About the normal heart
The heart is a muscle, an organ that pumps blood to the body. It has four chambers. The upper two chambers are the atria, and the lower two are the ventricles. There are also four heart valves:
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The tricuspid valve, located between the right atrium and the right ventricle;
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The pulmonary (pulmonic) valve, between the right ventricle and the pulmonary artery
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The mitral valve, between the left atrium and left ventricle; and
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The aortic valve, between the left ventricle and the aorta.
The valves all have leaflets or “cusps” which close and prevent blood from flowing backward.
Dark bluish blood (low in oxygen and under low pressure) flows back to the heart after circulating through the body. This returning blood relies partly on gravity to help it get back to the heart - it is not pumped back. It returns to the heart through veins (the superior and inferior vena cava or caval veins) and enters the right atrium. Blood from the right atrium flows through the tricuspid valve into the right ventricle. The right ventricle pumps the blood under low pressure through the pulmonary valve into the pulmonary artery. From there, the blood goes to the lungs where it receives oxygen. After the blood receives oxygen, it turns bright red. It then flows from the lungs and returns by the pulmonary veins to the left atrium, still under low pressure. From the left atrium, blood flows through the mitral valve and enters the left ventricle. This is a thick (large muscle) pumping chamber capable of pumping blood at the higher pressures needed to get it to the organs. The left ventricle pumps the oxygen rich blood through the aortic valve and into an artery called the aorta. The aorta branches into smaller arteries so that blood can go to all parts of the body through the arteries.
The normal fetal heart
In the fetus, after the heart is developed, the chambers are the same as in the normal heart with a few differences. Since the lungs have not yet opened in the fetus, very little blood flows to the lungs. In addition, blood flows to the mother's placenta which supplies oxygen for the blood. Because of these differences, a number of structures are present in the fetus that disappear after birth.
One is a ductus arterious, a blood vessel or duct between the aorta and pulmonary artery. Since it is open or patent, it is called a patent ductus arteriosus or PDA. Normally, this vessel closes within hours to days after birth. Another structure is an opening between the right and left atrium called a foramen ovale meaning “oval hole.” Since this hole is open or patent, it is called a patent foramen ovale or PFO.
With these structures, the blood flow in the fetus is different than in babies or children. Blood from the placenta (carrying oxygen) mixes with blood from the veins of the upper and lower half of the body (the superior and inferior vena cava or SVC and IVC for short), and enters the right atrium. Some of the blood (mostly the blood with more oxygen from the placenta) is directed to the left atrium through the PFO. The rest of the blood in the right atrium goes to the right ventricle. Usually, this blood has less oxygen in it. The blood then gets pumped from the right ventricle out to the pulmonary arteries, but the lungs are not open and therefore there is too much resistance for the blood to go to the lungs. Instead, the blood passes through the PDA and goes to the aorta.
From the there, it goes to the placenta where it receives oxygen. On the other hand, blood that passed into the left atrium (which has more oxygen) goes to the left ventricle where it is then pumped to the aorta. Most of this blood in the aorta goes to the upper half of the body (the brain), and only a smaller percentage goes to the lower half of the body. Of course, in the fetus, the lower half of the body gets blood that crosses the PDA as mentioned above.
Learn more about fetal cardiology and Children's Memorial's Institute for Fetal Health »