The circulatory system before and after birth

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Learn about blood circulation in the body, lungs and heart, before and after birth.

Key points

  • Blood flows (circulates) in the body through the heart, the lungs, and the veins and arteries (the blood vessels) that make up the circulatory system.
  • The blood flow in the whole body is called systemic circulation; the lungs’ blood flow is the pulmonary circulation; the heart’s is the coronary circulation.
  • After birth, the heart pumps blood through the whole body and through the lung, where it gets rid of carbon dioxide and collects oxygen.
  • Before birth, because the unborn baby does not use their lungs, they have two "short cuts" or “shunts” that help bypass the lungs and then directly send blood to the baby’s body.

The circulatory system refers to how the body keeps blood moving through the body (circulating). The heart, the lungs, and the veins and arteries (blood vessels) make up the circulatory system. Working together, they ensure that blood flows through the whole network that makes up the human body (systemic circulation), as well as within the lungs (pulmonary circulation) and the heart (coronary circulation). Blood high in oxygen appears red and blood low in oxygen, which is rich in carbon dioxide, appears blue.

Veins and arteries

Veins and arteries are known as blood vessels. Arteries carry blood away from the heart and veins carry blood toward the heart.

The major blood vessels that branch out of the heart are the aorta, the vena cava, and the pulmonary arteries and pulmonary veins. As well as these blood vessels, there are many smaller veins and arteries that carry blood to and from different parts of the body. At the very smallest level, thin tubes called capillaries allow oxygen, carbon dioxide, and other chemicals to pass between the blood and the tissues.

The vena cava

Blood that has circulated through the body has to come back to the heart to get more oxygen. The superior vena cava brings back the blood from the head and the limbs in the upper part of the body. The inferior vena cava brings back blood from the lower part of the body.

The pulmonary arteries

The pulmonary arteries are the major vessels that carry blood from the heart to the lungs. The right branch of the main pulmonary artery takes blood to the right lung, the left branch to the left lung.

The pulmonary veins

The pulmonary veins return oxygenated blood from the lungs to the heart.

The aorta

The aorta is the major artery that carries oxygenated blood from the heart to the rest of the body.

How does the heart pump blood?

Normal heart anatomyNormal heart showing placement of the atria, ventricles, aorta, superior vena cava, inferior vena cava, pulmonary artery and pulmonary veins
The heart has two upper (receiving) and two lower (pumping) chambers. Blood flows into the upper chambers (the right atrium and the left atrium). The lower chambers (the right and left ventricles) pump blood out of the heart. The heart valves open and close to keep blood flowing in the correct direction.

After blood has travelled through the body, it comes back to the heart through the vena cava and enters the right atrium. The blood is bluish in colour because it is low in oxygen. When the heart relaxes, the blood flows through the tricuspid valve into the right ventricle.

The right ventricle contracts to send the blood through the pulmonary valve into the pulmonary arteries. The blood travels to the lungs, where it gets fresh oxygen and turns bright red again. It returns to the left atrium through the pulmonary veins. Then it moves through the mitral valve to the left ventricle. The left ventricle pumps blood high in oxygen out through the aortic valve into the aorta. The aorta delivers the blood to the body and the process begins again.

How does the heart work before birth?

Before a baby is born, it gets oxygen from the mother’s placenta, through the umbilical cord. The baby does not use its lungs to breathe before birth, so the blood flow within the baby’s heart bypasses the lungs. This happens in two ways:

  • An opening in the atrial wall, called the foramen ovale, lets blood pass from the right atrium to the left atrium.
  • A blood vessel called the ductus arteriosus lets blood flow straight from the pulmonary artery into the aorta and the baby’s circulatory system.

These passages usually close after birth, typically sometime between a few days to a few weeks after the baby is born.

Last updated: January 28th 2021