Contracted elastic artery. Coloured transmission electron micrograph (TEM) of a section through an artery. Red blood cells (erythrocytes) are seen in the lumen (top centre). The lumen is lined with a layer of epithelium (yellow with red nuclei) and an inner elastic membrane (black). These together make up the tunica intima. This is surrounded by a layer of smooth muscle (pink) called the tunica media. The layer of muscle has contracted to decrease the volume of the lumen, causing the tunica intima to become folded. The outer layer of the artery, the tunica adventitia (brown), which contains collagen, is seen at bottom. An outer elastic membrane (black) lies at the junction of the tunica media and adventitia.