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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.
A pulmonary shunt occurs when there is re-diversion of blood from its usual path through pulmonary circulation. This can occur when there is an abnormal flow of blood from the right side of the heart to the left side of the heart, bypassing the lungs. Examples include conditions like patent ductus arteriosus, ventricular septal defect, and atrial septal defect. Sometimes a pulmonary shunt may be said when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused. This is because even though blood from the right ventricles passes through the lungs before reaching the left atria, it is still unoxygenated.
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