Thursday Tidbits: The Heart

The past two weeks of my human anatomy class have been spent studying and discussing the heart so I thought I'd share a bit of important information! The following picture for reference is from the National Heart, Blood and Lung Institute's website and the information has been cross referenced with the 5th ed. of Human Anatomy by Marieb, Mallatt and Wilhelm.

The circulatory system begins to form in the yolk sac during the fourth week of fetal development from mesodermal mesenchyme known as blood islands. The specific blood islands that create the heart form in the splanchnic mesoderm near what will become the future head and neck. Beginning as two endothelial tubes in the midline of the body, the heart it reaches out to the thoracic region and the tubes fuse near day 21 of development. Around day 22, the heart begins pumping due to the development of four bulges in the primitive tube (the premature and unpaired heart chambers.)

The heart folds into the characteristic shape in the thorax by being lifted up off the yolk sac by embryonic discs, around day 20-21.

The premature heart chambers are the:

• Sinus venosus - receiving chamber for all of the embryo's veins. This later becomes the SA node (sinoatrial node = pacemaker of the heart), smooth wall of the right atrium and coronary sinus and posterior wall of the left atrium.
• Atrium - which further develops into the right and left atria.
• Ventricle - this serves as the strongest pumping chamber of the primitive heart and later become the left ventricle
• Bulbus cordis - which gives rise to the pulmonary trunk (which carries oxygen poor blood to the lungs for gas exchange), the primary aorta (which carries oxygen rich blood in systemic circulation throughout the body) and the right ventricle.

By the second month of embryonic development the heart differentiates into its four respective chambers via the interventricular septum and valves.

By the third month, the fetus' blood is circulation through vessels in the same manner in which adult circulation occurs, except for two differences:

• The fetus supplies blood to the placenta (the organ by which oxygen and nutrients are obtained from the blood vessels in contact with the mother's uterus,) via umbilical cord vessels (arteries and vein) A very common misconception is that the mother's blood circulates through the fetus, when in actuality the fetus develops it's own circulatory system early on which does not come in contact with the mother's.
• The second major difference is that fetal respiration (breathing) does not occur by use of the lungs until birth. Thus, the lungs do not need as much blood and very little flows through the pulmonary circuit (the vessels that carry oxygen poor blood to the lungs for gaseous exchange for oxygen rich blood.)
• The lack of blood needed by the lungs calls for bypass mechanisms of the blood, one called the foramen ovale (a hole between the right and left atria, which closes at birth when the lings inflate and completely fuses around the child's first birthday.) The second bypass mechanism is the ductus arteriosus, a shunt which carries the blood pumped to the pulmonary trunk, to the aortic arch (this lets only the small amount of blood needed by the lungs to arrive there instead of being flooded by the total output from the right ventricle.) The ductus arteriosus closes at birth and completely fuses around three months of age.

During this period, complex developmental events take place and they do not always work out favorably, hence the formation of defects.

Congenital heart defects are the cause of nearly HALF of the total deaths due to birth defects and the most common that you will find listed below correlate to development during the second month of embryonic life. The most common defects can be narrowed down to one of these single causes, or a combination of these two causes:

• Defects from inadequately oxygenated blood reaching the body's tissues (because oxygen poor blood mixes with oxygen rich blood)
• The ventricles strain under an increased load of work (due to inadequate or narrowed valves and blood vessels.)

Common congenital heart defects include:

• Ventricular Septal  Defect - a hole between the right and left ventricles in the interventricular septum, causing oxygenated and deoxygenated blood to mix. This occurs in about 1 out of every 500 births.
• Transposition of the Great Vessels (Aorta and Pulmonary Trunk) - normally the aorta stems from the left ventricle and the pulmonary trunk from the right ventricle, but in this case they are switched thus the left side of the heart and systemic body are circulating deoxygenated blood, while the right side of the heart and lungs are circulating oxygenated blood. The bodily tissues are not receiving oxygen like they need to be and gas exchange is not occuring in the pulmonary circuit to recruit oxygenated blood for the body to receive because the tubes are mixed up. This occurs in about 1 out of every 1000 births.
• Coarctation of the Aorta - a portion of the aorta is narrowed, increasing the work the left ventricle has to perform to push the blood out. This occurs in about 1 out of every 1500 births.
• Tetralogy of the Fallot - four defects usually occur in this case, the pulmonary truck is too narrow, the pulmonary valve is stenosed, causing hypertrophy of the right ventricle (enlargement due to the volume of blood.) The ventricular septal defect accompanies this defect and the fourth defect is the thickening of the wall of the right ventricle due to in the increase in the amount of work it has to do (because the heart is a muscle.) This conglomerate of defects is commonly accompanied by other genetic defects; rarely is an infant born with this as a sole defect. Tetralogy of the Fallot occurs in about 1 out of every 2000 births.
• Pulmonary Stenosis - a narrowing of the pulmonary semilunar valve (which functions to prevent the back flow of pumped out blood into the right ventricle,) causes a decrease in the flow of blood to the lungs. This occurs in about 1 out of every 2800 births.

I have touched on just a few common congenital heart defects, however there are numerous known and they are a very real and frightening abnormality accounting for the majority of deaths in infants. For more information on congenital heart defects visit the National Heart, Blood and Lung Institute's website.

Also, learn more about congenital heart defects at Cora's Story, how one family lost their precious baby girl because of an undetected defect and who raise awareness and also at Baby Dickey! Search for Cora and CHD and while at Cora's Story, grab the buttom to raise awareness!