Heart, Description, Prenatal and Postanatal Circulation

The Heart

Heart View

The heart is a hollow organ with a base, apex, and three surfaces.

• The base of the heart is on the right, posteriorly, and superiorly, where the great vessels exit and enter.
• The apex is directed to the left, anteriorly, and inferiorly (created by the left ventricle).
• The three surfaces are described as anterior (sternocostal), posterior, and inferior (diaphragmatic).

• The anterior surface (sternocostal) is formed by the right ventricle which is separated from the left ventricle by the anterior interventricular sulcus  which is occupied by the left anterior descending artery (anterior interventricular branch of the left coronary artery) and the great cardiac vein.

Prenatal Circulation

Lungs of the fetus

Pressure is high in the right side of the heart when it comes to fetal circulation in comparison to adult circulation. Fetal lungs have no role in gas exchange as the pulmonary arterioles are in a hypoxic state. This is due to the fact that the lungs are not aerated (the fetus is not breathing air), the lungs are fluid-filled meaning they are collapsed, and the lungs are not yet expanded. This in-return physiologically causes hypoxic pulmonary vasoconstriction accompanied by high pulmonary vascular resistance and pressure within the pulmonary circulation causing the higher pressure in the right side of the heart of the fetus.

Due to what’s mentioned above, the blood tries to  avoid going into the pulmonary arteries, where the pressure is higher and no gas exchange is occurring, which leads to minimal flow in the pulmonary circulation of the fetus. Blood shunts away form the right ventricular outflow in two ways:

• Through the foramen ovale between the right and left atria
• Ductus arteriosus which is a connection between the pulmonary trunk and aorta.

The fetus relies on blood to be oxygenated from the mother rather than its own lungs. Gas exchange occurs in the placenta.

Prenatal Circulation

Prenatal Circulation

Blood from the placenta comes through the umbilical vein which has fully oxygenated blood. The umbilical vein makes an anastomosis  (gives off a branch that connects to the portal vein) which then sends oxygenated blood that mixes with the blood of the portal vein to supply the liver. The umbilical vein then drains into the inferior vena cava through ductus venosus (connects the umbilical vein to inferior vena cava). The blood mixes with deoxygenated blood of the inferior vena cava and drains into the right atrium, where fully deoxygenated  blood also drains from the superior vena cava.

Blood in the right atrium is partially oxygenated and avoids going into the pulmonary arteries in two ways. In the right atrium it can automatically go into the left atrium and avoids the high afterload of the right ventricle by passing through the foramen ovale into the left atrium. Blood that is in the right ventricle goes is pumped into the pulmonary trunk which branches into the pulmonary arteries but avoids going into the high pressure of the pulmonary arteries by going through the ductus arteriosus, a direct connection to the aorta.

• Blood coming from the inferior vena cava and going into the left atrium and left ventricle and then the aorta is more oxygenated and goes to the brain and upper limbs!

The blood that goes into the lungs returns to the left atrium through the pulmonary veins (which is little blood flow) goes into the left atrium and then the left ventricle and is then pumped out through the aorta to supply the fetus. Blood returns to the placenta for oxygenation through the umbilical arteries (branches of internal iliac arteries). Blood is barely introduced into the left atrium through the pulmonary veins.  

Fun Fact: The Ductus Arteriosus becomes the ligamentum arteriosum within three weeks after birth.

Postnatal Circulation

Circulation before birth - prenatal circulation.

After birth, when the newborn takes its first breath, the lungs are expanded and aerated and are now the essential organ for gas exchange; the placenta no longer plays a role. Vascular resistance in the pulmonary arteries falls drastically because the vessels of the expanded lungs are no longer vasoconstricted. The pressure in the right side of the heart and right atrium plummets, as it decreases drastically, and that causes an endocardial flap valve in the left atrium to close the foramen ovale, preventing blood from passing from the right atrium into the left atrium. Pressure in the left atrium is now higher.

The smooth muscle of the ductus arteriosus contraction causes it to occlude as well, preventing blood coming from the right ventricle to enter the aorta. Officially, the foramen ovale and the ductus arteriosus are occluded and the ductus arteriosus becomes the ligamentum arteriosum within three weeks after birth.

The umbilical vein also loses its anastomoses with the portal vein and the ductus venosus that connects it to the inferior vena cava is occluded as well. The ductus venosus becomes the ligamentum venosum. The umbilical vein eventually becomes fibrous along its whole entire length and forms the round ligament of the liver. The umbilical arteries, however, proximally only remain patent while the rest becomes obliterated and forms the medial umbilical ligament on both sides.

Circulation after birth - postnatal circulation

If the ductus arteriosus is stubborn and remains patent, blood will flow from the aorta to the pulmonary trunk and cause secondary damage to the pulmonary arteries.

Circulation:

• Blood flows into the right atrium from the inferior and superior vena cavas which carry deoxygenated blood. Blood then flows into the right ventricle where it is pumped out into the pulmonary trunk and the right and left pulmonary arteries which take blood to the lungs to allow the blood to get oxygenated.
• Blood returns to the heart’s left atrium through the pulmonary veins, now carrying oxygenated blood from the lungs into the left atrium. From the left atrium, blood flows into the left ventricle where it is then pumped out into the aorta, which distributes blood to the rest of the body.

Resources

Literature

• Schuenke, Michael, et al. Thieme Atlas of Anatomy: Neck and Internal Organs. Thieme, 2011.