Heart

The developing fetal heart accounts for a large percentage of the volume of the early thorax. About 20 days after fertilization, the heart develops from the fusion of paired endothelial tubes into a single tube. Heart growth subsequently involves the growth, expansion, and partitioning of this tube into four chambers separated by thickened septa of cardiac muscle and valves. Atrial development is initially more advanced than ventricular development. The left and right atria develop while the primitive ventricle remains a single chamber. As atrial separation nears completion, the left and right ventricles begin to form, then continue until the heart consists of its fully developed four-chambered structure.

Although the majority of the heart develops from mesoderm (splanchnic mesoderm) near the neural plate and sides of the embryonic disk, there are also contributions from neural crest cells that help form the valves.

Three systems initially return venous blood to the primitive heart. Regardless of the source, this venous blood returns to sinus venosus. Vitelline veins return blood from the yolk sac; umbilical veins return oxygenated blood from the placenta. The left umbilical vein enlarges and passes through the embryonic liver before continuing on to become the inferior vena cava that fuses with a common chambered sinus venosus and the right atrium of the heart. Especially early in development, venous return also comes via the cardinal system. The anterior cardinals drain venous blood from the developing head region. Subcardinal veins return venous blood from the developing renal and urogenital system, while supracardinals drain the developing body wall. The anterior veins empty into the common cardinals that terminate in the sinus venosus.

Movement of blood through the early embryonic vascular system begins as soon as the primitive heart tubes form and fuse. Contractions of the primitive heart begin early in development, as early as the initial fusion of the endothelial channels that fuse to form the heart.

The heart and the atrial tube that form the aorta develop by the compartmentalization of the primitive cardiac tube. Six separate septae are responsible for the portioning of the heart and the development of the walls of the atria and ventricles. A septum primum divides the primitive atria into left and right chambers. The septum secundum (second septum) grows along the same course of the primary septum to add thickness and strength to the partition. There are two holes in these septae through which blood passes, the foramen secundum and the foramen ovale. Specialized endocardinal tissue develops into the atrioventricular septum that separates the atrium and ventricles. The mitral and tricuspid valves also develop from the atrioventricular septum.

As development proceeds, the interventricular septum becomes large and muscular to separate the ventricles and provide strength to these high-pressure contractile chambers. The interventricular septum also has a membranous portion.

Initially, there is only a common truncus arteriosus as a channel for ventricular output. The truncus eventually separates into the pulmonary trunk and the ascending aorta.

Blood oxygenated in the placenta returns to the heart via the inferior vena cava into the right atrium. A valve-like flap in the wall at the juncture of the inferior vena cava and the right atrium directs the majority of the flow of oxygenated blood through the foramen ovale, then allows blood to flow from the right atrium to the left. Although there is some mixing with blood from the superior vena cava, the directed flow of oxygenated blood across the right atrium caused by the valve of the inferior vena cava means that deoxygenated fetal blood returning via the superior vena cava still ends up moving into the right ventricle.

While in the uterus, the lungs are non-functional. Accordingly, another shunt, the ductus arteriosis (also spelled ductus arteriosus) provides a diversionary channel that allows fetal blood to cross between the pulmonary artery and aorta and thus largely bypass the rudimentary pulmonary system.

Because only a small amount of blood returns from the pulmonary circulation, almost all of the blood in the fetal left atrium comes through the foramen ovale. The relatively oxygen-rich blood then passes through the mitral value into the left ventricle. Contractions of the heart, whether in the single primitive ventricle or from the more developed left ventricle, then pump this oxygenated blood into the fetal systemic arterial system.

In response to inflation of the lungs and pressure changes within the pulmonary system, both the foramen ovale and the ductus arteriosis normally close at birth to establish the normal adult circulatory pattern whereby blood flows into the right atrium, though the tricuspid valve into the right ventricle. The right atrium pumps blood into the pulmonary artery and pulmonary circulation for oxygenation in the lungs. Oxygenated blood returns to the left atrium by pulmonary veins. After collecting in the left atrium, blood flows through the mitral value into the left atrium where it is then pumped into the systemic circulation via the ascending aorta.