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Describes the structure and function, and blood flow of the heart.

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Where is your heart?

Place your hand on your heart. Did you put your hand on the left side of your chest? Most people do, but the heart is actually located closer to the center of the chest.

The Heart

What does the heart look like? How does it pump blood? The heart is divided into four chambers (Figure below), or spaces: the left and right atria, and the left and right ventricles. An atrium (singular for atria) is one of the two small, thin-walled chambers on the top of the heart where the blood first enters. A ventricle is one of the two muscular V-shaped chambers that pump blood out of the heart. You can remember they are called ventricles because they are shaped like a "V."

The atria receive blood and the ventricles pump blood out of the heart

The atria receive blood and the ventricles pump blood out of the heart.

The atria receive the blood, and the ventricles pump the blood out of the heart. Each of the four chambers of the heart has a specific job.

  • The right atrium receives oxygen-poor blood from the body.
  • The right ventricle pumps oxygen-poor blood toward the lungs, where it receives oxygen.
  • The left atrium receives oxygen-rich blood from the lungs.
  • The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body.

Blood Flow Through the Heart

Blood flows through the heart in two separate loops. You can think of them as a “left side loop” and a “right side loop.” The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it releases carbon dioxide and picks up oxygen. (Recall that carbon dioxide is a waste product that must be removed. It is removed when we exhale.) The left side carries the oxygen-rich blood back from the lungs into the left side of the heart, which then pumps the oxygen-rich blood to the rest of the body. The blood delivers oxygen to the cells of the body, where it is needed for cellular respiration, and returns to the heart oxygen-poor.

To move blood through the heart, the cardiac muscle needs to contract in an organized way. Blood first enters the atria (Figure below). When the atria contract, blood is pushed into the ventricles. After the ventricles fill with blood, they contract, and blood is pushed out of the heart. The heart is mainly composed of cardiac muscle. These muscle cells contract in unison, causing the heart itself to contract and generating enough force to push the blood out.

So how is the blood kept from flowing back on itself? Valves (Figure below) in the heart keep the blood flowing in one direction. The valves do this by opening and closing in one direction only. Blood only moves forward through the heart. The valves stop the blood from flowing backward. There are four valves of the heart.

  • The two atrioventricular (AV) valves stop blood from moving from the ventricles to the atria.
  • The two semilunar (SL) valves are found in the arteries leaving the heart, and they prevent blood from flowing back from the arteries into the ventricles.

Why does a heart beat? The “lub-dub” sound of the heartbeat is caused by the closing of the AV valves ("lub") and SL valves ("dub") after blood has passed through them.

Blood flows from atria to ventricles in the heart

Blood flows in only one direction in the heart. Blood enters the atria, which contract and push blood into the ventricles. The atria relax and the ventricles fill with blood. Finally, the ventricles contract and push blood around the body.


  • Blood enters the heart at the atria and then flows into the ventricles, which contract and push blood around the body.
  • Valves in the heart keep the blood flowing in one direction.

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Use the resource below to answer the questions that follow.

  1. How many chambers does a mammalian heart have? What are these chambers called?
  2. What are the smallest blood vessels in the body?
  3. What is the function of the circulatory system? What role does the heart play?
  4. What passes from the cells into the capillaries? What passes into the cells from the capillaries?


  1. What are the ventricles?
  2. Where does oxygen-poor blood first enter the heart?
  3. What part of the heart pumps blood to the rest of the body?
  4. What is the purpose of the valves in the heart?

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atrium Small, thin-walled chamber at the top of the heart where the blood first enters.
valve Structure in the heart that prevents back-flow and keeps the blood flowing in one direction.
ventricle Muscular V-shaped chamber that pumps blood out of the heart.

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