Fetal circulation is quite different from adult circulation because the digestive tract, lungs, and kidneys are not functioning. Oxygen and nutrients are obtained from the maternal blood in the placenta, while carbon dioxide and
other metabolic wastes are removed via the maternal blood. The pattern of fetal circulation is an adaptation to these conditions. Birth immediately separates the infant from its supply of nutrients and oxygen and stimulates cardiovascular changes to accommodate independent living as an air-breathing human.
Figure 18.12 illustrates the pattern of fetal circulation. Oxygenated and nutrient-rich blood is carried from the placenta to the fetus by the umbilical vein, which enters the fetus at the umbilicus (navel). Within the fetus, the umbilical vein passes toward the liver, where it divides into two branches. About half of the blood carried by the vessel enters the liver, while the other half bypasses the liver by flowing through the ductus venosus (duk’- tus ven-o’-sus) and into the inferior vena cava. Full blood flow through the fetal liver is not necessary because the fetal intestines are nonfunctional and the mother’s liver removes potentially hazardous substances before her blood enters the placenta. The oxygenated blood from the umbilical vein is mixed with deoxygenated blood in the inferior vena cava. The addition of blood from the ductus venosus increases the blood pressure within the inferior vena cava and the right atrium, which keeps the foramen ovale open.
Most of the blood entering the right atrium of the fetal heart passes directly through the foramen ovale (o-vah’-le), an opening in the interatrial septum, into the left atrium. The blood in the left atrium flows into the left ventricle and is pumped into the aorta for transport throughout the body. The blood that does enter the right ventricle is pumped through the pulmonary trunk. However, most of it bypasses the lungs by flowing through the ductus arteriosus (duk’-tus ar-te-re-o’- sus) into the aortic arch. These two lung bypasses work together to provide better oxygen and nutrient delivery to fetal tissues by providing additional blood for transport to the body. However, sufficient blood flows through the pulmonary circuit to maintain the nonfunctional lungs.
Blood is returned to the placenta by two umbilical arteries that branch from the internal iliac arteries. Trace the flow of blood through the fetal cardiovascular system shown in figure 18.12 .
Fetal Cardiovascular Adaptations
|Umbilical vein||Carries oxygenated and nutrient-rich blood from the placenta to the fetus|
|Ductus venosus||Carries about half of the blood in the umbilical vein into the inferior vena cava, bypassing the liver and mixing oxygenated and deoxygenated blood|
|Foramen ovale||Allows a large portion of the blood entering the right atrium to pass through the interatrial septum directly into the left atrium, bypassing the pulmonary circuit and providing as much oxygen and as many nutrients as possible for body cells via the systemic circuit|
|Ductus arteriosus||Carries most blood from the pulmonary trunk directly into the aorta, bypassing the nonfunctional lungs and providing more blood with available oxygen and nutrients for the systemic circuit|
|Umbilical arteries||Carry deoxygenated blood from the internal iliac arteries back to the placenta|