The hand is vascularized primarily through the ulnar and radial arteries. The radial passes through the wrist near the thumb, and the ulnar on the pinky finger side. Blood is returned through the veins which generally are closer to the surface but more centrally. You probably are familiar with them already as they appear blue though a thin tissues overlying your wrist on the palmar or anterior surface. There is some collateral circulation within the palm. That means that if circulation of one is cut off, the other can provide some circulation to the rest of the hand. When you put a lot of pressure on the base of the hand (resting the weight of your upper body through stiff arms on the handlebars) or squeezing tightly (death grip), you cut off some or all of that circulation. You in effect cause a temporary effect similar to carpal tunnel syndrome. Foam grips help spread out the pressure to avoid too much pressure on any one artery or veins. You can also help reduce this pressure by not deathgripping and by holding your posture erect with your back muscles instead of resting your upper body weight in a forward lean on straight arms. Larger diameter handlebars may also help. Cold weather or cold handlebars cause constriction of the vasculature as well, so keep them warm in cold weather. Source: I'm a physician assistant student senior.