pulmonary circulation starts with oxygen poor and carbon dioxide 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 arterials and then eventually into pulmonary capillaries which surround the Alvi which are of millions of tiny air sacks where gas exchange happens at that point oxygen enters the blood and carbon dioxide enters the Alvi 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 oxygen rich in carbon dioxide pore 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 thean veins rather
than draining into the Venus 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 broni receive systemic arterial blood from the bronchial arteries but the deoxygenated blood can flow or Anastos right into nearby pulmonary veins which are carrying oxygenated blood that has already traveled 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 BL 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's 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 Press left ventricle into the lower pressure right ventricle and an atrial seple 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 arteriosis the ductus arteriosis 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 all 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 press 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 that the right ventricle is doing a bit of extra work moving blood around that's already oxygenated on the flip side in of right to left 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 chamber of the
heart which reverses the gradient so for example in the congenital heart condition called tetrology of Flo there's 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 happens another example is eisener syndrome which is caused by pulmonary hypertension basically what happens is that
pulmonary hypertension which can result from a left to right cardiac shunt can get so severe that the pressures on the right side of the heart exceed the pressures on the left side leading to a right to left shunt ultimately in a right to left shunt oxygen Poe and carbon dioxide rich blood mixes with the oxygen rich and carbon dioxide Poe blood that's already gone through the pulmonary circulation the mixing causes oxygen levels to fall in the arterial blood leading to hypoxemia and supplemental oxygen does not bring levels back up because the blood going through the
pulmonary capillaries is already saturated and can't pick up any more oxygen in the short time that the blood is going through the pulmonary capillaries on the other hand if there's even a slight rise in carbon dioxide levels it's detected by Central chemo receptors in the brain which promptly increase the respiratory rate carbon dioxide in the lvi and blood equilibrates about 20 times faster than oxygen so unlike oxygen exchange which is physiologically Max out any excess carbon dioxide can quickly diffuse out of the blood and return to normal physiologic levels in fact it's the decrease in
oxygen levels that can actually be used to calculate the amount of blood flow through the shunt in liters per minute using the shunt fraction equation mathematically if the total amount of blood flow or cardiac output from the left ventricle is QT then it equals the amount that's shunted or Qs plus the amount that goes through the pulmonary capillaries or QC based on this the shunt fraction is the proportion of blood flow through the shunt over the total blood flow similarly the oxygen content in the cardiac output which is the arterial blood or cao2 is equal
to the oxygen content of the blood flowing through the shunt and remember that it's Venus blood that's diverted through the shunt or C VO2 plus the oxygen content of the blood flowing through the pulmonary capillaries or C CO2 now the more blood flow the more red blood cells there are to carry oxygen so oxygen content is affected by blood flow and we can multiply QT by C A2 Qs by CV2 and QC by C CO2 now we can arrange the total blood flow equation to substitute QC with QT minus Qs now the goal is to
solve for Qs / QT so we can first distribute C CO2 to to get QT multiplied cc2 minus Qs MTI cc2 now let's get all of the Qs parts of the formula on one side of the equation and all QT Parts on the other side next we can factor out Qs to get Qs * cc2 minus C V2 and do the same thing to QT to get QT * cc2 minus C ao2 now we divide both sides of the formula by QT Crossing it out from the right side leaving us with Qs * cco2 minus
C VO2 / by QT and then divide Again by C CO2 minus C VO2 on both sides to get rid of it on the left side so we're left with the shunt fraction equation Qs / QT the numerator has cco2 minus ca2 which is the oxygen content in the blood flowing through the pulmonary capillaries minus the oxygen content in the bacterial blood that difference represents the oxygen poor cvo2 blood coming from the shunt and mixing with the oxygen rich cco2 thereby causing cao2 to fall and the denominator is cco2 minus Z VO2 the oxygen content
of the blood flowing through the pulmonary capillaries minus the oxygen content of the Venus blood this value isn't affected by the shunt at all since oxygen rich C CO2 and oxygen poor C VO2 blood coming from the shunt represent values before the two get mixed together all right as a quick recap a shunt is a rediversion of blood from its usual path through the pulmonary circulation shunts can be physiologic or pathologic they can also result in a left to right or right to left diversion of blood across the heart the amount of blood that flows
through a right to left shunt can be calculated using the shunt fraction equation which is based on oxygen content of the blood helping current and future clinicians Focus learn retain and Thrive learn more
