the atrial septum separates the right and left atrium of the heart and an atrial septal defect describes where there's some opening in a baby's heart between these two upper chambers but how does this opening form well let's first run through how the septum develops in the first place when the heart's first developing a tissue called the septum primum between the left and the right atria grows downward slowly creating two separate chambers by closing this gap or ostium primum which means the first opening this septum primum then fuses with the endocardial cushion and closes the gap completely meanwhile a hole appears in the upper area called the ostium secundum or second opening now we also had the septum secundum which grows downward just to the right of the septum primum and covers the ostium secundum like a curtain leaving a small opening called the foramen ovale and essentially creating a makeshift valve that allows blood to go from the right atrium to the left atrium but not the other way the developing newborn gets oxygenated blood from the placenta which goes from the umbilical vein over to the right atrium which is different because after development only deoxygenated blood goes to the right atrium which is then sent to the lungs to pick up some oxygen in the developing fetus though it's already oxygenated so instead of going to the right ventricle into the lungs and back to the left atrium it just bypasses the right ventricle and lungs through the foramen ovale and into the left atrium the oxygenated blood then goes to the left ventricle to be pumped to the body at birth the septum secundum and septum primum slap shut and then fuse and close off this frame in a valley so we can rely on our own lungs for oxygen an atrial septal defect or ASD describes a condition where in some way the septum between the right and left atrium doesn't close up all the way and remains open even after birth about 90% of ASD cases are due to the ostium secundum in other words the septum secundum doesn't fuse with the septum primum and therefore the foramen ovale doesn't close quite right this actually accounts for about 10 to 15 percent of all congenital heart defects and is the most common congenital heart defect in adults 10% of ASD cases are due to the ostium primum where the first opening or ostium primum doesn't make it all the way down again leaving an opening between the right and left atria this primum type of congenital defect is found in around 25% of patients with Down syndrome atrial septal defects in general are also commonly associated with fetal alcohol syndrome okay so there's this opening the septum now what well you know how pressures higher on the left side that on the right well this pressure difference causes blood to be shunted from the left atrium to the right atrium meaning more oxygenated blood gets over to the right side which is usually deoxygenated blood right this type of defect is an a cyanotic heart defect which means not blue since oxygenated blood is going into the pulmonary circulation and essentially taking an extra trip to the lungs in contrast a cyanotic heart defect allows deoxygenated blood to bypass the lungs and make it out to the body causing a blue or purplish discoloration of the skin which is not the case here that being the case you'll see an increase in oxygen saturation of the blood in the right atrium the right ventricle and the pulmonary artery this extra blood volume passing by the pulmonic valve also causes a delay in the closure of the pulmonic valve relative to the aortic valve closure this slight delay can be heard via auscultation as a splitting of the s2 sound as well as a systolic murmur in some cases one potential complication for patients with an ASD is if the patient develops an embolus let's say a blood clot because that's the most common type that makes its way through the ASD so if we take the example of a person with a deep vein thrombosis part of that blood clot might break loose and become an embolus and usually this guy would travel into the right atrium then the right ventricle and then be pumped off to the lungs and lodged somewhere in the lung it's possible that the embolus crosses over to the left side in which case it can enter the systemic circuit and potentially lodge in the brain this situation is called paradoxical embolism paradoxical because the embolus starts off on right side but then crosses over into the left side sometimes children with ASDs will be monitored for a little while since smaller openings might close on their own in other cases though especially for larger openings surgery will be done to prevent complications in the future which typically involves plugging or patching the opening thanks for watching you can help support us by donating on patreon or subscribing to our channel or telling your friends about us on social media you