Swelling and stiffness in the lower legs are caused by increased fluid accumulation in the tissues, for a number of related physiological conditions and diseases. The fluids in the body are divided into intracellular fluid, which exists inside cells, and extracellular fluid, which exists outside cells. Extracellular fluid exists between tissue cells as tissue fluid and in the blood as plasma. Tissue fluid is formed by the filtration of blood plasma through the capillary walls and is basically the same as blood plasma in composition except that it does not contain large molecules of protein. The main cause of swelling and stiffness in the lower leg is an increase in intertissue fluid due to various factors. The main causes of fluid from blood into intertissue fluid include passive leakage of blood into tissue fluid, active leakage of blood into tissue fluid, and inability of tissue fluid to flow back. When there is an increase in the amount of blood circulating in the body, a decrease in the discharge of fluid, a decrease in the concentration of blood, an increase in the concentration of tissue fluid, or an increase in vascular pressure due to blockage of blood vessels in the lower extremities, fluid enters the tissue fluid from the blood, increasing the tissue fluid and causing swelling in the lower legs, such as cardiac insufficiency, renal insufficiency, and thromboembolism. When various inflammatory conditions occur in the lower leg blood actively exudes into the tissue fluid causing an increase in tissue fluid and swelling of the lower leg, such as dengue, cellulitis, etc. When there is blockage of lymphatic vessels in the calf, etc., tissue fluid cannot be returned and fluid accumulates in the tissues, which can also lead to swelling of the calf. Calf swelling and stiffness may be caused by a variety of reasons for increased tissue fluid. When swelling, stiffness and pain in the calf occur, it is important to seek medical attention to identify the cause and treat it as soon as possible.