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"Coronary artery. Computer coloured light micro- graph of a cross-section though a healthy human coronary artery. The coronary artery is found on the surface of the heart and supplies blood to the heart muscle. Heart muscle is coloured blue. The artery wall made of elastic fibres and smooth muscle appears red, surrounding the lumen (or space) at centre. The lumen is clear allowing for the free passage of oxygenated blood. Arterial walls are made to withstand the pressure exerted by the pumping action of the heart. At systole, blood is forced into the arteries and the walls expand; at diastole, the walls contract and push blood forward through the artery." (KEYSTONE/SCIENCE PHOTO LIBRARY/SCIENCE PHOTO LIBRARY)