How does transposition of the great arteries affect blood circulation?

Transposition of the great arteries (TGA) is characterized by a congenital heart defect in which the aorta and pulmonary artery are switched. This results in two separate circulatory systems that do not effectively communicate with each other: one system that circulates oxygen-poor blood from the body back to the heart and out to the lungs, and another that circulates oxygen-rich blood from the lungs back to the heart and out to the body.

By placing both circuits in parallel, TGA leads to a situation where the systemic and pulmonary circulations operate independently. This means that oxygenated blood can circulate back to the body without passing through the lungs, while deoxygenated blood circulates back to the lungs without returning to the body. This configuration prevents proper oxygenation of the blood and requires immediate medical intervention to establish some form of mixing between the two circuits, which is often accomplished through interventional procedures such as balloon atrial septostomy or the creation of a shunt.

Therefore, the correct understanding of how TGA affects blood circulation is the recognition that it places the pulmonary and systemic circuits in parallel, creating a critical need for mixing to ensure adequate oxygen delivery to the body.