Tricuspid atresia means that the tricuspid valve didn't develop normally. Because of this, the right ventricle may also not develop properly, and there may be only a single ventricle.

Because there is no tricuspid valve, blue blood returning to the heart from the body cannot enter the right ventricle. This blue blood then shunts across an atrial septal defect to the left atrium, and mixes with the red blood returning from the lungs. The mixture of blue and red in the left atrium enters the ventricular septal defect and from there is sent to the aorta and the pulmonary artery.