Your lungs contain numerous small, elastic air sacs called alveoli. With each breath, these air sacs take in oxygen and release carbon dioxide. Normally, the exchange of gases takes place without problems.

But in certain circumstances, the alveoli fill with fluid instead of air, preventing oxygen from being absorbed into your bloodstream. A number of things can cause fluid to accumulate in your lungs, but most have to do with your heart (cardiac pulmonary edema). Understanding the relationship between your heart and lungs can help explain why.

How your heart works

Your heart is composed of two upper and two lower chambers. The upper chambers (the right and left atria) receive incoming blood and pump it into the lower chambers. The lower chambers, the more muscular right and left ventricles, pump blood out of your heart. The heart valves — which keep blood flowing in the correct direction — are gates at the chamber openings.

Normally, deoxygenated blood from all over your body enters the right atrium and flows into the right ventricle, where it's pumped through large blood vessels (pulmonary arteries) to your lungs. There, the blood releases carbon dioxide and picks up oxygen. The oxygen-rich blood then returns to the left atrium through the pulmonary veins, flows through the mitral valve into the left ventricle, and finally leaves your heart through another large artery, the aorta. The aortic valve at the base of the aorta keeps the blood from flowing backward into your heart. From the aorta, the blood travels to the rest of your body.

Heart-related (cardiac) pulmonary edema

Cardiac pulmonary edema — also known as congestive heart failure — occurs when the diseased or overworked left ventricle isn't able to pump out enough of the blood it receives from your lungs. As a result, pressure increases inside the left atrium and then in the veins and capillaries in your lungs, causing fluid to be pushed through the capillary walls into the air sacs.

Congestive heart failure can also occur when the right ventricle is unable to overcome increased pressure in the pulmonary artery, which usually results from left heart failure, chronic lung disease or high blood pressure in the pulmonary artery (pulmonary hypertension).

Medical conditions that can cause the left ventricle to become weak and eventually fail include:

  • Coronary artery disease. Over time, the arteries that supply blood to your heart can become narrow from fatty deposits (plaques). A heart attack occurs when a blood clot forms in one of these narrowed arteries, blocking blood flow and damaging the portion of your heart muscle supplied by that artery. The result is that the damaged heart muscle can no longer pump as well as it should.

    Although the rest of your heart tries to compensate for this loss, either it's unable to do so effectively or it's weakened by the extra workload. When the pumping action of your heart is weakened, blood backs up into your lungs, forcing fluid in your blood to pass through the capillary walls into the air sacs.

  • Cardiomyopathy. When your heart muscle is damaged by causes other than blood flow problems, the condition is called cardiomyopathy. Because cardiomyopathy weakens the left ventricle — your heart's main pump — your heart may not be able to respond to conditions that require it to work harder, such as a surge in blood pressure, a faster heartbeat with exertion, or using too much salt that causes water retention or infections. When the left ventricle can't keep up with the demands placed on it, fluid backs up into your lungs.
  • Heart valve problems. In mitral valve disease or aortic valve disease, the valves that regulate blood flow in the left side of your heart either don't open wide enough (stenosis) or don't close completely (insufficiency). This allows blood to flow backward through the valve. When the valves are narrowed, blood can't flow freely into your heart and pressure in the left ventricle builds up, causing the left ventricle to work harder and harder with each contraction. The left ventricle also dilates to allow more blood flow, but this makes the left ventricle's pumping action less efficient. Because it's working so much harder, the left ventricle eventually thickens, which puts greater stress on the coronary arteries, further weakening the left ventricular muscle.

    The increased pressure extends into the left atrium and then to the pulmonary veins, causing fluid to accumulate in your lungs. On the other hand, if the mitral valve leaks, some blood is backwashed toward your lung each time your heart pumps. If the leakage develops suddenly, you may develop sudden and severe pulmonary edema.

  • High blood pressure (hypertension). Untreated or uncontrolled high blood pressure causes a thickening of the left ventricular muscle, and worsening of coronary artery disease.

Noncardiac pulmonary edema

Not all pulmonary edema is the result of heart disease. Fluid may also leak from the capillaries in your lungs' air sacs because the capillaries themselves become more permeable or leaky, even without the buildup of back pressure from your heart. In that case, the condition is known as noncardiac pulmonary edema because your heart isn't the cause of the problem. Some factors that can cause noncardiac pulmonary edema are:

  • Lung infections. When pulmonary edema results from lung infections, such as pneumonia, the edema occurs only in the part of your lung that's swollen.
  • Exposure to certain toxins. These include toxins you inhale — such as chlorine or ammonia — as well as those that may circulate within your own body, for example, if you inhale some of your stomach contents when you vomit.
  • Kidney disease. When your kidneys can't remove waste effectively, excess fluid can build up, causing overload pulmonary edema.
  • Smoke inhalation. Smoke from a fire contains chemicals that damage the membrane between the air sacs and the capillaries, allowing fluid to enter your lungs.
  • Adverse drug reaction. Many drugs — ranging from illegal drugs such as heroin and cocaine to aspirin and chemotherapy drugs — are known to cause noncardiac pulmonary edema.
  • Acute respiratory distress syndrome (ARDS). This serious disorder occurs when your lungs suddenly fill with fluid and inflammatory white blood cells. Many conditions can cause ARDS, including severe injuries (trauma), systemic infection (sepsis), pneumonia and shock.
  • High altitudes. Mountain climbers and people who live in or travel to high-altitude locations run the risk of developing high-altitude pulmonary edema (HAPE). This condition — which typically occurs at elevations above 8,000 feet (about 2,400 meters) — can also affect hikers or skiers who start exercising at higher altitudes without first becoming acclimated. But even people who have hiked or skied at high altitudes in the past aren't immune.

    Although the exact cause isn't completely understood, HAPE seems to develop as a result of increased pressure from constriction of the pulmonary capillaries. Without appropriate care, HAPE can be fatal.

  • Near drowning. Inhaling water causes noncardiac pulmonary edema that is reversible with immediate attention.
Jul. 29, 2011