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How can heart problems lead to shortness of breath from fluid in the lungs?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — respiratory disease The bronchioles of the lungs are the site where oxygen is exchanged for carbon dioxide during the process of respiration. Inflammation, infection, or obstruction of the bronchioles is often associated with acute or chronic respiratory disease, including bronchiectasis, pneumonia, and lung abscesses.

When the heart is unable to pump blood efficiently, it can lead to fluid accumulation in the lungs, a condition known as pulmonary edema. This process is most commonly associated with left-sided heart failure, where the left ventricle fails to pump sufficient blood into the general circulation.12

As the heart struggles, blood backs up into the pulmonary vessels that deliver oxygenated blood to the heart. This creates high pressure within the alveolar capillaries, causing them to distend. Eventually, this pressure becomes so great that fluid—typically blood plasma—is forced out of the capillary walls and into the alveoli, the tiny air sacs responsible for gas exchange.13

This fluid buildup stiffens the lungs and impairs the exchange of oxygen and carbon dioxide, which directly increases the work of breathing and results in shortness of breath (dyspnea). Symptoms often worsen during physical exertion or when lying down, as the latter position mobilizes fluid into the bloodstream, further increasing the load on the heart.423