Pulmonary oedema. Light micrograph of a section through a lung with pulmonary oedema, an accumulation of fluid in the lungs. The capillaries have become congested with red blood cells (erythrocytes, red) and dilated, and the alveoli (air spaces) have filled with fluid. The presence of fluid in the lungs interferes with gaseous exchange, causing shortness of breath, anxiety and can lead very quickly to respiratory arrest and death. Pulmonary oedema is most often caused by heart failure, but may also be caused by kidney failure.