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 30-40 μmAbundance

Heart, liver > TendonArterio-venous shunts

Direct connections between arterial + venous system

Types of endothelium

1. Continuous: 1. Most common 2. Basement membrane is present3. Uninterrupted lining with tight junctions

2. Fenestrated: 1. Found in gut, kidney, endocrine glands2. 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: endotheliumTunica media: elastic and/or smooth muscleTunica adventitia: collagen, elastic fibres

Outer

ARTERIESARTERIOLI

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 adventitiaLoose connective tissue

MUSCLE/MEDIUM SIZED ARTERIES

Blood flow to organs

Tunica intima:1. Endothelium2. Either Lamina elastica interna

or Subendothelial layer of thin loose connective tissue

MUSCLE/MEDIUM SIZED ARTERIES

Tunica media: 1. Most prominent2. Spirally arranged smooth muscle3. Little connective tissue4. Lamina elastic externa

MUSCLE/MEDIUM SIZED ARTERIES

Tunica adventitia: Usually thinner, loose connective tissue

lamina elastica interna

smooth muscle of tunica medialamina 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:1. 40-70 fenestrated concentric layers of

elastic fibre2. Amorphic ground substance with collagen

& smooth muscle cells

Tunica adventitia: 1. Irregular connective tissue, few elastic fibers

endotheliumsubendothelium layerlamina elastica interna

tunica media

tunica adventitia

tunica intima

THE VENOUS SYSTEM

Thinner walls than arteries, larger lumen

VENULESTunica intima:

Endothelial layer

Tunica media: Pericytes ____ smooth muscle

fibres Tunica adventitia:

Longitudinal collagen fibres

SMALL /MEDIUM VEINS

Small= 0.2-1 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 tissue3. Elastic fibres4. 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 fibresSmooth muscle Numerous vasa vasorum

endothelium

subendothelial layer

lamina elastica interna

smooth muscle bundles

collagen fibers

tunica media tunica adventitia

tunica intima

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

LYMPHATIC SYSTEM

Begin blind in connective tissue as lymph capillariesLarge 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 fibres3. Outer layer of collagen fibres

From Basic Histology, 4th 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: SUBENDOCARDIUM1. Loose connective tissue

2. Fat cells 3. Purkinje fibers4. Small blood vessels+ nerves +

branches of conducting system

C: MYOCARDIUMBundles 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 tissue4. 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 myocardiumDepolarization and contraction

LA

RA

LVRV

SA Node

AV Node

Bundle of Hiss

Right branch

Left branch

A: SINOARTRIAL (SA) NODE

Group of collagen & elastic fibresWell 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

Practical

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