|MadSci Network: Medicine|
You have asked a great question, and touched on an area in which I have a particular interest. I'll warn you that my curiosity regarding early heart development has resulted in a rather lengthy answer! :)
Before I answer your question, I think it is helpful to first review some basic anatomy and physiology of the heart.
Anatomy and physiology of the adult heart
The adult human heart is usually positioned on the left side of the body and has four compartments called chambers--you might think of them like the different rooms in a house. There are two different types of chambers, referred to as atria (singular: atrium) and ventricles. The human heart has two atria (a right and a left atrium) and two ventricles (a right and a left ventricle). The right side of the heart (atrium and ventricle) is responsible for sending 'deoxygenated' blood from the body to the lungs, while the left side of the heart accepts blood from the lungs (which is now 'oxygenated') and pumps it throughout the body. In general, blood flows from the atria to the ventricles, but needs to first pass through a specialized door called a valve. The valve on the right side of the heart, between the right atrium and ventricle, is called the tricuspid valve. The equivalent structure on the left side is known as the mitral valve. Once blood has reached the ventricles, it is pumped out through very large blood vessels. Blood from the right ventricle is directed through the pulmonary artery and into the lungs, where it picks up oxygen. However, its passage is again regulated by another valve, this one known as the pulmonary valve. Switching now to the left side of the heart, once blood has reached the left ventricle, it passes through the aortic valve, into a very large vessel called the aorta and subsequently travels throughout the body. So, tying all this information together, we can think of blood flow as travelling in a series: oxygen-poor blood is returned to the right side of the heart, travels through the lungs where it acquires oxygen, and then flows through the left side of the heart and into the circulatory network of the body. It is important to understand that normally, blood does not enter the left side of the adult heart until it has first traveled through the right atrium and ventricle, and into the lungs.
The atria and ventricles: keeping things separate
The atrial chambers have thin walls in comparison to the thick, muscular walls of the ventricles. These anatomical differences between the atria and ventricles reflect the differences in their function. For the most part, the atria act as large receptacles for blood coming from the lungs or the from the rest of the body, while the ventricles are responsible for pumping blood throughout the network of blood vessels. In the adult, the right and left atria are separated from each other by a structure called a septum--this is analogous to the walls that separate the rooms in a house. This wall is very important for preventing the blood from these two chambers from mixing. If we think back to how blood flows in the normal heart, the right atrium contains oxygen-poor blood while the left atrium contains oxygen-rich blood. If there were a hole in the atrial septum (referred to as an atrial septal defect), this would allow the blood in these two chambers to mix. This means that deoxygenated blood, which is normally confined to the right side of the heart, could now flow to the left side via this hole, and travel through the body--bypassing the lungs completely. If this hole is very large, the result is that the tissues throughout the body (such as the brain, muscles, etc), which rely on the oxygen supplied by circulating blood for normal function, now receive less of this critical nutrient. This hole could also allow oxygenated blood from the left side of the heart, which has already been to the lungs, to flow back to the right side. However, the consequence is that this oxygen-rich blood merely flows through the lungs again, which is not nearly as serious as the previous circumstance.
The left and right ventricles are also separated by a septum, similar to the one that divides the left and right atria. Any structural defect in this septum (referred to as a ventricular septal defect) can lead to problems similar to the ones I have already discussed.
So, why are babies with 'holes' in their hearts called "blue babies"? Well, first I should tell you that oxygen-rich blood, that is blood that has traveled through the lungs, appears bright red. On the other hand, oxygen-poor blood appears more purplish. These children are called "blue babies" because they actually appear blue. Major structural defects in the heart can cause blood flow to be disrupted, such that a large proportion of blood bypasses the lungs and is transported through the body in its purplish, deoxygenated form. These infants have a characteristic blue hue (called 'cyanosis') because oxygen-poor blood contained in the vessels appears blue through the skin.
Unfortunately, children can be born with a variety of heart defects. The ones I have described, atrial and ventricular septal defects, are only a small number. Generally, when people refer to "holes in the heart", they are talking about holes in the atrial or ventricular septum. However, "blue babies" rarely have just a septal problem. Often, there are other serious problems with the valves (particularly the tricuspid and mitral valves) as well as the major vessels of the heart. In general, defects that impair the normal flow of blood to the lungs can result in a "blue baby". In addition, there are a number of other heart defects that do not cause cyanosis, but which result in serious circulatory problems. Fortunately, many of these heart defects, from both the cyanotic and the acyanotic categories, can be surgically repaired, thus restoring normal circulatory function.
I hope you find this information helpful! There are a number of websites dedicated to congenital heart defects--I have listed my favorite below. I would be more than happy to clarify any information I have discussed, or to answer any further questions! You can contact me at the email address listed below.
This is a great place for learning more about normal heart function as well as many of the heart defects that plague young children.
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