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.
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.
But the deoxygenated blood can flow, or anastomose, right into nearby pulmonary veins which are carrying oxygenated blood that has already travelled through the pulmonary capillaries.
So, once again, deoxygenated blood might flow right into the pulmonary veins and mix in with the rest of the oxygenated blood, bypassing the pulmonary circulation.
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.
So, for example, a ventricular septal defect allows blood to flow down its pressure gradient from the high-pressure left ventricle into the lower pressure right ventricle.
And an atrial septal defect allows for the same thing, only blood is shunted from the left atrium to the right atrium.
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.
This pathway is supposed to close at birth, and allow blood to flow normally from the right ventricle into the pulmonary artery and into the lungs.
But in some babies, it remains open, or patent, and that allows blood to flow from the high-pressure aorta into the lower-pressure pulmonary artery.
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.
The right ventricular outflow tract stenosis increases the resistance to blood going into the pulmonary circulation, and that increases right ventricular pressure.
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.
