Normally, the heart is set up so that the left ventricle pumps oxygenated blood out to the body through the aorta; deoxygenated blood comes back to the right atrium, flows into the right ventricle, and is pumped to the lungs through the pulmonary artery. From the pulmonary artery, it comes back to the left atrium, flows into the left ventricle, and the whole process restarts. The “great arteries,” are the two main arteries taking blood away from the heart: the aorta and pulmonary artery. “Transposing” means that two things switch places with each other. So, transposition of the great arteries, or TGA, is when these two arteries swap locations.
Normally, blood flows through all of these chambers and blood vessels in a big circuit, but if you switch these two main arteries, you switch from one big circuit to two smaller circuits. On the left side, blood is now pumped from the left ventricle, to the pulmonary artery, and to the lungs; it then comes back to the left atrium and left ventricle, and restarts the circuit. On the right side, blood is pumped out of the right ventricle through the aorta, and then goes to the body; blood comes back the the right atrium and right ventricle, and restarts the circuit. Blood on the right side therefore never gets oxygenated, and blood on the left side never gets deoxygenated. This isn’t good. This situation is actually called complete TGA, or sometimes dextro-TGA or d-TGA; dextro means “right,” because, in this case, the aorta is in front of and primarily to the right of the pulmonary artery.