Tricuspid Valve Atresia consists of the following abnormalities:

The Tricuspid Valve did not develop so the blood cannot get to the Right Ventricle and that ventricle in turn also is severely underdeveloped.

Usually only a small part of the Right Ventricle is developed and is supplied with blood through a hole from the Left Ventricle called a Ventricular Septal Defect (VSD).

A hole between the two atriums called a Atrial Septal Defect (ASD) is essential for the blue blood coming back from the body to get into the heart to circulate.

The blood which is coming back to the Right Atrium has nowhere to go other than to go through the Atrial Septal Defect. When it gets through the ASD it mixes with the pink blood coming back from the lungs into the Left Atrium.

The Pulmonary Artery and Valve are also severely underdeveloped. Since the Right Ventricle did not develop normally the artery coming out of it and its branches do not develop normally.

Ductus Arteriosus
The Ductus Arteriosus is a little artery-like structure which has muscles in its walls located between the main Pulmonary Artery and the Aorta and is usually present in all babies before birth.

When a baby is exposed to oxygen the Ductus closes on its own. It's okay for the Ductus to close in a baby with a normal heart, but for a baby with Tricuspid Atresia this will almost be fatal to the baby, so doctors try to keep the Ductous open until surgery can be performed.