Why do blood clots form in hospital patients or patients who are on bed rest? Is there a chemical change?
Normally, blood constantly flows through vessels with the help of body movement and muscle contraction, which squeeze veins and promote blood flow. However, blood can pool in the legs during long periods of immobility. Pooling of the blood can lead to the accumulation of clotting factors and the formation of a clot inside a vessel. Clot formation inside vessels is dangerous because the clots can break off and, for example, go to the lungs, leading to pulmonary embolism, a blockage of an artery in the lung.
National Heart Lung and Blood Institute: