Fluid builds up in the body when the normal balance between what pushes fluid out of your blood vessels and what pulls it back in gets disrupted. This condition, called edema, can result from problems with your heart, kidneys, liver, veins, or lymphatic system, as well as from medications, diet, and hormonal changes. Understanding the specific cause matters because the swelling itself is a symptom, not a diagnosis.
How Your Body Normally Controls Fluid
Your smallest blood vessels, the capillaries, constantly exchange fluid with surrounding tissues. Two main forces govern this process: the pressure of blood pushing fluid outward through capillary walls, and proteins in your blood (primarily albumin) pulling fluid back in. When these forces are balanced, fluid moves out at one end of a capillary and gets reabsorbed at the other end. Whatever small amount of fluid remains in the tissues gets picked up by your lymphatic system, a network of vessels that drains excess fluid back into your bloodstream.
Edema develops when something tips this balance. That can happen four ways: pressure inside blood vessels rises too high, protein levels in the blood drop too low, blood vessels become leaky from inflammation or injury, or the lymphatic drainage system gets blocked. Most causes of fluid buildup trace back to one or more of these mechanisms.
Heart Failure
When the heart can’t pump blood efficiently, blood backs up in the veins. This raises pressure inside the capillaries and forces more fluid into surrounding tissues than can be reabsorbed. The location of swelling depends on which side of the heart is affected. Right-sided heart failure typically causes swelling in the legs, ankles, and abdomen. Left-sided failure pushes fluid into the lungs, causing shortness of breath and a condition called pulmonary congestion.
Heart failure also triggers a chain reaction in the kidneys. Reduced blood flow signals the body to activate hormonal systems that retain sodium and water, increasing blood volume further. This creates a cycle: the extra fluid raises venous pressure even more, worsening the congestion. Fatigue, rapid weight gain over a few days, and difficulty breathing when lying flat are common signs that heart-related fluid retention is getting worse.
Kidney Disease
Your kidneys filter blood through tiny clusters of blood vessels called glomeruli. When these filters are damaged, they can leak albumin into the urine instead of keeping it in the bloodstream. Since albumin is the main protein responsible for pulling fluid back into blood vessels, losing it leads to widespread swelling. This pattern, known as nephrotic syndrome, causes severe edema around the eyes, in the ankles and feet, and often noticeable weight gain from retained fluid.
Kidney disease can also impair the organ’s ability to excrete sodium. When sodium accumulates, water follows, expanding blood volume and raising blood pressure. The combination of protein loss and sodium retention makes kidney-related edema particularly stubborn.
Liver Disease
The liver produces most of the albumin in your blood. When the liver is severely scarred (cirrhosis), two things happen simultaneously. First, albumin production drops, reducing the pull that keeps fluid inside blood vessels. Second, scar tissue blocks normal blood flow through the liver, raising pressure in the portal vein, the large vessel that carries blood from the intestines to the liver. This elevated pressure pushes fluid out of blood vessels in the abdomen, causing a distinctive form of fluid accumulation called ascites, where the belly swells noticeably with trapped fluid.
Vein and Lymphatic Problems
Veins in your legs contain one-way valves that push blood upward against gravity. In chronic venous insufficiency, those valves become damaged and blood flows backward, pooling in the lower legs. The increased pressure eventually becomes so high that the tiniest blood vessels burst, and fluid leaks steadily into surrounding tissues. Over time, severe swelling can cause scar tissue to form in the skin and trap even more fluid, leading to skin changes, discoloration, and sometimes ulcers.
Your lymphatic system handles the fluid that capillaries don’t reabsorb. When lymphatic vessels are blocked or damaged, protein-rich fluid accumulates in the tissues. This is called lymphedema, and it has several causes. Cancer surgery that removes lymph nodes is one of the most common in developed countries, particularly after breast cancer treatment, where one study found 33.5% of patients treated with certain chemotherapy drugs developed lymphedema. Radiation therapy can scar lymph vessels. Tumors can grow large enough to physically compress lymphatic drainage. In tropical regions, parasitic worm infections that clog lymph nodes remain the leading cause.
Pitting vs. Non-Pitting Edema
One useful distinction is what happens when you press a finger into the swollen area. If the pressure leaves a temporary dent, that’s pitting edema, which typically signals fluid with low protein content. This is the kind you see with heart failure, vein problems, or kidney disease. If the skin springs back immediately with no indentation, that’s non-pitting edema, which is more characteristic of advanced lymphedema, where the tissues have become thickened and fibrotic. Early lymphedema can still pit, though, so a dent doesn’t rule it out.
Medications That Cause Fluid Retention
Several widely prescribed drug classes cause edema as a side effect, and this is a more common culprit than many people realize.
- Blood pressure medications (calcium channel blockers): About 25% of users develop peripheral edema, with rates climbing as high as 80% at high doses over extended periods. Swelling typically appears in the ankles, feet, and lower legs.
- Anti-inflammatory painkillers (NSAIDs): Ibuprofen, naproxen, and similar drugs cause visible edema in 3% to 5% of users by reducing the kidney’s ability to excrete sodium.
- Steroids: Edema affects roughly 20% of corticosteroid users. Doses taken daily for more than a year substantially increase the risk.
- Nerve pain medications: Gabapentin and pregabalin cause edema in 5% to 8% of users.
- Certain antidepressants: Escitalopram, mirtazapine, paroxetine, and venlafaxine are the ones most associated with swelling.
- Hormonal contraceptives: Oral contraceptives cause sodium and fluid retention in about 5% of users.
- Diabetes medications (thiazolidinediones): These cause mild edema, mostly in the arms and legs.
If you notice new swelling after starting a medication, the timing is often the biggest clue. Drug-related edema usually improves when the medication is adjusted or changed.
Dietary Sodium
Sodium directly attracts water. A high-sodium diet draws extra water into the bloodstream, increasing blood volume and raising pressure in your vessels. The recommended daily limit is less than 2,300 milligrams, roughly one teaspoon of table salt. Most people consume well above that, primarily from processed and restaurant foods rather than the salt shaker. In someone whose heart, kidneys, or veins are already compromised, excess sodium can be the difference between manageable swelling and a sudden flare.
Pregnancy and Preeclampsia
Mild swelling in the feet and ankles is normal during pregnancy, especially in the third trimester, as the growing uterus puts pressure on pelvic veins and blood volume increases. Sudden or severe swelling in the hands and face, however, can signal preeclampsia, a serious condition diagnosed by high blood pressure (140/90 or above) and protein in the urine after 20 weeks of pregnancy. Preeclampsia affects blood vessel function throughout the body, causing them to leak fluid into tissues. It requires medical monitoring because it can progress to dangerous complications for both mother and baby.
Other Contributing Factors
Prolonged sitting or standing allows gravity to pull fluid into the lower legs, which is why ankles often swell on long flights or after a full day on your feet. This is usually temporary and resolves with movement and elevation. Severe malnutrition or very low-protein diets can reduce albumin levels enough to cause generalized swelling, a pattern sometimes seen in eating disorders or conditions that impair nutrient absorption. Burns, allergic reactions, and infections cause localized edema by making blood vessel walls more permeable, allowing fluid and immune cells to flood into damaged tissue as part of the inflammatory response.
Blood clots in the deep veins of the leg (deep vein thrombosis) can cause sudden swelling in one leg by physically obstructing blood flow. This type of swelling tends to come on quickly, often with pain, warmth, or redness, and needs prompt evaluation because the clot can travel to the lungs.