Circulatory System

The circulatory system is made up of the heart, blood and blood vessels known as arteries, capillaries and veins. The heart pumps blood throughout your body through the blood vessels. Blood delivers oxygen and nutrients to cells and carries away carbon dioxide and other waste materials.

© Microsoft Corporation. All Rights Reserved.

Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.


Heart and Circulation

Did you ever send a valentine with the shape of a heart on it? Did you ever hear someone say, “That came straight from my heart?” People talk about hearts a lot. People have always known that hearts are very important.

You have a heart. Your heart does not look like a valentine heart. Your heart is a pump. When you run very fast, your heart pumps hard and fast. You can feel your heart pumping, or beating.


Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.

Your heart looks like an upside-down pear. It is about the size of your closed fist. It is almost in the middle of your chest. It is just off to the left side.

Your heart is made of muscle. It is divided into four parts called chambers. The chambers are hollow inside. The two chambers on top are called atria. The chambers on the bottom are called ventricles. Your heart also has four valves that let blood in and out of the chambers.

Tubes called arteries come out of your heart. Tubes called veins go into your heart. Arteries and veins are also called blood vessels.


Your heart pumps blood. Blood comes into the atria or top chambers of your heart. Your ventricles, or bottom chambers, pump blood out to every part of your body.

Blood going out of your heart carries food and oxygen. Every part of your body needs food and oxygen for energy. You need energy for your body to work and for you to stay alive. Your heart pumps blood carrying food and oxygen through your arteries. Big arteries carry the blood to your legs and arms. The arteries get smaller and smaller the farther out they go. Little blood vessels called capillaries take blood to your cells. Everything in your body is made of tiny cells.

Your cells give off waste products when they make energy from food and oxygen. One of these waste products is a gas called carbon dioxide. The blood in your capillaries picks up the waste products. Capillaries connect to bigger veins. The pumping of your heart pushes the blood through your veins.



Your veins carry blood back to your heart. The chambers on the right side of your heart take care of blood coming back through your veins. First, the blood comes into your right atrium, the top chamber. Your right atrium pumps the blood into your right ventricle, the bottom chamber. Your right ventricle pumps the blood through an artery into your lungs.


Your blood has to get rid of carbon dioxide. It has to get a fresh supply of oxygen. Your lungs take care of both jobs. Carbon dioxide from your blood goes into your lungs. Your lungs get rid of the carbon dioxide when you breathe out.

Then you breathe in. Your lungs get oxygen from breathing in air. Your lungs fill up with oxygen. Your blood picks up a new supply of oxygen from your lungs. Now your blood is ready to go out through your arteries to all the parts of your body.

The chambers on the left side of your heart take care of blood going out through your arteries. Special veins send blood from your lungs to your left atrium, or top chamber. The blood goes from the left atrium to the left ventricle. The left ventricle pumps the blood out through your arteries to every part of your body.



Make a fist. Open your fist slightly, and then squeeze it closed. Open and close your fist again and again. This is sort of how your heart pumps blood. The muscles in your heart squeeze the chambers.

To open and close your fist, you have to think about doing it. You don’t have to think about squeezing your heart muscles. Your brain tells your heart to pump over and over again. Your heart pumps when you are awake. Your heart pumps when you are asleep. Your heart pumps faster when you run fast. Your body needs more oxygen when you run.

Your heart is better than any pump made. It beats over and over again, day and night. The heart of a 76-year-old person has beaten nearly 2.8 billion times. It has pumped about 179 million quarts (169 million liters) of blood. No one can live if their heart stops beating for more than a few minutes.


Artery, one of the tubular vessels that conveys blood from the heart to the tissues of the body. Two arteries have direct connection with the heart: (1) the aorta, which, with its branches, conveys oxygenated blood from the left ventricle to every part of the body; and (2) the pulmonary artery, which conveys blood from the right ventricle to the lungs, whence it is returned bearing oxygen to the left side of the heart (see Heart: Structure and Function). Arteries in their ultimate minute branchings are connected with the veins by capillaries. They are named usually from the part of the body where they are found, as the brachial (arm) or the metacarpal (wrist) artery; or from the organ which they supply, as the hepatic (liver) or the ovarian artery. The facial artery is the branch of the external carotid artery that passes up over the lower jaw and supplies the superficial portion of the face; the hemorrhoidal arteries are three vessels that supply the lower end of the rectum; the intercostal arteries are the arteries that supply the space between the ribs; the lingual artery is the branch of the external carotid artery that supplies the tongue. The arteries expand and then constrict with each beat of the heart, a rhythmic movement that may be felt as the pulse.






Veins are blood vessels that return blood to the heart from other parts of the body. This false-color electron micrograph shows red blood cells packed into a capillary, the smallest type of blood vessel. Blood flows from the capillaries into veins after oxygen has been exchanged.




Capillary, one of the minute blood vessels that form the connection between the arteries and the veins. These tiny vessels vary in diameter from 0.0127 to about 0.2032 mm (0.0005 to about 0.008 in) and are present in great numbers throughout the entire body. The walls of capillaries are exceedingly thin and readily permeable. They are surrounded by lymph, and there is a constant interchange between the substances in the blood within the capillaries and the waste products in the body tissues and lymph outside. This interchange facilitates the processes of nutrition and elimination and enables the exchange of oxygen and carbon dioxide to take place. Lymph capillaries assist the blood capillaries in this process.