Humans and many other animals depend on blood flowing through their bodies to keep them alive. The blood travels through a system that includes the heart and a network of blood vessels. This is called a cardiovascular system.

The human heart is a pear-shaped organ about the size of a fist. It is made up of a special type of muscle called cardiac muscle, which is not found anywhere else in the body. The heart acts as a pump to push the blood throughout the body.

The heart is separated into four chambers, or parts. The upper chambers are called atria, and the lower chambers are called ventricles. A valve, or a flap that can open and shut, connects each atrium to the ventricle below it. The valves control the movement of blood through the heart.

The blood vessels are a system of tubes that carry the blood throughout the body. The main vessels are arteries, veins, and capillaries. Arteries carry blood out from the heart. Veins return blood to the heart. Arteries are the thickest of all blood vessels. They expand and contract to keep blood moving away from the heart. Veins contain valves that prevent blood from flowing backward. Capillaries are tiny passages that connect the arteries and the veins to the body’s tissues.

Blood flows through the vessels in two very specific paths. In the first path, called pulmonary circulation, blood travels from the heart to the lungs. In the lungs carbon dioxide and other waste gases leave the blood. The blood takes in oxygen and then returns to the heart.

In the second path, called systemic circulation, the blood travels away from the heart throughout the body. The arteries carrying the blood branch out into smaller vessels called arterioles and finally into capillaries. In the capillaries the blood transfers oxygen and nutrients to cells in the body’s tissues. The blood in the capillaries also collects waste products from the cells. From the capillaries the blood flows into small vessels called venules. These unite to form veins. The blood flows through the veins back to the heart.

In addition to the lungs, many other organs in the body interact with the cardiovascular system. Blood carries away nutrients from the intestines, which are part of the digestive system. The spleen and the liver filter, or clean, the blood by removing old blood cells and bacteria. The kidneys take water, minerals, and other waste products from the blood.

When the force of the blood against the walls of the blood vessels is too high, a person has hypertension, or high blood pressure. Hypertension can weaken the heart and damage other organs.

Sometimes fatty deposits thicken and harden the walls of the arteries. When this happens a person has arteriosclerosis. Arteriosclerosis slows the flow of blood, which can lead to a heart attack.

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