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A large muscle that pumps blood through your body as long as you're alive.

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What's the most active muscle in the body?

The human heart. An absolutely remarkable organ. Obviously, its main function is to pump blood throughout the body. And it does this extremely well. On average, this muscular organ will beat about 100,000 times in one day and about 35 million times in a year. During an average lifetime, the human heart will beat more than 2.5 billion times.

The Circulatory System

The circulatory system can be compared to a system of interconnected, one-way roads that range from superhighways to back alleys. Like a network of roads, the job of the circulatory system is to allow the transport of materials from one place to another. As described in Figure below, the materials carried by the circulatory system include hormones, oxygen, cellular wastes, and nutrients from digested food. Transport of all these materials is necessary to maintain homeostasis of the body. The main components of the circulatory system are the heart, blood vessels, and blood.

Circulatory system relative to body

The function of the circulatory system is to move materials around the body.

The Heart

The heart is a muscular organ in the chest. It consists mainly of cardiac muscle tissue and pumps blood through blood vessels by repeated, rhythmic contractions. The heart has four chambers, as shown in Figure below: two upper atria (singular, atrium) and two lower ventricles. Valves between chambers keep blood flowing through the heart in just one direction.

The chambers of the heart and the valves between them are shown here.

Blood Flow Through the Heart

Blood flows through the heart in two separate loops, which are indicated by the arrows in Figure above. 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 the blood to the lungs. In the lungs, carbon dioxide is released and oxygen obtained by the blood. The left side of the heart carries the oxygen-rich blood back from the lungs and pumps it to the rest of the body. The blood delivers oxygen to the body's cells, returning the oxygen-poor blood back to the heart.

You can watch an animation of the heart pumping blood at this link: http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_pumping.html.

  1. Blood from the body enters the right atrium of the heart. The right atrium pumps the blood to the right ventricle, which pumps it to the lungs. 
  2. Blood from the lungs enters the left atrium of the heart. The left atrium pumps the blood to the left ventricle, which pumps it to the body.


Unlike skeletal muscle, cardiac muscle contracts without stimulation by the nervous system. Instead, specialized cardiac muscle cells send out electrical impulses that stimulate the contractions. As a result, the atria and ventricles normally contract with just the right timing to keep blood pumping efficiently through the heart. You can watch an animation to see how this happens at this link: http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_electrical.html.


  • The heart contracts rhythmically to pump blood to the lungs and the rest of the body.
  • Specialized cardiac muscle cells trigger the contractions.

Explore More

Use these resources to answer the questions that follow.

Explore More I

  1. How does blood travel through the heart?
  2. List the three main types of blood vessels.
  3. What is the difference between the right side and left side of the heart?
  4. Describe the general structure of the heart.
  5. Why does the heart need its own arteries?

Explore More II

  1. Where is your heart located?
  2. What is the pericardium?
  3. Describe the left ventricle.
  4. What is the role of the pulmonary valve?
  5. What does the heart have a natural pacemaker?


  1. What are the main components of the circulatory system?
  2. Describe how blood flows through the heart.
  3. What controls heartbeat?

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(plural, atria): upper chamber of the heart


Fluid connective tissue that circulates throughout the body through blood vessels.

blood vessel

Vessel that transports blood; includes the arteries, veins, and capillaries.

circulatory system

Organ system consisting of the heart, blood vessels, and blood that transports materials around the body.


Muscular organ in the chest that that pumps blood through blood vessels when it contracts.


Lower chambers of the heart

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