SN6 SHORT NOTE 5M

Describe the histological structure of arteries, veins and capillaries.

Anatomy · 1st Professional — Pre-Clinical

Blood vessels have three coats — tunica intima, media and adventitia — whose relative thickness identifies the vessel.

Feature	Artery	Vein
Lumen	Small, round, patent	Large, irregular, often collapsed
Wall	Thick relative to the lumen	Thin
Thickest coat	Media	Adventitia
Internal elastic lamina	Prominent, often wavy	Absent or poorly developed
Valves	Absent	Present in limb veins
Contents in a section	Usually empty	Often full of blood

Type	Media	Examples
Elastic (conducting)	40 to 70 fenestrated elastic laminae with smooth muscle between	Aorta, pulmonary trunk, brachiocephalic, common carotid, subclavian
Muscular (distributing)	Up to 40 layers of smooth muscle; a very prominent internal elastic lamina	Most named arteries — radial, femoral, splenic
Arteriole	1 to 3 layers of smooth muscle; wall about equal to the lumen	The chief site of peripheral resistance
Metarteriole	Discontinuous muscle; precapillary sphincters	Controls flow into the capillary bed

The elastic laminae give the aorta its "wavy" appearance in a section and are stained black by Verhoeff
Elastic arteries convert pulsatile into steady flow — the Windkessel effect; their recoil in diastole maintains the diastolic pressure

Type	Endothelium	Basement membrane	Sites
Continuous	Unbroken; tight junctions	Continuous	Muscle, lung, skin, CNS (with the tightest junctions)
Fenestrated	Pores 60–80 nm, usually with a diaphragm	Continuous	Intestinal mucosa, endocrine glands, glomerulus (without diaphragms)
Sinusoidal (discontinuous)	Large gaps between cells	Absent or incomplete	Liver, spleen, bone marrow

Vasa vasorum — "vessels of the vessel"; supply the adventitia and the outer media of large vessels, the inner layers being nourished by diffusion from the lumen
More extensive in veins than arteries, since venous blood is less nutritive
Nervi vasorum — sympathetic vasomotor fibres ending in the adventitia and outer media

Atherosclerosis begins in the intima of elastic and large muscular arteries; endothelial dysfunction, lipid deposition and a fibrous cap that may rupture
Arteriosclerosis of arterioles (hyaline and hyperplastic) occurs in hypertension and diabetes, and damages the kidney and retina
Aortic dissection — blood enters the diseased media through an intimal tear; the media is weakened in Marfan syndrome and in hypertension
Syphilitic aortitis attacks the vasa vasorum of the ascending aorta, causing medial necrosis and aneurysm — a direct consequence of the blood supply of the wall
Varicose veins show fibrosis and irregular hypertrophy of the media with incompetent valves
Identifying artery from vein in a section rests on the thickness of the media relative to the lumen and the presence of the internal elastic lamina, not on the size of the vessel

All questions in Systemic Histology