Venous stasis is the slowing or pooling of blood in the veins, most commonly in the lower legs. It happens when the one-way valves inside your veins stop working properly, allowing blood to flow backward and collect instead of returning efficiently to the heart. Over time, this persistent pooling raises pressure inside the veins and can lead to swelling, skin changes, and in severe cases, open wounds called venous ulcers.
How Blood Pools in the Legs
Your veins rely on a combination of small internal valves, muscle contractions, and pressure gradients to push blood upward against gravity. The calf muscle pump is a major driver of this process. Every time you flex your calf (walking, standing on your toes, climbing stairs), it squeezes blood out of the deep veins and sends it toward the heart. One-way valves snap shut behind the blood to prevent it from falling back down.
When those valves weaken, widen, or get damaged, blood leaks backward. This creates a cycle: the pooled blood stretches the vein walls further, which makes the valves fit together even more poorly, which allows more blood to pool. The resting pressure inside your leg veins climbs, and the condition gradually worsens.
Valve failure can happen in three places. Superficial veins near the skin surface often fail because the valves are misshapen or the vein has stretched too wide for the valve leaflets to meet. Deep veins usually lose function after a blood clot (DVT) causes inflammation that scars and stiffens the valves. Perforator veins, which connect the deep and superficial systems, can also fail and allow high-pressure blood to flood into smaller surface veins that aren’t built to handle it.
What Causes It
About 70% of cases are considered “primary,” meaning no single triggering event caused them. Instead, the vein walls gradually lose structural integrity over time. Research has identified reduced elastin content, increased tissue remodeling, and chronic low-grade inflammation in these veins, all of which promote stretching and valve failure. The remaining 30% of cases are “secondary,” typically triggered by a prior DVT that damages the vein from the inside.
A population-based study identified several independent risk factors: advancing age, higher body mass index, prior DVT (especially in the left leg), longer time since the clot occurred, and existing varicose veins. Obesity likely contributes by raising central venous pressure, which makes it harder for blood to return from the legs. Prolonged immobility is another major factor. A meta-analysis found that lower limb immobilization after trauma or surgery carries up to a 40% risk of DVT, and in one study, 33% of patients developed a clot after just two weeks of leg immobilization.
Symptoms at Each Stage
Venous stasis progresses through recognizable stages. Early on, you might notice small spider veins or slightly enlarged reticular veins near the skin surface. These are cosmetic and painless for most people. As the condition advances, veins dilate to 3 mm or more and become visibly raised varicose veins. At this point, many people start experiencing heaviness, aching, or fatigue in the legs, especially after long periods of standing or sitting.
Persistent swelling (edema) comes next, usually around the ankles and lower calves. The swelling tends to worsen throughout the day and improve overnight when the legs are elevated. Left untreated, the high venous pressure forces red blood cells out of tiny capillaries and into the surrounding tissue. As those cells break down, they release hemoglobin, which the body converts into an iron-storage compound called hemosiderin. This hemosiderin, along with increased melanin production, stains the skin a brownish or reddish-brown color, typically around the inner ankle. The discoloration is permanent in many cases.
In later stages, the skin and the fat layer beneath it harden and thicken, a process called lipodermatosclerosis. The skin may feel woody or tight. The weakened, poorly nourished tissue eventually becomes vulnerable to breakdown, and venous ulcers can form. These ulcers typically appear on the inner lower leg as shallow, irregularly shaped wounds with well-defined edges and a fibrinous (yellowish-white) base. They heal slowly because the same poor circulation that caused them also starves the tissue of oxygen and nutrients.
How Venous Ulcers Differ From Other Leg Wounds
Not all leg ulcers are the same, and the differences matter for getting the right treatment. Venous ulcers sit on the inner lower leg, are relatively shallow, and the surrounding skin often shows the hallmark brown discoloration and swelling of chronic venous disease. Arterial ulcers, caused by poor blood supply rather than poor blood return, tend to appear on the toes, top of the foot, or front of the shin. They look deeper and drier, and the surrounding skin feels cold with weak pulses. Diabetic ulcers form over bony pressure points on the sole of the foot, often surrounded by callused skin, and result from nerve damage that prevents the person from feeling injury.
Compression Therapy
Compression stockings are the cornerstone of managing venous stasis. They work by applying graduated pressure, tightest at the ankle and decreasing toward the knee, to counteract the effects of gravity and support the calf muscle pump. The right pressure level depends on the severity of your condition.
For people with early-stage disease or occupational swelling from prolonged sitting or standing, light compression in the 10 to 15 mmHg range is effective at preventing edema and reducing leg complaints. A systematic review found this pressure level works well for prevention, while lower pressures are ineffective and higher pressures may offer no additional benefit for this group. Stockings in the 15 to 20 mmHg range provide moderate support for people with visible varicose veins or mild swelling. For more advanced disease with significant edema, skin changes, or healed ulcers, 20 to 30 mmHg or higher is typically recommended.
Calf-length stockings with pressures between 11 and 21 mmHg can reduce or completely prevent edema in people whose jobs require long periods of sitting or standing. The key is wearing them consistently. Stockings that stay in a drawer don’t help.
Keeping Blood Moving
Because the calf muscle pump is so central to venous return, anything that activates it helps counteract stasis. Regular walking is the simplest intervention. Calf raises, ankle circles, and flexing your feet while seated all engage the pump to some degree. If you have a desk job, getting up and walking for a few minutes every hour makes a measurable difference in how much fluid accumulates in your lower legs by the end of the day.
Elevating your legs above heart level when resting allows gravity to assist venous return instead of working against it. This is especially useful in the evening when swelling peaks. Even propping your legs on a footstool while sitting reduces venous pressure compared to having them flat on the floor.
Preventing Blood Clots During Immobility
Hospitalization and surgery create a perfect environment for venous stasis. Prolonged bed rest slows blood flow dramatically, and surgical injury to blood vessels adds a second risk factor. Hospitals use two main strategies to prevent clots in these situations: blood-thinning medications that reduce the blood’s tendency to clot, and mechanical devices like intermittent pneumatic compression sleeves that inflate and deflate around the calves to mimic the muscle pump. Both approaches require consistent use to work, and many hospitals now build clot-prevention assessments into routine patient care, reassessing risk throughout a hospital stay.
Outside the hospital, long flights, car rides, or any period of forced immobility carry similar (though lower) risks. Moving your ankles frequently, staying hydrated, and wearing compression stockings during travel all help keep blood flowing.