📝 STANDARD DESCRIPTION:
3 Layers in vessels & heart:
• Tunica intima — endothelium + thin subendothelial CT.
• Tunica media — smooth muscle ± elastic fibres (thick in arteries).
• Tunica adventitia / externa — CT.
HEART layers: Endocardium (= intima), Myocardium (= cardiac muscle), Epicardium (= visceral pericardium).
Vessel types:
• Elastic (large) arteries — many elastic laminae (aorta).
• Muscular (medium) arteries — thick smooth muscle; distribution arteries.
• Arterioles — small; control resistance.
• Capillaries — single endothelial layer; site of exchange.
• Venules + Veins — thinner walls, larger lumen, valves.
📚 DETAILED DESCRIPTION:
CIRCULATORY SYSTEM HISTOLOGY
I. HEART WALL:
• Endocardium = endothelium + subendothelial CT + subendocardial layer (containing Purkinje fibres).
• Myocardium = cardiac muscle (the bulk).
• Epicardium = visceral pericardium = mesothelium + CT + adipose with coronary vessels.
Conduction system histology:
• SA node + AV node — small modified cardiomyocytes with few myofibrils; pace-setting.
• Purkinje fibres — large pale-staining cells with peripheral myofibrils; lots of glycogen → 'empty' look on H&E. Conduct rapidly.
II. BLOOD VESSEL WALL — 3 LAYERS:
1. Tunica Intima:
• Endothelium — single layer of squamous cells lining lumen; very thrombogenic if damaged.
• Subendothelial CT.
• Internal elastic lamina (IEL) — wavy fenestrated sheet between intima and media (clearly visible in muscular arteries on H&E).
2. Tunica Media:
• Smooth muscle in concentric layers + elastic fibres.
• Thickness defines vessel type.
• External elastic lamina (EEL) often visible at outer edge.
3. Tunica Adventitia (Externa):
• Loose CT with collagen + elastic fibres.
• Contains vasa vasorum (vessels of vessels) supplying outer wall of large vessels.
• Nervi vasorum (nerves).
III. VESSEL TYPES:
A. ARTERIES:
• Elastic (Conducting) Arteries: aorta, brachiocephalic, common carotid, pulmonary trunk. Tunica media has many concentric ELASTIC LAMINAE (visible as wavy lines on H&E). Smooth muscle between laminae. Recoil during diastole drives blood forward.
• Muscular (Distributing) Arteries: most named arteries. Tunica media is mostly SMOOTH MUSCLE (~30 layers). Prominent IEL + EEL. Diameter 1-10 mm.
• Arterioles: <0.1 mm; 1-3 layers smooth muscle. Major resistance vessels — control blood flow & pressure.
B. CAPILLARIES:
• Smallest vessels (~5-10 μm); RBCs squeeze through single file.
• Single layer of endothelium + basement membrane + occasional pericyte.
• 3 types:
– Continuous — endothelium intact; tight junctions; muscle, brain, lung, skin.
– Fenestrated — small windows; kidney glomerulus, gut mucosa, endocrine glands.
– Sinusoidal (discontinuous) — large gaps; liver, spleen, bone marrow.
C. VEINS:
• Larger lumen + thinner walls than companion arteries.
• Three tunics still present but media is THINNER and adventitia is THICKEST.
• VALVES — semilunar folds of intima; prevent backflow (well developed in limb veins).
D. LYMPHATIC VESSELS:
• Begin as blind-ended capillaries with very thin endothelium.
• Carry colourless lymph + chyle.
• Larger lymphatics have valves + thin smooth muscle wall.
// COMPARATIVE ANALYSIS
| ALL |
Same plan; size of vessels scales with body size. Equine aorta is famously large (matches cardiac output). |
| BIRD |
Right aortic arch (mammals = left). |
CLINICAL RELEVANCE
Atherosclerosis — intimal lipid plaques in muscular/elastic arteries. Aneurysm: Weakened wall ballooning; risk of rupture. Thrombus / Embolus: Vessel occlusion.
