Expansion
Tunica intima, media and adventitia, in differing proportions by vessel type
Mnemonic
Three coats, from the lumen out:
- Tunica intima - endothelium plus a thin subendothelial layer and the internal elastic lamina
- Tunica media - smooth muscle and elastic tissue. The layer that varies most
- Tunica adventitia - collagen, containing the vasa vasorum and nerves
“The media makes the difference”:
- Elastic (conducting) arteries such as the aorta have an elastin rich media, giving the Windkessel effect that converts pulsatile flow into continuous flow
- Muscular (distributing) arteries have a muscle rich media
- Arterioles are almost all muscle, and are the main site of peripheral resistance
- Veins have a thin media, a wide lumen and valves, and hold about 65 per cent of the blood volume as a capacitance reservoir
- Capillaries have endothelium and basement membrane only
Aortic dissection is a tear in the intima allowing blood into the media, which is why connective tissue disorders such as Marfan’s and Ehlers-Danlos predispose to it.
Expansion
Three layers
- Tunica intima: endothelium, basement membrane, subendothelial connective tissue, and the internal elastic lamina
- Tunica media: smooth muscle and elastic tissue; the external elastic lamina bounds it
- Tunica adventitia: connective tissue, vasa vasorum and nerves
By vessel type
| Vessel | Distinguishing feature |
|---|---|
| Elastic (conducting) artery | Aorta and great vessels: media rich in elastic lamellae for the Windkessel effect |
| Muscular (distributing) artery | Prominent smooth muscle media, clear internal elastic lamina |
| Arteriole | One to three layers of smooth muscle; the resistance vessels |
| Capillary | Endothelium and basement membrane only, with pericytes |
| Venule and vein | Thin media, wide lumen, prominent adventitia, valves in limb veins |
Capillary types are worth distinguishing: continuous (muscle, brain), fenestrated (gut, endocrine, glomerulus) and sinusoidal (liver, spleen, marrow), in increasing order of permeability.
Vasa vasorum supply the outer media of large vessels, and their obliteration in tertiary syphilis causes the thoracic aortic aneurysm.