The heart is the main pump to get oxygenated blood to tissues via the arteries.  The calf muscle is the main pump to get blood back to the heart via the veins.  To be effective, compression must be graduated.  The strongest is at the ankle and decreases in the opposite direction.  Compression is a prescription from your MD or Nurse Practitioner typically for:  improve blood flow velocity/to prevent and treat reflux/to reduce and prevent edema (swelling)/prevent and/or treat Chronic Venous Disorders.