Venous pooling

Blood leaving the heart travels down through the arteries to the toes, then returns to the heart through the veins. When you are standing, this return journey goes upward against gravity. The calf muscles help with this. Every time you walk or lift your toes, your calves squeeze the veins, while the venous valves inside prevent blood from flowing back down, keeping it moving only upward.

But when you stand or sit for a long time with very little calf muscle movement, this pump takes a break. Blood then lingers around the feet and ankles and gradually pools there. This is called venous pooling. Rather than a disease, it is a common circulatory phenomenon that can happen to anyone who stays in the same position for a long time.

When the calf muscles do not contract while you are standing, the veins are not compressed, so gravity draws blood down toward the feet and ankles, where it pools. Without rhythmic muscle contractions to push blood upward, the amount of blood returning to the heart decreases. Venous circulation flows more slowly than usual.

If this pooling continues, pressure inside the blood vessels in the lower legs remains high. This pressure causes fluid to gradually leak from the capillaries—the smallest blood vessels—into the spaces between tissues. This can lead to lifestyle-related edema, with swelling in the feet, ankles, and calves. This is usually why shoes that feel loose during the day feel tight in the evening.

Hydrostatic pressure is the physical concept that explains the “pressure” mentioned in the mechanism section. It follows the same principle as a long, upright tube filled with water: pressure is higher toward the bottom. Something similar happens when a person stands. The blood vessels in the legs are full of blood, and the feet and ankles are far below the heart, so gravity increases the pressure on the vessels there. Pressure rises toward the ends of the legs, increasing the amount of fluid that moves out of the capillaries into the tissues.

“Blood pooling” does not mean that blood stops moving entirely. Blood continues to flow, but the conditions of that flow have changed: it moves upward toward the heart more slowly, and pressure is higher lower down. Swollen or heavy legs can have various causes, so it is difficult to draw a firm conclusion about the cause based only on how your legs feel that day.

• When standing for a long time — It is particularly noticeable in jobs that involve standing, such as retail, cooking, hairdressing, and teaching. You can read more in Lower limb strain from occupational standing.
• When sitting for a long time — Sitting at a desk or in a car without being able to move your legs leaves the calf pump inactive. The key is staying in one position for a long time, rather than whether you are standing or sitting.
• Toward evening — As time spent standing and sitting accumulates throughout the day, leg swelling is generally most noticeable in the evening. It also commonly decreases after a night's sleep.
• In hot weather or warm places — Blood vessels widen, allowing blood to pool more easily, and your legs may feel more swollen than usual.

• Stand up frequently. Research has found reduced calf swelling simply from breaking up periods of sitting by standing up. Try to avoid sitting continuously for more than an hour.
• Move your ankles. Even while sitting, doing ankle pump exercises—pulling your toes up and then pointing them down—allows your calf muscles to compress the veins rhythmically. While standing, try rising onto your toes and lowering your heels, or shifting your weight from one foot to the other.
• Elevate your legs. Raising your legs above your heart at the end of the day allows pooled blood to drain more easily. See Leg elevation for how to do this.
• Go for an easy walk. Walking is the easiest way to activate the calf pump.
• Check your clothing and shoes. Clothing that tightly constricts your calves or thighs can interfere with the upward flow of blood. Some people use aids such as compression stockings when they need to stand for long periods.
• Avoid sitting with your legs crossed for a long time. Keeping one leg compressed for a prolonged period slows venous flow on that side even further.

If just one leg suddenly swells with pain or warmth; if swelling does not subside after several days; or if both legs swell along with shortness of breath or a feeling of facial swelling, there may be a cause other than ordinary lifestyle-related pooling. If you notice these signs, take a careful look at your health beyond your usual lifestyle management. For information related to the structure of the veins themselves, see Chronic venous insufficiency.