15.3 Circulation in the Human Embryo

15.3 Circulation in the Human Embryo

Lungs and liver are not yet doing the adult job

After birth, the right side of the heart pumps the cardiac output through the lungs, and blood from the digestive tract passes through the liver. A human fetus cannot use that plan. The lungs are fluid-filled and uninflated, and they are not the source of oxygen. The placenta has already loaded oxygen into fetal blood. Forcing every drop of blood through the lungs would add resistance without adding oxygen. The liver also does not need to process the entire stream arriving from the placenta, because that blood is not a fresh meal absorbed from the gut. Fetal circulation therefore bypasses the lungs, and it bypasses much of the liver.

Three shunts do the bypassing. The ductus venosus is the liver shunt. The foramen ovale and the ductus arteriosus are the lung shunts. The inbound vessel for this oxygen-rich blood is the umbilical vein. An umbilical artery is not that inbound road. Arteries in the cord carry oxygen-poor blood back toward the placenta, the reverse of adult systemic arteries, which carry oxygen-rich blood away from the heart.

The liver shunt

Ductus venosus

Much of the blood in the umbilical vein enters the ductus venosus and empties into the inferior vena cava, skipping the capillary bed of the liver. The inferior vena cava then carries that oxygen-rich stream up to the heart, mixed with ordinary venous blood from the lower body. Some placental blood does enter liver tissue, which is why the bypass is partial. The accurate claim is that much of the stream skips the liver, not that the liver receives no placental blood at all.

The ductus venosus does not bypass the lungs. It ends at the inferior vena cava, still upstream of the heart. Lung bypasses begin once blood reaches the heart and the pulmonary artery. Keeping the three shunts in anatomical order prevents the common swap of a liver shunt for a lung shunt.

Two shunts that spare the lungs

Foramen ovale

Blood arriving in the right atrium from the inferior vena cava can cross the foramen ovale into the left atrium. Flow then continues into the left ventricle and out the aorta. The brain and the heart can receive this blood with no stop in the lungs. The foramen ovale is an opening between the atria. It is not an opening between the ventricles. A hole in the ventricular wall is a different and abnormal defect. It is not the normal fetal shunt, and it does not carry the name foramen ovale.

Before birth, pressure favors passage from the right atrium into the left atrium through a flap that acts as a one-way door. That door lies between the atria only.

Ductus arteriosus

Blood that does enter the right ventricle is pumped into the pulmonary artery. Resistance in the collapsed lungs keeps most of that flow from entering lung tissue. The blood instead crosses the ductus arteriosus from the pulmonary artery into the aorta. That crossing is the second lung bypass. The ductus arteriosus is a vessel between the pulmonary artery and the aorta. It is not the opening between the atria, and it is not a continuation of the umbilical vein.

The diagram in this section shows the foramen ovale and the ductus arteriosus, the two shunts that bypass the lungs. The ductus venosus is the liver shunt and sits earlier in the circuit.

Fetal hemoglobin

Fetal hemoglobin binds oxygen more tightly than adult hemoglobin. Maternal blood at the placenta is the fetus's only outside source of oxygen, and fetal blood has to load oxygen across the villus barrier from maternal hemoglobin that has already given up part of its load. A tighter hold lets fetal hemoglobin take on oxygen under those conditions. After birth the infant gradually replaces fetal hemoglobin with adult hemoglobin, which releases oxygen to the tissues more readily. The prenatal advantage is loading at the placenta, not a stronger lung.

The circuit changes when the lungs fill

At birth the newborn breathes and the lungs inflate. Pulmonary resistance falls, so blood in the pulmonary arteries flows through the lungs and returns to the left atrium through the pulmonary veins. The added return raises pressure in the left atrium relative to the right atrium. The flap of the foramen ovale is pressed shut. The opening closes, and the sealed site later becomes the fossa ovalis, a depression in the wall between the adult atria rather than a passage between them.

The ductus arteriosus closes as well. Rising oxygen and the loss of the relaxing prostaglandin signal cause its muscular wall to constrict. The closed vessel becomes the ligamentum arteriosum, a fibrous cord between the pulmonary trunk and the aorta. Blood no longer crosses from the pulmonary artery into the aorta through that cord.

Umbilical flow stops once the placenta separates and the cord no longer carries blood. The umbilical vessels become ligaments. The umbilical vein becomes the round ligament of the liver, and the umbilical arteries become the medial umbilical ligaments along the inside of the abdominal wall. The ductus venosus also closes and becomes the ligamentum venosum. In a healthy infant these remnants are closed landmarks, not open shunts. Blood now runs in series through the right heart, the lungs, the left heart, and the body.