The membrane that keeps blood and air apart
Gas exchange depends on the alveolar–capillary membrane — a barrier so thin that oxygen and carbon dioxide cross it in a fraction of a second. Its thinness is its gift and its weakness: the same delicacy that lets gases pass freely also lets it fail when pressure rises too high or its lining is injured. Both flooding and bleeding are failures of this one membrane.
Pulmonary edema: fluid where air should be
Pulmonary edema is fluid leaking into the air sacs, so the lung becomes heavy and stiff and oxygen struggles to get through. By far the most common form is cardiogenic pulmonary edema: when the left heart cannot keep up, pressure backs up through the lung's veins and capillaries, and fluid is squeezed out across the membrane — pure plumbing, not infection.
- [[orthopnea|Orthopnea]] — breathlessness when lying flat, relieved by sitting up or piling on pillows, because lying down shifts fluid toward the lungs.
- [[paroxysmal-nocturnal-dyspnea|Paroxysmal nocturnal dyspnea]] — waking abruptly an hour or two into sleep, gasping for air and needing to sit or stand.
- [[crackles|Crackles]] at the lung bases — the popping sound of fluid-filled air sacs reopening with each breath.
- Frothy, sometimes pink sputum — fluid (occasionally blood-tinged) churned up by breathing.
When the air sacs bleed
The opposite failure is diffuse alveolar hemorrhage: blood, not fluid, spilling across the membrane into the air sacs throughout both lungs. It often comes from inflammation that attacks the tiny vessels — some autoimmune diseases, certain drugs, or severe infection. The classic triad is worsening breathlessness, drop in blood count, and patchy shadows across both lungs.
A puzzling feature is that coughing up blood may be absent in up to a third of cases — the blood stays down in the air sacs rather than being coughed up, so a normal-looking cough does not rule it out. This is also a useful reminder that hemoptysis has many causes across this track: a pulmonary embolism with infarction, severe edema, and alveolar hemorrhage can each produce blood, and telling them apart relies on the whole picture rather than the single symptom.