There are many different causes of swelling (edema) in the feet and legs. These include vein disease, lymphatic disease, injury and inflammation, as well as heart disease, kidney disease, and fluid retention from certain medications.

The most common cause of lower leg swelling is a disease of the veins. The veins are the part of the circulatory system that returns unoxygenated blood back to the heart, from which it then receives fresh oxygen in the lungs and is then pumped back through the body to deliver nutrients through the arteries. It is the pumping action of the heart, along with muscles in the arteries, that pushes blood to the far reaches of the body. The veins do not have the same pressure in them, and blood has to travel against gravity to get out of the legs on the way back to the heart. Assisting the veins in this problem are a series of one-way valves within the vein lining that stop blood from leaching back into the legs and feet. The leg muscles also help by squeezing the veins when they contract, pumping the blood up past the next series of valves in the veins. Eventually, the blood reaches the heart again through this process of pumping and valve restriction. Some people have faulty valves that do not function properly, and blood pools in the veins of the feet and legs as a result. This can start off subtle, involving only the small superficial veins under the skin surface. This is seen externally as spider veins, also known as telangiectasias. Once the deeper veins begin to fail, they can become varicose, and lumpy dark tubes can be seen under the skin surface. Eventually, this fluid will leach out of the veins and settle in the soft tissue of the leg, causing leg, ankle, and foot swelling. This can increase and decrease depending on the activity or the time of the day, with most people having greater edema at the end of the day. Other symptoms can eventually develop, including skin discoloration from iron pigment leaching out of the blood, as well as skin flakiness, wounds, and pain. Treatment usually involves compression therapy, although some people may need surgical intervention. Diuretic medication (water pills) is also sometimes used.

Related to leg swelling associated with vein disease is swelling of one leg due to a blood clot in the vein. People who have preexisting vein disease, are smokers, are obese, have had surgery, are immobilized or inactive for an extended period of time, have a clotting disorder, or those taking birth control pills are generally at greater risk for developing a clot in a leg vein. If in one of the superficial veins, this clot can produce superficial warmth, limited swelling, mild pain, and skin redness. If in one of the deeper veins, the clot can produce more amplified symptoms, with leg or calf swelling, redness, warmth, and significant pain. In some cases, these classic symptoms may be absent but some sign of inflammation is still usually present. Deep vein clots need immediate medical treatment, as they have the potential to break off and get lodged in the lungs. This can be fatal. Superficial clots should also still be treated, but this can usually be done at home.

Circulation-related swelling due to lymphatic system damage is also common. The lymphatic system is essentially the body's sewer system. It consists of lymph channels and lymph nodes. When fluid is spilled in tissue out of the bloodstream due to the body's response to injury or damage, or some other process, it eventually finds its way back into the general circulation by traveling through lymph channels. These sewer line-like channels pass through the body and drain extra fluid back into the largest vein in the chest before eventually heading to the heart. Along the way, the fluid is filtered through lymph nodes, which essentially remove things that could be harmful to the body if left in the fluid to go to the general circulation again. When either the lymph channels or lymph nodes become blocked or damaged (due to scarring from surgery or deliberate removal in the case of lymph node biopsy), then the fluid will remain more easily trapped in the legs. This can lead to fairly severe swelling of the legs and feet. Along with this swelling, other symptoms that can eventually appear include severe skin disease with wrinkling and oozing of the skin, as well as leg and foot deformity from the intense fluid retention. Specialty treatment is necessary to keep this type of foot and leg swelling under control.

A far more common cause of foot and leg swelling is simply inflammation from either an injury or an infection. Swelling due to these reasons typically occurs in only one part of the foot or leg, except in the case of severe infection. The body responds to injury or the infection of a microscopic organism by delivering healing cells and chemicals to the injured area. With these compounds come fluid and warmth. In the case of infection, the immune system is also activated which increases the swelling further. The acute inflammation process of an injury generally lasts a week and slowly resolves after this time as the injury begins to heal. Rest, ice, leg elevation, and compressive wraps can shorten the overall time of swelling. Infection-related swelling will last until the infection is resolved, and sometimes a little while longer. Antibiotics are needed for treatment, and severe cases may need surgical drainage if a deep abscess is present.

Yet another cause of leg swelling includes organ disease. Heart disease, such as congestive heart failure, can have leg swelling as one of its symptoms. Kidney disease or kidney failure can also produce leg swelling. Treatment revolves around controlling the underlying disease.

Finally, leg swelling can be due to the use of certain medications. There are a great number of medications that list leg swelling as a potential adverse effect. This can be due to a variety of mechanisms, all depending on how the medication interacts with the body and other medications. Usually discontinuing the medication resolves the leg swelling, although it can persist for a little while following the discontinuation.

All the above conditions are the most common causes of leg and foot swelling. There are numerous other causes that are relatively uncommon and are usually specific to certain diseases like cancer, diabetes, or genetic abnormalities. If you have developed swelling in your feet and legs, a medical evaluation is recommended, especially if the swelling is new, due to the number of potential conditions that can cause leg and foot edema.