Pulmonary edema is due to the excess fluid inside the lungs. With the presence of fluid in the lungs, there is less area of the respiratory membrane for gas exchange. The fluid also adds another "layer" to the respiratory membrane for gas to pass through making the diffusion slower. This will decrease the lung's ability for effective diffusion of gasses across the respiratory membrane which can lead to shortness of breath, pain, fatigue, coughing, sweating, increased heart rate, and irregular breathing.