The circulatory system can be viewed as two circuits - the pulmonary circuit and systemic circuit.
Pulmonary circulation involves the transport of blood to and from the lungs. Deoxygenated blood returns to the right side of the heart. This blood is then pumped by the right ventricle into the pulmonary artery (deoxygenated blood) and into the capillaries of the lungs. In the capillaries of the alveoli, oxygen enters the blood and binds to hemoglobin in red blood cells as carbon dioxide diffuses out of the blood into the lungs for removal. The oxygenated blood then travels in the pulmonary vein back to the left atrium of the heart where it joins the systemic circuit.
Systemic circulation involves the transport of blood to and from all the tissues of the body. This circuit is much larger than the pulmonary circuit and so the walls of the left ventricle of the heart are much larger than on the right side. This thicker muscle generates the force required to pump blood all around the systemic circuit. Oxygenated blood is pumped by the left ventricle into the aorta. The aorta then branches into many smaller arteries that carry blood to all areas of the body. In the capillaries, oxygen is delivered to cells and carbon dioxide is picked up for removal. The deoxygenated blood then returns to the anterior vena cava from the upper body and the posterior vena cava from the lower body. Both vessels enter the right atrium of the heart and blood is returned back to the pulmonary circuit for oxygenation.
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