Fetal circulation is the circulatory system of the foetus which includes the umbilical cord and blood vessels within the placenta that carry fetal blood. Fetus lungs are non-functional during pregnancy so oxygen and other necessary nutrients are supplied by the mother through the placenta and umbilical cord to the foetus.
Before birth, circulation is different from after birth circulation. The placenta is the main source of nutrition and elimination of waste. There are many temporary structures in addition to the placenta and umbilical cord and these enable the fetal circulation to take place which include-
- The umbilical vein(one)
- The umbilical arteries(two)
- The foramen ovale
- The ductus arteriosus
- The ductus venosus
Process of Fetoplacental circulation
The umbilical vein carries the oxygenated blood from the placenta and enters the foetus at the umbilicus. In the liver, it gives off branches to the left lobe of the liver and receives deoxygenated blood from the portal vein. Here portal venous blood gets mixed with a greater portion of oxygenated blood. Through the ductus venosus, the blood enters the inferior vena cava and then to the right atrium of the heart.
In the right atrium, most of the blood flows through the foramen ovale(oval opening between atria) into the left atrium. Here it is mixed with a small amount of venous blood returning from the lungs through the pulmonary veins. The blood passes into the left ventricle through the mitral valve.
Some amount of blood from the superior and inferior vena cava, after reaching the right atrium passes into the right ventricle through the tricuspid valve.
During ventricular contraction(systole), the left ventricular blood is pumped into the aorta and distributed to the heart, head, neck, brain and arms. The right ventricular blood with low oxygen content is passed into the pulmonary system but during fetal life resistance in the Pulmonary artery is very high, so the main portion of the blood passes directly through the ductus arteriosus into the descending aorta bypassing the lungs.
The mixed blood is distributed by the descending aorta to the lower parts of the fetus and leaves the body by two umbilical arteries to reach the placenta where it is oxygenated and gets ready for recirculation. The cardiac output in the fetus is estimated to be 255 ml per kg/min.
Changes occur at birth in fetal circulation
Soon after birth, initiation of respiration and cessation of the placental blood flow occurs. The following changes occur in the vascular system-
- Closure of the umbilical arteries
- Closure of the umbilical vein
- Closure of the ductus arteriosus
- Closure of the foramen ovale
After birth, the cardiac output is estimated to be about 500ml/min and the heart rate ranges between 120-140/min.
In the above post, we discussed that the fetus lungs are not functional during pregnancy, oxygen and nutrition are supplied by the mother through the cord and placenta; the process of fetoplacental circulation and changes occur at birth.