'The heart is a four-chambered organ. The upper two (small) chambers are called auricles and the lower two (larger) are called ventricles. The auricles on each side are connected with their respective ventricles through an orifice/ opening, the left one is called mitral orifice and the right one is called tricuspid orifice. The orifices are guarded by valves (called the mitral valve and the tricuspid valve) so that blood flows in one direction only i.e. from auricles to ventricles, on each side.

The left side of the heart contains pure or oxygenated blood. After oxygenation, the blood comes from the lungs to the left upper chamber i.e. the left auricle through the pulmonary veins. From left auricle, the blood goes to the left ventricle through the mitral orifice, and from the left ventricle, the blood is pushed/pumped into the aorta (through the aortic orifice, guarded by the aortic valve) i.e. the main blood vessel which supplies blood to the entire body through its several branches.

From the body, the blood has to return to the heart for further oxygenation. This blood enters into the upper right chamber of the heart (right auricle), through superior vena cava which returns the blood from the upper half of the body and through the inferior vena cava, which returns the blood from the lower half of the body. The blood then goes to the right ventricle through the tricuspid orifice. The right ventricle further pushes the blood through the pulmonary orifice (guarded by pulmonary valve) into the pulmonary trunk, which divides into right and left pulmonary arteries for carrying the blood further to the right and left lung respectively, for oxygenation.

It is obvious from the above that the left side chambers contain oxygenated blood while the right side chambers contain impure blood. The right and left side chambers of the heart are not connected in any way, so that there is no mixing of pure and impure blood.

The main pulmonary artery/trunk which carries blood from right ventricle to the lungs for oxygenation may be involved. If this vessel is stenosed, called pulmonary stenosis (PS), the whole blood from the right ventricle will not be able to go to the lungs for purification. This defect may be associated with a septal defect either between the two auricles called atrial septal defect (ASD) or between the two ventricles called ventricular septal defect (VSD), or both, so that impure blood goes to the left side (as in such cases pressure is more in the chambers on the right side), i.e. into the left auricle or the left ventricle.

In this way, left side chambers of the heart instead of containing pure oxygenated blood, contain mixed blood i.e. both pure and impure blood. This mixed blood is supplied to the body through the aorta, so that the body instead of getting pure oxygenated blood, gets mixed blood, and hence each organ/tissue of the body suffers, and the child may be born. blue/ cyanosed, or becomes blue with a slight exertion.

If pulmonary stenosis (PS) is associated with VSD With resultant right ventricular hypertrophy (RVH), and the aorta also gets connected with the right ventricle (as a result of VSD), called dextroposition of the aorta (DA), the condition is called tetralogy of Fallot (PS, VSD, RVH, DA). And when tetralogy of Fallot is associated with ASD, the condition is called pentalogy of Fallot. In such cases the child is expected to be markedly blue (cyanosed) right from birth i.e. a blue baby may be born.