HEART AND VASCULAR SYSTEM

CAPILLARIES Function = exchange of gas, nutrients, metabolites, water Structure = Endothelium +pericyte + basement membrane Pericyte =undifferentiated cells that becomes smooth muscle or fibroblast

CAPILLARY SYSTEMS Vary regarding: Diameter 3-4 μm to μm Abundance Heart, liver > Tendon Arterio-venous shunts Direct connections between arterial + venous system

Types of endothelium 1.Continuous: 1. Most common 2. Basement membrane is present 3. Uninterrupted lining with tight junctions 2.Fenestrated: 1. Found in gut, kidney, endocrine glands 2. Basement membrane is present 3. Large pores

From: Wheater’s Functional Histology, B Young and JW Heath

3. Discontinuous: No basement membrane Large diameter Continuous interface between lumen and surrounding tissue Sinusiods of liver, spleen and bone marrow

ARTERIES AND VEINS Inner Tunica intima: endothelium Tunica media: elastic and/or smooth muscle Tunica adventitia: collagen, elastic fibres Outer

ARTERIES ARTERIOLI Control blood flow by dilation or constriction Tunica intima Endothelium Lamina elastica interna (2 or more layers of a fenestrated elastic membrane) Tunica media Few layers smooth muscle cells Tunica adventitia Loose connective tissue

MUSCLE/MEDIUM SIZED ARTERIES Blood flow to organs Tunica intima: 1.Endothelium 2.Either Lamina elastica interna or Subendothelial layer of thin loose connective tissue

MUSCLE/MEDIUM SIZED ARTERIES Tunica media: 1.Most prominent 2.Spirally arranged smooth muscle 3.Little connective tissue 4.Lamina elastic externa

MUSCLE/MEDIUM SIZED ARTERIES Tunica adventitia: Usually thinner, loose connective tissue

lamina elastica interna smooth muscle of tunica media lamina elastica externa

ELASTIC ARTERIES Pulmonary trunk/aorta + major branches Stretch/systole/heart contracts/blood ejected from ventricles – diastole/recoil Tunica intima: 1.Endothelium 2. Subendothelium layer 3.Lamina elastica interna

ELASTIC ARTERIES Tunica media: fenestrated concentric layers of elastic fibre 2.Amorphic ground substance with collagen & smooth muscle cells Tunica adventitia: 1.Irregular connective tissue, few elastic fibers

endothelium subendothelium layer lamina elastica interna tunica media tunica adventitia tunica intima

THE VENOUS SYSTEM Thinner walls than arteries, larger lumen VENULES Tunica intima: Endothelial layer Tunica media: Pericytes ____ smooth muscle fibres Tunica adventitia: Longitudinal collagen fibres

SMALL /MEDIUM VEINS Small= mm + Medium =1-10 mm Tunica intima: 1.Endothelium 2.Subendothelial layer 3.Lamina elastica interna (poorly developed) Tunica media: 1. Layers of circular smooth muscle fibres 2.Loose connective tissue

SMALL /MEDIUM VEINS Tunica adventitia: 1. Thick, longitudinal collagen fibres 2.Loose connective tissue 3.Elastic fibres 4.Fibroblasts and macrophages

Valves Folds intima strengthened with connective tissue & elastic fibres NB! Note difference to heart valve structure

LARGE MUSCULAR VEINS Tunica intima: Similar to medium veins Tunica media: Similar to medium veins Tunica adventitia: Thick, bundles longitudinal collagen Elastic fibres Smooth muscle Numerous vasa vasorum

endothelium subendothelial layer lamina elastica interna smooth muscle bundles collagen fibers tunica mediatunica adventitia tunica intima

From Basic Histology, 4 th Ed, LC Junquera, J Carneiro

LYMPHATIC SYSTEM Begin blind in connective tissue as lymph capillaries Large pores- cells, bacteria and macromolecules

LYMPH CAPILLARIES Similar structure to blood capillaries Large lumen No pericytes Basal membrane is often absent

SMALL/LARGER LYMPH VESSELS Layers difficult to distinguish Tunica intima: Similar to large veins Tunica media: Concentric/diagonal muscle bundles Tunica adventitia: 1.Longitudinal/diagonal bundles of smooth muscle 2.Bundles of collagen fibres 3.Outer layer of collagen fibres

From Basic Histology, 4 th Ed, LC Junquera, J Carneiro

NERVE SUPPLY Most blood vessels have a well developed nerve supply Efferent, Afferent, Motor, Baro and chemoreceptors

HEART Tunica intima= Endocardium (inner) Tunica media = Myocardium (middle) Tunica adventitia= Epicardium (outer)

pericardium Epicardium (t.a.) Myocardium (t. m.) Subendocardium (s.e.) Endocardium (t.i.)

A: ENDOCARDIUM (inner/contact with blood) 1.Endothelium 2.Delicate layer of collagenous tissue 3.Robust fibro-elastic layer -contains smooth muscle cells

B: SUBENDOCARDIUM 1. Loose connective tissue 2.Fat cells 3.Purkinje fibers 4.Small blood vessels+ nerves + branchesof conducting system C: MYOCARDIUM Bundles of cardiac muscle (thickest)

Pericardium Epicardium (t.a) Myocardium (t. m.) Subendocardium (s.e.) Endocardium (t.i.)

D: EPICARDIUM (Outer) External 1.Mesothelium - flattened epithelial cells 2.Fibroelastic tissue (thin) 3.Broad layer adipose tissue 4.Coronary vessels + autonomic nerves pass through the epicardium to supply the myocardium

pericardium Epicardium (t.a.) Myocardium (t. m.) Subendocardium (s.e.) Endocardium (t.i.)

SKELETON OF THE HEART Thick fibrous rings at the origin of aorta + pulmonary arteries Connected by trigonum fibrosum with rings at arterio-ventricle openings

VALVES Plate/flap of fibroelastic connective tissue Extending from the fibrous skeleton Covered by endocardium

CONDUCTING SYSTEM The coordinated contraction of the myocardium is mediated by a specialized conducting system of modified cardiac muscle fibers

1.SINOARTRIAL (SA) NODE - PACEMAKER REGION (Right atrium (top)) 2.ATRIOVENTRICULAR (AV) NODE (Right atrium (bottom)) 3.ATRIOVENTRICULAR BUNDLE (BUNDLE OF HISS) (interventricular septum)

SA Node AV Node

HOW DOES IT WORK? Impulse starts at SA node Spreads throughout atrium Causes contraction Blood into ventricles

Then! Impulse spreads to AV node Passed via bundle of Hiss Divides into smaller branches bundles of Purkinje fibers

Then! Passes to subendocardial connective tissue Penetrates the ventricular myocardium Depolarization and contraction

LA RA LV RV SA Node AV Node Bundle of Hiss Right branch Left branch

A: SINOARTRIAL (SA) NODE Group of collagen & elastic fibres Well developed capillary system In middle: Nodal myocyotes or P cells Borders on narrow transitional cells Border of the Purkinje Cardiac muscle cells Region contains many axons of sympathetic and parasympathetic nervous system

Impulse moves from the → P-cells (Pale cells/pacemakers) → Transitional cells:fewer myofibrils → Purkinje cells: large, abundant glycogen, few myofilaments, extensive gap junctions → Cardiac muscle cells

B: ATRIOVENTRICULAR(AV) NODE Right atrium, microscopically similar to the SA node

C: ATRIOVENTRICULAR BUNDLE (BUNDLE OF HISS) Longitudinal transitional cells of AV node Down the interventricular septum Divide into left and right bundle Moves through subendocardium

C: ATRIOVENTRICULAR BUNDLE (BUNDLE OF HISS) Half way down septum, transitional cells are replaced with Purkinje cells Two branches divide further, form the Purkinje cell network Contact between Purkinje cells and cardiac muscles cells of myocardium

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