Page 3 of 5

AN69.1-3 | Blood Vessels — Part 2

Arterioles, Capillaries, and Venules

Arterioles (diameter 10–100 µm):
- Tunica intima: endothelium only (no subendothelial layer; no distinct elastic lamina in the smallest arterioles)
- Tunica media: 1–3 layers of smooth muscle — this makes arterioles the primary resistance vessels controlling blood pressure
- Adventitia: thin

Capillaries (diameter 7–10 µm — barely large enough for a red cell):
- No tunica media or adventitia
- Wall = single layer of endothelial cells + basement membrane + scattered pericytes (contractile cells that regulate capillary blood flow)
- Three structural types:
1. Continuous capillaries — tight junctions between endothelial cells; found in muscle, lung, skin, CNS (blood-brain barrier). Most restrictive.
2. Fenestrated capillaries — endothelial cells have pores (fenestrae) bridged by a thin diaphragm; found in kidney glomerulus, intestinal villi, endocrine glands. Allow rapid exchange.
3. Sinusoidal capillaries (sinusoids) — large diameter, incomplete basement membrane, gaps between endothelial cells; found in liver, spleen, bone marrow. Allow passage of cells and large proteins.

Venules (10–100 µm):
- Pericyte-covered endothelium; thin or absent smooth muscle
- Site of leukocyte diapedesis (white cell emigration) during inflammation — postcapillary venules are most permeable

Veins (compared to arteries of similar diameter):
- Larger lumen, thinner wall, less organised layers
- Tunica media: sparse smooth muscle, more collagen → veins collapse when empty; arteries remain patent
- Intraluminal valves in veins of limbs (absent in veins of head, neck, thoracic viscera) — prevent retrograde flow against gravity

Ultrastructure of Blood Vessels (AN69.3)

Under electron microscopy, blood vessel walls reveal key ultrastructural features:

Endothelial cell ultrastructure:
- Tight junctions (zonula occludens) between adjacent endothelial cells — the structural basis of the blood-brain barrier and selective permeability. Absent or incomplete in sinusoids.
- Weibel-Palade bodies — elongated membrane-bound organelles unique to endothelial cells; store von Willebrand factor (coagulation) and P-selectin (leukocyte adhesion)
- Caveolae — flask-shaped invaginations of the plasma membrane facilitating transcytosis (transport of molecules across the endothelial cell)
- Endothelial cells contain intermediate filaments (vimentin) and actin microfilaments enabling shape change

Basement membrane (basal lamina):
- Produced by endothelial cells and pericytes/smooth muscle
- Components: type IV collagen (structural scaffold), laminin (cell adhesion), heparan sulphate proteoglycans (charge barrier in kidney)
- Thickened in diabetes (a major cause of diabetic microangiopathy)

Smooth muscle cell ultrastructure (tunica media):
- Spindle-shaped cells with central oval nucleus
- Contractile filaments: actin + myosin (no striations — smooth muscle)
- Cells linked by gap junctions enabling coordinated vasoconstriction
- Dense bodies — anchoring structures for actin filaments (analogous to Z-discs in striated muscle)

Elastic fibres:
- Core of elastin protein surrounded by fibrillin microfibrils
- Fenestrations in elastic lamellae allow diffusion across the media