ST BARTHOLOMEW HOSPITAL / SCIENCE PHOTO LIBRARY ST BARTHOLOMEW HOSPITAL / SCIENCE PHOTO LIBRARY
Pulmonary artery wall. Light micrograph of a section through part of the wall of a normal human pulmonary artery, an elastic artery. This wall is composed of three layers. The pale, innermost tunica intima surrounds the lumen (white). It possesses a fine elastic sheet known as the internal elastic lamina. The middle layer, the thick tunica media (dark pink), is mainly composed of elastic and smooth muscle fibres. This enables it to carry blood under pressure from the heart to the lungs. The external tunica adventitia, mainly composed of connective tissue, is just visible upper left.
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