Pulmonary shunts

Pulmonary circulation starts with oxygen (O2) poor and carbon dioxide (CO2) rich blood in the right atrium that flows into the right ventricle.

From there - blood is pumped into the large pulmonary trunk, which splits to form the two pulmonary arteries – one for each lung.

The pulmonary arteries divide into smaller arteries known as pulmonary arterioles and then eventually into pulmonary capillaries which surround the alveoli - which are the millions of tiny air sacs where gas exchange happens.

At that point, O2 enters the blood and CO2 enters the alveoli.

The pulmonary capillaries drain into small veins known as pulmonary venules that flow into two pulmonary veins exiting each lung, and these pulmonary veins complete the circuit by delivering O2-rich and CO2-poor blood into the left atrium, which flows into the left ventricle and then into the aorta where it enters systemic circulation.

Normally, about 2% of the blood follows a slightly different path. It’s diverted, or shunted, so that it bypasses the pulmonary capillaries, and this is called a physiologic shunt.

There are two main ways this happens. First, when blood goes out to the heart muscle itself - it returns through tiny veins called thebesian veins.

Rather than draining into the venous system and going into the right atrium, these veins sometimes dump that blood into the closest chamber of the heart.

So, for example, if blood that goes out the aorta and through the coronary arteries to the muscle in the left ventricle of the heart, then the deoxygenated blood might then drain directly into the left ventricle chamber of the heart. At that point it would mix with the rest of the oxygenated blood and get squeezed right back out through the aorta.