Ascites or pulmonary hypertension was originally reported as high altitude disease, as it occurred especially when broiler were grown in elevated areas of 1000m altitude or more. Over time, the disease occurred at lower altitudes as well, and at this moment, it often occurs at sea level.

Ascites (also called water-belly or high altitude disease) is a metabolic imbalance between the demand of oxygen by the growing tissues and the supply of oxygen to these tissues. If the supply of oxygen is less than the demand, the metabolism of the bird will try to compensate that by pumping the blood faster through the veins. This leads to an increase in blood pressure. As a result of this increased blood pressure, the veins start leaking and the fluid fraction of the blood leaks through the walls of the veins and heart and into the body cavity.

The bigger blood cells, which are too big to pass through the wall of the veins will be kept in the blood circulation. However, they will actually thicken the blood, making it more difficult for the blood to be pumped around and reach the tissues that are in demand of oxygen. This will then accelerate the problem.

Due to the higher pressure that the heart has to pump the blood around, the heart will be enlarged, especially at the right ventricle which has to deliver the pumping power (see pictures). As a result, the body cavity will be filled with fluid that can reach an amount of a liter or more (see pictures).

Moreover, due to the thickening of the blood, the tissues will be supplied with less and less oxygen, and the growth of the bird will slow down and eventually stop. The accumulation of huge amounts of fluid in the body cavity will make the situation worse which results to the eventual death of the bird.

Ascites does not only occur in broilers. The disease is reported in humans as well, where it is also called, ascites.