Water retention is annoying when it is mild, like during a woman’s period or after standing for a long time. But a fluid buildup, called edema, can also be much more severe and a sign that something is going wrong in your body. Organ failure, malnutrition, or even a negative reaction to high altitude can all cause swelling both inside and out.

The U.S. National Library of Medicine describes edema as a part of the body swelling because fluid has gathered in the tissue, most commonly in the arms, legs, ankles and feet. That “peripheral edema” shows early signs of “a fully or heavy feeling” in the limb, swelling that when pressed with a finger leaves a dent, a tight or warm feeling in the skin, hindered mobility, and pain. The Mayo Clinic adds “stretched or shiny skin” as a symptom of the condition.

Although uncomfortable, it can be temporary and resolve itself, as it does when limbs swell in people who have sat on a long flight or eaten too much salt or when menstruating women become bloated. But it can also appear in people with serious health issues, like heart and circulatory problems. The library of medicine says congestive heart failure makes the heart too weak to pump blood, so blood builds up around the heart and leaks into surrounding tissue in the abdomen, causing swelling, or affects the legs. The swelling in the feet and ankles can also come from veins that are not properly transporting blood back up to the chest, causing the local oversupply to leak into tissue.

People with liver diseases may also see swelling in their abdomen because the organ’s tissue is so scarred or inflamed that it becomes congested and pressure in the blood vessels increases, leading to fluid seepage. Malnutrition has the same swelling effect because the body does not have enough of the protein albumin in its blood to shore up the blood vessels. And kidney disease prevents the kidneys from removing salt and water from the body, resulting in a buildup and increased vessel pressure. In that case, the legs could become swollen or even the area around the eyes.

That’s not the only kind of swelling that can strike the eyes. The National Eye Institute describes macular edema as a fluid buildup in the middle of the retina, which is the light-sensitive tissue at the back of the eye that allows us to see. When edema causes the center of it to swell and thicken — such as through a complication of diabetes, an inflammatory disease or other conditions — your sight will be distorted, with “blurry or wavy vision near or in the center of your field of vision. Colors might also appear washed out or faded.”

One type of edema that is dangerous in and of itself, not just due to its underlying condition, is in the lungs. The life-threatening pulmonary edema occurs when the lungs fill up with fluid “because the left side of the heart is not strong enough to pump the blood returning from the lungs,” according to the U.S. National Library of Medicine. When the backed up blood causes fluid to seep into lung tissue and blocks oxygen from being absorbed, people can experience shortness of breath, difficulty breathing, coughing and chest pain. Usually this is the result of heart-related problems, but it doesn’t have to be. The Mayo Clinic lists trauma, infection, a bad reaction to drugs, smoke inhalation and a near-drowning as factors.

High altitudes are another cause because the capillaries in the lungs become constricted or weakened: “Mountain climbers and people who live in or travel to high-altitude locations run the risk of developing high-altitude pulmonary edema,” the organization says. “This condition — which generally 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, which can take from a few days to a few weeks. But even people who have hiked or skied at high altitudes in the past aren't immune.”

For other persistent kinds of edema outside the lungs, leaving it untreated can also be dangerous. The Mayo Clinic says the swelling will become more painful and may cause stiffness and difficulty walking, and the person could be at an increased risk of infection and skin ulcers, experience decreased circulation, and have tissue scarring.