Ventricular septal defect

If you look at the heart, you’ve got the right and left atrium up top, and the right and left ventricles down low. Each of these pairs is separated by a wall, called a septum. A ventricular septal defect is when this lower wall—the ventricular septum—has a gap in it after development. 

The septum is formed during development as this muscular ridge of tissue grows upward from the apex, or the tip, and then fuses with a thinner membranous region coming down from the endocardial cushions. Voila—two separate chambers. If these don’t fuse though, then a gap is left between the two chambers; in other words, a ventricular septal defect, or VSD. The majority of cases are caused by a defect in the membranous portion of the septum.

Among babies, VSDs are actually the most common congenital defect overall, but 30 to 50% of VSDs can spontaneously close during childhood, which makes ventricular defects less common with adults. VSDs are associated with fetal alcohol syndrome and Down syndrome, and are often associated with other cardiac deformities as well. 

Alright so now let’s check out what happens with blood flow, now that there’s this opening between the two ventricles. I’m going to actually switch to this super duper simplified heart instead, just because it’s easier to show what’s going on with blood flow. Alright, so deoxygenated blood comes from the body to the right atrium, and then flows down into the right ventricle, where now it can either be pumped out to the lungs, as normal, or pop over to the left ventricle. Since the pressure on the left side of the heart is actually higher than on the right, and blood likes to flow from high pressure to low pressure, it actually prefers to just keep going on to the lungs. When oxygenated blood comes back from the lungs to the left atrium, and then the left ventricle, now again, it’s got two choices: it can either be pumped out to the body, or flow over to the right ventricle through the gap. Since now it’s in the left ventricle which has higher pressure, some of the blood flows over to the lower-pressure right ventricle, so a left-to-right shunt has been set up, where oxygenated blood takes an extra trip to the lungs.