circulatory system

Our circulatory system can be thought of as a network of living pipes that allows our body to transport oxygen as well as nutrients to the tissues. Chemical wastes produced by our cells enter the blood to be filtered and eliminated by our body (largely by the kidney and liver). Our body circulates about 5 liters (5.3 quarts) of blood; at rest, the heart moves it at a rate of 5 liters per minute. In summary, even without activity, blood can run the entire circuit of the body in one minute. During heavy exercise, the whole loop can be completed in ten seconds.
The main components of the circulation are pictured on the left:

1. Arteries - the red vessels represent arteries. Hemoglobin, the oxygen carrying chemical in red blood cells, takes on blood's distinctive red color when it is combined with oxygen. Arteries carry blood away from the heart and branch into smaller vessels (first arterioles, then capillaries) in order to facilitate delivery of its cargo to the organs and other tissues.
2. Veins - the blue vessels represent veins. This bluish tint is also due to hemoglobin. After releasing oxygen to living cells, hemoglobin undergoes a chemical change that gives it a distinctive reddish-blue tone. Veins also have a series of one-way valves built into their structure. This is crucial for proper circulation, as gravity could cause blood to pool below the heart otherwise!