Explain how blood circulates to and from the placenta and the fetus. How is blood shunted away from the lungs? (pages 401,403 )

During fetal development, circulation of blood between the fetus and the placenta is essential for supplying oxygen and nutrients to the growing fetus, and for removing waste products. Additionally, fetal blood bypasses the lungs as they are not yet fully developed and functional.

Some key structures involved in this circulation process include the umbilical cord, umbilical vein, and umbilical arteries. Inside the umbilical cord, there is one umbilical vein and two umbilical arteries. The umbilical vein carries oxygen-rich blood from the placenta to the fetus, while the umbilical arteries return deoxygenated blood from the fetus to the placenta.

The following steps illustrate the circulation of blood between the placenta and the fetus: 1. Oxygenated blood (rich in nutrients and oxygen) from the mother's blood supply diffuses into fetal blood vessels present in the placenta. 2. This oxygenated blood is transported through the umbilical vein towards the fetus. 3. A portion of the blood is directed towards the liver, bypassing it via the ductus venosus, and the remaining blood enters the inferior vena cava. 4. The blood from the inferior vena cava enters the right atrium of the fetal heart.

Since the fetus receives oxygenated blood from the placenta, and the lungs are not fully developed and functional, there are special structures that help bypass the lungs. These structures are: 1. The foramen ovale: A small hole in the septum between the right and left atria of the fetal heart. The blood entering the right atrium from the inferior vena cava is diverted through the foramen ovale to the left atrium, bypassing the pulmonary circulation. 2. The ductus arteriosus: A small connection between the pulmonary artery and the aorta. The blood that does enter the right ventricle is pumped into the pulmonary artery, but most of it is diverted away from the lungs through the ductus arteriosus and into the aorta, further bypassing the pulmonary circulation.

After oxygen and nutrients have been used by the fetus, the deoxygenated blood (rich in carbon dioxide and waste products) follows this path: 1. Blood from the fetal body drains into the superior and inferior vena cavae, entering the right atrium of the heart. 2. Blood from the right atrium passes into the right ventricle and is pumped into the pulmonary artery. 3. Most of the deoxygenated blood is shunted through the ductus arteriosus into the aorta, bypassing the lungs. 4. Deoxygenated blood is returned to the placenta through the two umbilical arteries present in the umbilical cord. This continuous circulation process between the placenta and the fetus allows for the exchange of oxygen, nutrients, and waste products until the fetus is born. At birth, the lungs fill with air and start functioning, the foramen ovale closes, and the ductus arteriosus constricts, transitioning the baby to a normal circulatory system.