Question

1. 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

CHANGES IN CIRCULATORY SYSTEM AT THE TIME OF BIRTH:

1. Foremen ovale is closed as the lungs become functional.

2. Ductus venosus is constricted and the ductus arteriosus is closed.

3. (Not so important change) placental vessels are occluded.

1.FORAMEN OVALE–

FEATURE IN PRENATAL ANIMAL:

Foramen ovale allows most of the oxygenated blood entering the right atrium from inferior venacava to pass into the left atrium.

It prevents passage of blood in opposite direction because the septum primum closes against the rigid septum secundum.

CHANGE AT THE TIME OF BIRTH:

–Closes at birth due to decreased flow from placenta and inferior venacava to hold open foramen.

–More importantly because of the increased pulmonary blood flow and pulmonary venous return to the left heart causing the pressure in left atrium to be higher than in right atrium.

– The increased left atrial pressure then closes the foranen ovale against the septum secundum.

– The output from the right ventricle now flows entirely into pulmonary circulation.

AFTER BIRTH-

–foramen ovale is no longer functional and is rudimentary and is called FOSSA OVALIS.

2.DUCTUS VENOSUS AND DUCTUS ARTERIOSUS-

IN PRENATAL ANIMAL :

DUCTUS VENOSUS– it is a fetal blood vessel connecting the umbilical vein to inferior venacava.

Blood flow is regulated through sphincter.

Carries mostly highly oxygenated blood.

DUCTUS ARTERIOSUS – it protects lungs against circulatory overload and allows the right ventricle to strengthen.

Carries mostly med. oxygen saturated blood.

CHANGE AT THE TIME OF BIRTH–

DUCTUS VENOSUS- the sphincter is constricted ,so that all the blood entering the liver passes through the hepatic sinusoids.

DUCTUS ARTERIOSUS – constrcts at the time of birth. When the PO2 of blood passing through ductus arteriosus reaches less than 50 mm of Hg it constricts.

As pulmonary vascular resistance reduces the pulmonary arterial pressure falls below the systemic level and the blood flows through the ductus arteriosus is diminished as a result of constriction.

AFTER BIRTH–

DUCTUS VENOSUS –It becomes rudimentary and is called LIGAMENTUM VENOSUM.

DUCTUS ARTERIOSUS–It also becomes rudimentary and is called LIGAMENTUM ARTERIOSUM.

3. PLACENTAL VESSELS–

BEFORE BIRTH –umbilical vein provides oxygenated blood and nutrients to the fetus

Umbilical arteries takes deoxygenated blood and excretory wastes from the fetus.

AT BIRTH –no longer is of use

It constricts and is occluded to prevent the loss of fetal blood.

AFTER BIRTH–is not present as the umbilical cord is cut off after the birth.