To understand why a PDA can cause problems for premature babies, you first need to understand a bit about how the blood circulates through the heart.
Basically, the heart can be divided into two halves – one on the right, and one on the left. In the adjacent pictures, the right half is colored blue and the left half is colored red.
The blood that has been circulating around the body comes first to the right half of the heart. It comes into the right atrium (1), and runs down into the right ventricle (2) before it is pumped out into the pulmonary artery (3) which takes the blood to the lungs.
The blood then returns from the lungs to the left half of the heart. The blood comes into the left atrium (4), continues down into the left ventricle (5) and is pumped out via the aorta (6-7) to all the organs of the body before it returns again to the right half of the heart (1).