Content Reviewers:Rishi Desai, MD, MPH
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.
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.
So - this blood basically bypasses the pulmonary circulation.
Second, the conducting airways of the lungs, like the bronchi, receive systemic arterial blood from the bronchial arteries.
Now, in addition to these naturally occurring physiologic shunts, there are also some pathological defects that can lead to more shunting of blood.
In most left-to-right shunts, blood flows from the left side of the heart to the right side of the heart. This can happen when there is a gap in the wall, or septa, that divides the left and right chambers of the heart.
Another type of left-to-right shunt happens with a patent ductus arteriosus.
The ductus arteriosus is a fetal blood vessel that creates a pathway for blood to flow from the pulmonary artery into the aorta. During fetal development, this is important because the lungs are not working, and are fluid-filled and compressed.
So oxygenated blood coming from the placenta bypasses the lungs and goes directly into fetal systemic circulation.
The end result of any of these left-to-right shunts is that oxygenated blood is making a second loop through pulmonary circulation, which means the right ventricle is doing a bit of extra work - moving blood around that’s already oxygenated.
On the flip side, in a right-to-left shunt, blood flows from the right side of the heart to the left. Normally this wouldn’t happen because blood would not want to flow up its pressure gradient.
But, right-to-left shunts typically involve changing pressures in the chambers of the heart which reverse the gradient.
So, for example, in the congenital heart condition called Tetralogy of Fallot, there is a large ventricular septal defect and stenosis, or narrowing, of the right ventricular outflow tract into the pulmonary artery.
If the right ventricular pressure exceeds left ventricular pressure, then blood can flow down the new pressure gradient, and a right to left shunt occurs.
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