Question

Describe the path of blood flow in the upper part of the body BEFORE birth, starting at the upper right side of the heart (the right atrium) and ending when the blood is pumped into the body’s main circulation (into the aorta). Name all relevant structures and indicate the direction of flow.

Answer 1

• When oxygenated blood from the mother enters the right side of the heart it flows into the upper chamber (the right atrium). Most of the blood flows across to the left atrium through a shunt called the foramen ovale.

• From the left atrium, blood moves down into the lower chamber of the heart (the left ventricle). It's then pumped into the first part of the large artery coming from the heart (the ascending aorta).

• From the aorta, the oxygen-rich blood is sent to the brain and to the heart muscle itself. Blood is also sent to the lower body.

• Blood returning to the heart from the fetal body contains carbon dioxide and waste products as it enters the right atrium. It flows down into the right ventricle, where it normally would be sent to the lungs to be oxygenated. Instead, it bypasses the lungs and flows through the ductus arteriosus into the descending aorta, which connects to the umbilical arteries. From there, blood flows back into the placenta. There the carbon dioxide and waste products are released into the mother's circulatory system. Oxygen and nutrients from the mother's blood are transferred across the placenta. Then the cycle starts again.

At birth, major changes take place. The umbilical cord is clamped and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of air, the lungs start to expand, and the ductus arteriosus and the foramen ovale both close. The baby's circulation and blood flow through the heart now function like an adult's.

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