Transposition of the Great Vessels (of the Heart)

Sometimes lots of stuff goes wrong in a disease, but  other times a relatively small change can have massive consequences. In this diseasese a small switch leads to a massive problem! Here it is, Transpositions of the Great Vessels:

Short 'n Sweet:
This is a congenital malformation in which a child is born with the Aorta attaching to the Right Ventricle and Pulmonary Artery attaching to the Left Ventricle.

Background Anatomy&Physio:
As we all know, the heart is a four-chambered pump that propels blood around the body. Blood circulates inside large tubes called 'blood vessels'. Those blood vessles that lead away from the heart are called 'arteries', and the ones that lead back to the heart are called 'veins'. The blood flows in one direction through the circulatory system, then through the four chambers of the heart, and back out. We need blood in order to transfer the oxygen we breathe in from our lungs to the rest of the body, and then the carbon dioxide we make in the rest of our body back out through our lungs.

Since we know the blood flows through the circulatory system, let's trace the path that it takes! The heart has two sides - the right side and left side. Each side is further divided into an atrium and ventricle, so the four chambers are Right Atrium, Right Ventricle, Left Atrium, and Left Ventricle. Our blood follows the following path (red=oxygen, blue=no oxygen): 

Right Atrium -> Right Ventricle -> Pulmonary Artery -> Lungs (where blood gets Oxygen) ->

Pulmonary Vein -> Left Atrium -> Left Ventricle -> Aorta (huge artery) ->

Rest-of-the-Body (delivers oxygen, picks up waste) -> Veins -> Right Atrium. (and it continues)

And so our circulatory system forms this nice "circuit", with oxygen-supply in the lungs (on the right sided heart circulation) and oxygen demand/delivery in the body (left sided heart circulation).


What Goes Wrong:
So your heart and my heart look like I've just described, right? But it wasn't always like that. In early embryologic stages, while the fetus is still growing, the heart is originally just a single large tube. Bit by bit it grows and develops like the rest of the body. Any error happening in this phase is called a congenital defect and Transposition of the Great Vessels is an example of such a defect. Due to a small mistake with spiralling, the Aorta attaches to the Right Ventricle instead of the left, and the Pulmonary Artery attaches to the Left Ventricle instead of the right.

Sounds like a small mistake, right? But think about it for a second . . . what does this mistake do to your circulation pathway? Let's trace it again and see :). Remember, the only change in this baby is that we have Right Ventricle->Aorta and Left Ventricle->Pulmonary Artery. . .

Right Atrium -> Right Ventricle -> Aorta ->

Rest of the body (picks up CO2) -> Veins-> Right Atrium  (circles)

Whoa! Wait a minute! What happened to all the rest of the stuff that's supposed to happen? Let's start at the Left Atrium this time instead (it's supposed to be a circle, right?)

Left Atrium -> Left Ventricle -> Pulmonary Artery ->

Lungs (gets oxygen) -> Pulmonary Vein -> Left Atrium (circles)

Now the situation is clear! Bizzare as it is, we have not one, but *TWO* completely distinct circulations that are totally disconnected! Instead of having once circulatory system to supply our body with O2 and pick up CO2 to get rid of, we now have two circuits! Effectively we have two two-chambered hearts instead of one 4-chambered one.

But it's even more serious than that. Because unlike a healthy system, where you get oxygen to give the body (we need oxygen to survive, remember?) and get rid of CO2 which can be acidic and poisonous if it builds up, we now have these two separate circulations that aren't working together. The left-sided circuit will have all the oxygen it wants, but nothing to deliver the oxygen to. And the right-sided circuit - which supplies the rest of the body including the brain, stomach, arms, legs, liver, kidneys and more - will have all the CO2 and no oxygen! It's a very serious condition that requires immediate care; and without an additional heart 'defect' or *immediate* care, the child will die very shortly after birth!

Treatment:
Luckily for us, this condition isn't necessarily lethal. Using proper technology, we can detect signs of the malformation while the child is still in the mother's womb and being oxygenated by her circulation. At the time of birth, the doctors know what's coming and will be ready to perform surgery on the newborn baby. A surgeon removes the two ends of the vessels and re-attaches them to the correct ventricles, totally fixing the problem.

Thanks for reading! As this is the first, please let me know if I'm talking at too high (or too low) a level! I remind first-time readers that this blog is aimed at a 9th-12th grade level of knowledge of english, biology and chemistry; so if you can't follow along or understand it easily, then I'm not doing my job right!