Some of the symptoms are subtle and subjective, and therefore might be missed by an untrained person. The assumption is, that if these symptoms are detectable past a certain stage by an untrained person, an emergency situation probably exists. The heart functions in two ways. It pushes blood out through the arteries, and sucks it in from the veins. The greatest pressure is on the arterial side.
Pressure is minimal on the venous side.
The greater volume of blood builds up on the venous side, waiting to be pumped back through. When the heart is failing to function up to a certain level of efficiency, there is a buildup on the venous side in excess of what the heart can draw in. This is the congestion that is referred to in congestive heart failure. Location of the congestion varies, and can be an indication of where the defect of the heart lies. One form, which Coco had, results in clear fluid leaking into the pleural space, outside the lobes of the lung, but inside the pleura which contains the lung.