Although there are many reasons for fluid in the lungs, the most common are related to the heart, according to Mayo Clinic. The medical term for this condition when it relates to the heart is cardiogenic pulmonary edema. Coronary artery disease, damage to the heart muscle, heart valve problems and high blood pressure lead to cardiogenic pulmonary edema. Other causes of fluid in the lungs include acute respiratory distress syndrome, high altitudes, exposure to certain toxins and near drowning.
The alveoli are small air sacs within the lungs that expand and allow the exchange of carbon dioxide and oxygen to occur with respiration. When fluid collects in these structures, they cannot effectively exchange these gases, indicates Mayo Clinic.
Cardiogenic pulmonary edema occurs when the left ventricle of the heart is unable to pump the blood it receives from the body effectively. The pressure builds in the left atrium of the heart, and eventually affects the veins and capillaries of the lungs. As the pressure increases, it pushes fluid from the blood into the air sacs, reports Mayo Clinic.
With noncardiogenic pulmonary edema, the capillaries weaken to the point that they leak fluid into the alveoli without any additional pressure. Toxins, such as smoke, irritate the lining of the air sacs so they begin to leak fluid, according to Mayo Clinic.