L28: Histology And Functions Of Blood Vessels Flashcards
Function of circulatory system
- Transport blood, nutrients, gases and waste
- Regulate blood flow and BP
- Protect from infection
- Thermoregulation
- Fluid balance
Histological organisation of Heart
Epicardium (Visceral pericardium)
- mesothelium (fibrous CT, nerves, coronary vessels, adipose tissue)
Myocardium
- cardiac muscle + vasculature
Endocardium
- endothelium (向住心內)
- sub-endothelial CT
- middle CT
- smooth muscle
- sub-endocardium (Purkinje fibres, nerve fibres)
Histology of cardiac muscle
- Central nucleus
- Branched arrangement
- connected by intercalated disc
Histology of Heart valve (3 layers)
- Spongiosa
- loose CT
- atrial / blood vessel side of each valve
- covered by endothelium - Fibrosa
- core
- dense irregular CT
- fibres from cardiac fibrous skeleton - Ventricularis
- dense CT
- covered by endothelium
- many layers of elastic fibres
- continues into Chordae tendineae (in AV valves)
***Histology and organisation of blood vessels
3 layers:
- Tunica adventitia (thickest in veins)
- loose collagen and elastic fibre (longitudinally arranged)
- vasa vasorum
- nervi vasorum - Tunica media (thickest in arteries)
- vascular smooth muscle (circumferentially arranged, regulate blood flow, structural support)
- fenestrated lamellae of elastin and collagen in between SMC
- external elastic membrane (separate media and adventitia) - Tunica intima (thinnest)
- simple squamous epithelium with basal lamina (from endocardium to capillaries 都有)
- sub-endothelial layer (loose CT, SM)
- internal elastic membrane (arteries and arterioles, fenestrated, separate intima and media)
From inner to outer:
Simple squamous endothelium with basal lamina —> sub-endothelial layer —> internal elastic membrane —> vascular smooth muscle with elastin, collagen —> external elastic membrane —> elastic fibre, collagen, nervi vasorum, vasa vasorum
Organisation of arteries and veins by size
4 classes each Arteries: - Elastic arteries - Muscular arteries - Small arteries - Arterioles
Veins:
- Large veins
- Medium veins
- Small veins
- Venules
Route for blood circulation
Heart —> aorta —> muscular arteries —> small arteries —> arterioles —> capillaries —> venules —> small veins —> medium veins —> IVC/SVC —> heart
Atheromatous plaque
Content:
- scattered SMC —> foam cells
- macrophage —> foam cells
- foam cells
- T lymphocytes
- cholesterol crystal
- cell debris
Formation:
Oxidised LDL —> macrophage + SMC (foam cells) —> fatty streak —> fibrofatty plaque —> necrosis —> thrombosis
Arteries vs Veins
Arteries:
- thicker than veins (thicker tunica media)
- circular in shape
Veins:
- thinner walled (much thinner tunica media)
- irregular in shape
- tunics less distinct
- less robust
- valves to prevent backflow
- larger lumen (blood reservoir: 60-70%)
- venous return aided by:
1. Musculovenous pump
2. Respiratory pump
3. Artery pulsation
Elastic arteries
- Conducting arteries (facilitate continuous and uniform movement of blood)
- Aorta, Pulmonary trunk and main branches
- Rich elastic materials:
—> accommodate pressure changes
—> Recoil during diastole
—> Tense up during systole
—> Maintain continuous flow during diastole (Windkessel effect)
Tunica adventitia:
- thin
- loose CT
Tunica media:
- MANY layers of elastin in fenestrated concentric manner between muscle
- NO fibroblast
- external elastic membrane NOT obvious
Tunica intima:
- Thick sub-endothelial layer with SM, collagen, elastic fibres
- internal elastic membrane NOT obvious
Medium / muscular arteries
- Distribute blood and regulate blood flow and BP
—> by Vasoconstriction and Vasodilation - More smooth muscle
- Less elastin
- Prominent internal and external elastic membrane (less elastin to distract, prominent boundaries)
Tunica adventitia:
- Thicker than elastic artery
- fibroblast, collagen, elastic fibre
Tunica media:
- entirely smooth muscle
- LITTLE elastic fibre
- SPIRAL fashion
- NO fibroblast
- Prominent external elastic membrane
Tunica intima:
- Thinner than elastic artery
- PROMINENT internal elastic membrane
- expanded by lipid deposit in aging
Small arteries and arterioles
Small arteries: 3-8 SMC layers
Arterioles: 1-2 SMC layers
Arterioles: —> flow regulators —> alter vascular resistance by Vasodilation / Vasoconstriction —> Distribution of blood flow —> affect Systemic arterial pressure
Structure:
- Internal and external elastic membrane less and less defined —> disappear
- Arterioles have NO external elastic membrane
- Tunica adventitia: thin and blend with surrounding CT
- Smooth muscle at the end of arteriole linking capillary bed —> Precapillary sphincter
Capillaries
- smaller diameter than RBC
- single layer of endothelial cells
- NO tunics
- one RBC passage at a time
- allow gases, metabolite, waste to move through thin wall between blood and tissue
Structure: Pericyte
- surrounding capillary (enclosed within basal lamina)
- branching cytoplasmic process
- vascular support + stability to capillary
- undifferentiated mesenchymal cells —> differentiate into endothelial and SCM
Diapedesis: Process of leukocyte extravasation
3 types of capillaries
- Continuous capillaries (muscle, lung, CNS)
- continuous basal lamina
- complete endothelial cells
- very limited amount of substances can pass through
- large molecules by pinocytosis (transcellular) - Fenestrated capillaries (endocrine glands + metabolite exchange: gall bladder, kidney, GI tract)
- continuous basal lamina
- fenestrated endothelial cells (small pores) (fenestration covered by non-membranous diaphragm)
- greater range of substances can pass through
- thickness of wall and number of fenestration can change - Discontinuous capillaries / Sinusoids (liver, spleen, bone marrow)
- discontinuous basal lamina
- incomplete endothelial cells (large pores)
- allow protein even cells to pass freely (no barrier)
- large in diameter, irregular shape
Large veins
- diameter > 10mm (subclavian, portal, vena cava)
Tunica adventitia:
- MUCH THICKER layer
- Longitudinally arranged SMC: help to force blood back to heart
- Myocardial sleeves (SVC, IVC, pulmonary trunk)
Tunica media:
- thin
- circumferentially arranged
