Your cardiovascular system is made up of the heart, blood vessels and blood. It is a system that never rests. It pumps 5 to 6 liters of blood per minute through your body and can pump as many as 30 liters per minute during times of extreme stress. In humans, the cardiovascular system is a closed system, meaning that the blood never leaves the blood vessels. This simple-seeming closed system, however, carries out a multitude of complex tasks. It transports nutrients, oxygen and hormones where they need to go. It protects your body from infections, toxins and blood loss. It even helps regulate body temperature, fluid balance and pH.
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The heart is a muscular organ in your chest, which controls the movement of blood in your cardiovascular system. Acting as a pump, it rhythmically moves all the blood in your body where it needs to go, through networks of blood vessels that branch out to and from all parts of your body. The heart is divided into two sides: right and left. The blood leaving the right side of your heart creates a circuit to and from the lungs, known as the pulmonary circulation. The blood leaving the left side of your heart creates a circuit to and from the rest of your body, known as the systemic circulation.
Pulmonary circulation is a route your blood takes from your heart to the lungs -- to collect more oxygen -- and back again. Oxygen-poor blood leaves the right side of the heart and travels through the pulmonary arteries toward your lungs. When the blood reaches the lungs, it releases carbon dioxide, a waste product, and picks up more oxygen. The newly oxygen-rich blood can now travel back to the left side of the heart, via the pulmonary veins, to complete the circuit. From there, the blood is ready to go into systemic circulation.
In systemic circulation, the oxygen-rich blood leaves the left side of the heart and travels to all other parts of your body through the arteries. From there, it goes into smaller and smaller blood vessels and eventually enters tiny blood vessels called capillaries. At the level of capillaries, cells send their carbon dioxide and other wastes into the blood, and receive the blood's oxygen. From there, the oxygen-poor blood travels back to the heart via your veins. These veins empty into the right side of the heart, where the blood enters the pulmonary circulation again, finishing the circuit.
Blood has many components. The majority of your blood is plasma, a liquid that makes up more than half of the blood volume. It is critical to maintain blood pressure and regulate body temperature as it carries all the cells and nutrients throughout the body. There are three main types of cells in the body. The red blood cells carry oxygen and carbon dioxide to and from the cells. White blood cells play a role in fighting off infections. Platelets help promote clotting at sites of injury.