After the Fontan procedure, the blood without oxygen comes back from the body directly in the lungs, without being pushed by the heart. There are two main driving forces allowing this flow into the lungs.

The first one is an increased pressure in the veins. Instead of a pressure of 0 to 5 millimeter of mercury (the equivalent of the weight of a column of water of 1 square centimetre over a height of 5 centimeter, very little), the pressure in the veins after a Fontan procedure is around 15 to 20 milimeter of mercury.

The second driving force is the breathing. As you breathe in, the size of the inside of the chest is increased, and the air is sucked in the airways. At the same time, the blood is sucked into the lungs. When you breathe out, the opposite occurs. The size of what is inside of the chest is reduced and the air is pushed out of the lungs. At the same time, the blood is pushed out of the lungs. The breathing acts like a pump for the blood flowing passively in and out of the lungs. That is why it is important for patients who had a Fontan operation to have good lungs. It has been shown that after Fontan, the blood circulates better in the body of those who keep exercising.

For the Fontan operation to work well, it is also important to have good lung arteries, without restriction, and to have well a functioning heart, even if the heart is working as a single pump instead of two.