Question

Describe two major changes that happen in a placental mammal’s circulatory system at birth. Specifically, for each change describe the feature in the prenatal animal, the change that happens at birth, and the feature in the postnatal animal.

Answer 1

At the birth of placental mammals , their circulatory systems undergo certain changes which enables the newborn for independent life. The fetus depends on the mother for nourishment and oxygen. After birth, the lungs are engaged, becoming the primary source of oxygen, replacing the placental barrier as a means for blood-gas exchange. In the fetus, lungs are not being used. The liver is not fully developed. Circulating blood bypasses the lungs and liver by flowing in different pathways and through special openings called shunts. The blood flow pathway in the fetus is as follows: Oxygen and nutrients from maternal blood are transferred to the fetus through the umbilical cord across the placenta. The blood flows through the umbilical vein toward the baby’s liver and there it moves through a shunt called the ductus venosus allowing some of the blood to enter the liver. But most of this highly oxygenated blood flows to a large vessel called the inferior vena cava and then into the right atrium of the heart.

At birth some major changes occur. The umbilical cord is clamped and the baby stops receiving oxygen and nutrients from the mother. With the first breath of air, the lungs start to expand, and the ductus arteriosus and the foramen ovale both close. Following birth, for the next few months of post-birth development, the veins and arteries associated with the umbilical cord disintegrate as their lumens fill with fibrous tissue. In two to three months, permanent ligaments are formed in their respective locations.The foramen ovale, temporary valve structure between the right and left atrium, is sealed completely within a year after birth. Absolute closure never happens in 20–25% percent of the population, resulting in a probe patent foramen ovale, a condition that is rarely serious. The lumen of the ductus arterosis is eventually replaced by the ligamentum arteriosum, a band of fibrous tissue that runs from the center of the pulmonary arteries to bottom of the aortic arch.The closure of the ductus arteriosus and foramen ovale completes the transition of fetal circulation to newborn circulation.