How the Heart Works

The heart is a complex muscular organ in the middle of the chest. A child's heart is about the size of his fist. A primary function of the heart is to pump oxygen-rich blood to vital organs and tissues.

How it works

  • Step 1: Blood comes to the right side of the heart through two large veins, the superior vena cava and the inferior vena cava. This is blood that has been used by the body. It contains very little oxygen and also contains waste in the form of carbon dioxide.
  • Step 2: The blood then passes through the tricuspid valve to the right ventricle. From here, it is pumped out through the pulmonary valve and pulmonary artery to the lungs. 
  • Step 3: In the lungs, the carbon dioxide is removed from the blood. It is exchanged for oxygen that has been breathed in through the nose and mouth.
  • Step 4: The oxygen-rich blood leaves the lungs and returns to the left atrium through the pulmonary veins. This process is called pulmonary circulation.
  • Step 5: Blood flows from the left atrium through the mitral valve into the left ventricle. The left ventricle is a thick, powerful muscle.
  • Step 6: The left ventricle pumps oxygen-rich blood through the aortic valve to the aorta and out through the body. This is the way the body and organs receive blood and oxygen.

The heart repeats this cycle of receiving and pumping blood to the body many times a minute. This is called the heart rate or pulse.

Just as a child's tissues need oxygen-rich blood for energy, his heart needs its own blood supply for energy. The coronary arteries are blood vessels that wrap around the heart and supply it with oxygenated blood.

View our heart transplant handbook for patients and families

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