According to experts, your calf muscles — owing to their role in promoting blood circulation in the lower extremities — are often referred to as the ‘second heart’.
Like the heart, which receives venous or deoxygenated (impure) blood from the body and then pumps arterial (pure) blood to the whole body for oxygen supply, the calf muscles play a major role in returning the venous blood from the legs to the heart.
But when these valves become defective or weak, the second heart can get overwhelmed — such that the blood can pool in veins (and the legs due to gravity) — and lead to conditions like varicose veins, spider veins, and Chronic Venous Insufficiency (CVI).
Not exercising the calf muscles, during long flights or drives or sitting at one place without moving for long, thus leads to pooling and clot formation.
This is the reason for deep vein thrombosis in the legs, which can be very dangerous.
The calf muscles act as a venous pump, contracting and relaxing as you move, which helps push blood back up towards the heart against gravity.