Final answer:
Pulmonary edema involves excess fluid in the lung's air sacs, often due to heart failure that causes increased pressure in the pulmonary capillaries, pushing fluid into lung tissues.
Step-by-step explanation:
Pulmonary edema is the condition where there is excess fluid in the air sacs of the lungs. In cases resulting from heart failure, this is due to a chain reaction that begins with the left ventricle's inability to pump blood effectively. This inefficiency leads to an accumulation of blood in the pulmonary circulation, starting from the left atrium, then to the pulmonary veins, and subsequently into the pulmonary capillaries. The increased pressure within these capillaries forces fluid out, which then infiltrates the lung tissues, hindering proper gas exchange.