenated" and contains a significant amount more Carbon Dioxide than the oxygenated blood. This blood is then returned to the right side of the heart via the Vena Cava ready to start the cycle again. The fact that the blood is pumped heart to lungs, heart to body makes it very efficient. This is known as a Double Circulatory System.