eli5: how does heart failure cause fluid build up?

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My husband just spent 5 days in the hospital getting IV diaretics to remove fluid …lost 10 lbs. He has congestive heart failure. I know that the blood and ergo the oxygen is not flowing freely, by how does this cause fluid buildup and what IS the fluid. How is this different from fluid in lungs from pneumonia?

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The right side of the heart is a drain for fluid from the body. It takes fluid from the body as its input, and then pumps this fluid to the lungs as its output.

The left side of the heart is a drain for fluid from the lungs. It takes fluid from the lungs as its input, and then pumps this fluid to the body as its output.

What happens if you clog a drain? The fluid gets backed up.

In right-sided heart failure, fluid gets backed up in the body. This causes swelling of the legs, ankles, feet, or abdomen. Usually lower extremities are more affected than upper extremities because of gravity pulling fluid down.

In left-sided heart failure, fluid gets backed up in the lungs. This causes shortness of breath, coughing (often a productive cough with mucus/phlegm), and difficulty breathing when lying flat.

What is this fluid? The short answer is interstitial fluid. Most cells in the body are bathing in interstitial fluid all the time. This fluid is where cells get their nutrients and oxygen. This fluid is also where cells dump their wastes and carbon dioxide. All day long, this fluid circulates around cells to give them what they need and remove what they do not need. This fluid enters and exits the circulatory system by passing filters called capillaries.

The fluid build-up in heart failure is edema. It is called pulmonary edema when in the lungs, and peripheral edema when in the extremities. At a microscopic level, edema caused by heart failure is mostly interstitial fluid. It can build up in the lungs when it leaks out of microscopic capillaries in the lungs, since the drain is backed up.

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