Light micrograph of a cross-section of a normal human coronary artery, which supplies blood to the heart. The micrograph shows the arterial wall as a circle of pink-stained tissue, 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 & the walls expand; at diastole, the walls contract & push the blood forward through the artery. The walls consist of elastic fibres & smooth muscle, the latter of which is under the control of the sympathetic nervous system.