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Heart bypass surgery. Surgeon preparing a harvested leg vein which will be used in coronary artery bypass surgery. The branches leading off this vein are tied off here. The surgeon is cutting the vein to the correct length for the procedure. Bypass surgery is performed when a coronary artery (an artery supplying the heart muscle) is blocked or narrowed. The length of vein is attached to the heart muscle to provide an alternative route for blood flow. Blocked coronary arteries can lead to angina pectoris (heart pain) or a heart attack. (KEYSTONE/SCIENCE PHOTO LIBRARY/ANTONIA REEVE)