Circulatory System
The blood the the system of transportation by which oxygen and nutrients reach the body's cells, and waste materials are carried away. Blood also carried hormones, which control body processes, and antibodies to fight invading germs. The heart is the pump that keeps this transport system moving. The upper atrium receives blood coming in from the veins. This blood flows through a one-way valve, into the larger, lower chamber called the ventricle. It has thick strong walls that contract to squeeze blood through another valve, out into the arteries.

The blood circulation has two parts to it; the heart acts as a double pump.  Blood from the right side of the body is dark red (blue in pictures) and is low in oxygen or is deoxygenated.  This blood travels along the pulmonary arteries to the lungs where it receives oxygen and becomes bright red.  Then it flows along the pulmonary veins back to the heart�s left side pump.

Blood travels from the left side of the heart through arteries, which eventually divide into capillaries.  In the capillaries, food and oxygen are released to the body cells, and carbon dioxide and other waste products are returned to the bloodstream. The blood then travels in veins back to the right side of the heart, and the whole process begins again.
For an awesome link on circulatory system diseases, click here.
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