The center of the circulatory, or cardiovascular system, is the heart, a powerful pump organ designed to beat many millions of times over the lifetime of an organism. The heart circulates blood throughout the veins and arteries. Arteries carry oxygen-rich blood away from the heart, providing it to tissues, then returning the depleted red bloods cells back to the heart through the veins for reoxygenation.
All the body's stationary cells are surrounded by interstitial fluid, also known as extracellular fluid, which is designed to draw oxygen and nutrients from red blood cells passing by. Red blood cells float in a medium called plasma which is similar to interstitial fluid, and makes up most of the volume of the blood, the primary fluid of the circulatory system.
The largest artery in the human body is the aorta, running through the neck and immediately proximate to the heart. The heart oxygenates red blood cells in its ventricles, or compartments, regulated by valves. The lungs receive fresh oxygen from the air outside, then relaying it to the heart. Complex multicellular organisms such as human beings need air with a fair amount of oxygen (15-25%) in it to survive. Plants, and many microbes, can survive in oxygen-free environments - unlike animals, they require carbon dioxide for respiration.
If the operation of the heart is interrupted, the organism is likely to quickly die, after brain damage begins to set in. This sometimes happens during major heart attacks. By using artificial heart stimulation systems, modern medicine is able to keep the circulatory systems of such victims alive long enough for surgery.
Anthropods and molluscs lack typical circulatory systems - in their bodies, there is no distinction between blood and the interstitial fluid - a material taking both properties simply bathes the organs in the necessary oxygen.