Vascular Anatomy & Physiology
MRCS Part A & B — Comprehensive Study Notes
Histological Layers of Blood Vessels
All blood vessels — except capillaries — share a trilaminar wall structure. The relative proportions of elastin, smooth muscle, and collagen differ profoundly between vessel types, reflecting their specific mechanical demands.
| Layer | Components | Function | Clinical Correlate |
|---|---|---|---|
| Tunica Intima | Single layer of endothelium (resting on a basement membrane of type IV collagen and laminin) + thin subendothelial connective tissue. Internal elastic lamina (IEL) forms the boundary with media in most arteries. | Non-thrombogenic barrier. Releases vasoactive mediators: NO and prostacyclin (vasodilators, platelet-inhibiting), endothelin-1 (vasoconstrictor), von Willebrand factor, tissue plasminogen activator, PAI-1. | Endothelial dysfunction is the initiating step in atherosclerosis. Intimal hyperplasia is the principal cause of late vein graft failure. |
| Tunica Media | Vascular smooth muscle cells (VSMCs) arranged in helical layers around the lumen. Embedded in a matrix of elastin, collagen type III, and proteoglycans. External elastic lamina (EEL) separates media from adventitia. | Vasomotor tone via VSMC contraction/relaxation (controlled by sympathetic innervation and local mediators). Structural integrity and elasticity via elastin/collagen ratio. | Cystic medial necrosis (in Marfan’s, bicuspid aortic valve) weakens the media → aortic dissection. Medial calcification (Mönckeberg sclerosis) stiffens arteries → falsely high ABPI in diabetics. |
| Tunica Adventitia | Loose connective tissue rich in collagen type I, fibroblasts, mast cells, macrophages, vasa vasorum (small nutrient vessels for the outer wall), and nervi vasorum (sympathetic autonomic supply). | Structural anchoring to surrounding tissues. Vasa vasorum supply nutrition to the outer two-thirds of the wall in large vessels. Nervi vasorum regulate VSMC tone. | Disruption of vasa vasorum may contribute to medial ischaemia in aortic dissection. Perivascular inflammation (e.g., inflammatory AAA) is predominantly an adventitial process. |
Vessel Type Comparison — Structural Differences
| Vessel Type | Examples | Dominant Layer / Feature | Function | Disease Susceptibility |
|---|---|---|---|---|
| Elastic (Conducting) Arteries | Aorta, brachiocephalic, common carotid, pulmonary trunk, subclavian | Media rich in 50+ fenestrated elastin lamellae (more elastin than smooth muscle). Thick wall relative to lumen. | Windkessel effect: expand during systole (storing ~50% of stroke volume), then recoil during diastole to maintain forward flow and diastolic pressure. | Aneurysm (elastin degradation by MMPs), dissection (cystic medial necrosis), atherosclerosis at branch ostia |
| Muscular (Distributing) Arteries | Femoral, brachial, radial, coronary, mesenteric | Media with up to 40 layers of smooth muscle. Well-defined IEL and EEL. Less elastin than elastic arteries. | Regulate distribution of blood to organs via active vasoconstriction/vasodilation. Control peripheral resistance. | Atherosclerosis (plaques preferentially at bifurcations/bends), Mönckeberg medial calcification, fibromuscular dysplasia (mid-distal RAS) |
| Arterioles | Pre-capillary arterioles in skeletal muscle, kidney glomerulus | 1–2 layers of smooth muscle. Lumen <300 μm. No IEL. High smooth muscle:lumen ratio. | Principal resistance vessels — responsible for 60–70% of total peripheral resistance. Regulate capillary perfusion pressure. | Hypertensive arteriosclerosis (hyaline arteriosclerosis), thrombotic microangiopathy |
| Capillaries | Systemic and pulmonary beds | Endothelium only (no media or adventitia). Lumen ~8 μm (RBC squeezes through). | Exchange of O₂, CO₂, nutrients, water between blood and tissues (Starling forces). | Diabetic microangiopathy (basement membrane thickening), capillary leak in SIRS/sepsis |
| Veins | Femoral, saphenous, portal, vena cava | Thin, relatively underdeveloped media (thin smooth muscle layer). Collagen-dominant adventitia is the thickest layer. Bicuspid valves every 2–4 cm in limb veins. | Capacitance vessels — contain ~70% of total blood volume at rest. Venous return driven by skeletal muscle pump, respiratory changes, venomotor tone. | Varicose veins (valve incompetence → venous hypertension), DVT (virchow’s triad), CVI with lipodermatosclerosis and venous ulcers |
| Venules | Post-capillary venules in all tissues | Near-endothelium only. Pericytes replace smooth muscle in post-capillary venules. | Primary site of leucocyte extravasation during inflammation (ICAM-1, VCAM-1, selectins expressed here). Post-capillary fluid reabsorption. | Venulitis in vasculitides (particularly small-vessel vasculitis, e.g., HSP) |
Laplace’s Law and Vessel Pathology
Wall tension (T) = Pressure (P) × Radius (r) / Wall thickness (w) — known as the Law of Laplace for hollow cylinders. This explains why: (1) aneurysms enlarge progressively — as radius increases, wall tension rises, further damaging the wall; (2) the aorta has thick walls to withstand high systolic pressure; (3) venous walls are thin because low venous pressure requires less wall tension. In AAA, once the radius exceeds a critical point (~5.5 cm), wall stress exceeds the tensile strength of the weakened elastin matrix and rupture risk increases sharply.
Endothelial Function — Key Mediators
| Mediator | Released By | Action | Clinical Note |
|---|---|---|---|
| Nitric Oxide (NO) | Endothelial cells (eNOS) | Potent vasodilator; inhibits platelet aggregation; inhibits VSMC proliferation | Reduced in hypertension, DM, smoking → endothelial dysfunction → atherosclerosis. GTN is an exogenous NO donor. |
| Prostacyclin (PGI₂) | Endothelial cells (COX pathway) | Vasodilator; inhibits platelet aggregation (↑cAMP in platelets) | Counterbalanced by thromboxane A₂ (from platelets). Iloprost (prostacyclin analogue) used in critical ischaemia and Raynaud’s. |
| Endothelin-1 (ET-1) | Endothelial cells | Powerful vasoconstrictor (ETA receptors on VSMCs); promotes VSMC proliferation | Elevated in pulmonary arterial hypertension, sepsis, CCF. Bosentan (ET-1 receptor antagonist) used in PAH. |
| von Willebrand factor (vWF) | Endothelial cells (Weibel-Palade bodies) | Platelet adhesion (bridges GPIb on platelets to exposed collagen); carries factor VIII | vWF deficiency = von Willebrand disease. High shear stress (stenotic vessels) causes vWF cleavage deficiency → acquired VWD in LVAD patients. |
| Thrombomodulin | Endothelial cells | Binds thrombin → converts protein C to activated protein C (aPC) → inactivates Va and VIIIa → anticoagulation | Loss of endothelial thrombomodulin in inflammation → prothrombotic state. |
| tPA (tissue plasminogen activator) | Endothelial cells | Converts plasminogen → plasmin → fibrinolysis | Recombinant tPA (alteplase) used therapeutically in stroke, PE, peripheral arterial thrombolysis. |
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