• The structure of the arteries • The structure of the veins • The
structure of the lymphatic vessels
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• Junqueira's Basic Histology, twelfth edition, text and atlas,
Anthony L. Mescher, McGraw-Hill Companies
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Summary
VESSELS • LYMPHATIC VASCULAR SYSTEM
Blood vascular system: composed heart : pump blood arteries:
efferent vessels, carry blood with
nutrients & oxygen tissue capillaries: smallest blood vessels,
interchange veins: convergence of capillaries, convey blood
heart
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return fluid of tissue space blood
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macrovasculature: vessels more than 0.1mm in diameter
large arterioles, muscular & elastic arteries, muscular
veins
microvasculature: arterioles, capillaries, post-capillary venules
functional important:
site of interchange
A muscular organ contract rhythmically, pumping blood
R & L ventricles: blood lung & rest of body R& L atria:
receive blood from body & lung Walls of heart:
internal endocardium middle mycardium external epicardium
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Endocardium A single layer of squamous endothelial cells Thin layer
of loose connective tissue:
elastic & collagen fibers & smooth muscle cells
Subendocardial layer: veins & nerves, branch of
impulse-conducting system
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Myocardium Thickest, consist of cardiac muscle cells Arranged in
layers: surround heart chambers
complex spiral Ventricles thicker than atria Epicardium Simple
squamous epithelium (mesothelium)
support by a thin layer of connective tissue Subepicardial
layer:
veins, nerves, adipocytes
(containing elastic & collagen fibers) both sides: endothelial
layers bases: attach to strong fibrous rings
part of fibrous skeleton site of origin & insertion of cardiac
muscle fibers
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Consist: 2 nodes located in right atrium:
sinoatrial (SA) node: pacemaker atrioventricular (AV) node
Atrioventricular bundle (of His)
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SA node: a small mass of modified cardiac muscle cells fusiform,
smaller & fewer myofibrils
AV node: similar to SA node cytoplasmic projections branch in
various
directions AV bundle:
originate from node pass along interventricular septum split into L
& R bundles branches to both ventricles
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Distal fibers of AV bundles: larger than ordinary cardiac muscle
fibers
conducting myofibers or Prukinje fibers 1 or 2 nuclei cytoplasm:
rich in mitochondria, glycogen myofibrils: sparse, restricted to
periphery
Both parasympathetic & sympathetic neural components
heart
Ganglionic nerve cells & nerve fibers: present close to SA
& AV nodes affect heart rate & rhythm
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afferent free nerve endings sensibility & pain
Partial obstruction of coronary arteries: reduce supply of oxygen
cause pain (angina pectoris)
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STRUCTURAL PALN OF BLOOD VESSELS
3 layers of vascular wall: Tunica intima One layer of squamous
epithelial cells Subendothelial layer: loose connective
tissue
smooth muscle found in In arteries:
internal elastic lamina most external component of intima composed
of elastin holes (fenestrated) allow diffusion
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smooth muscle cells Among smooth muscle cells:
elastic fibers & lamellae, reticular fibers, proteoglycans,
glycoprotein
In arteries: external elastic lamina separate from adventitia
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Tunica adventitia (tunica externa) Consist of type I collagen &
elastic fibers Continuous with stromal connective tissue of
organs
Large vessels: vasa vasorum consist of arterioles, capillaries,
venules in tunica adventitia & outer part of media provide
metabolites to cells of those layers large vessels: walls too
thick
to be nourished by diffusion from lumen
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neurotransmitter – diffusion smooth muscle : gap junctions
Thinner walled veins: nerve ending: in adventitia & media
overall density : less than encountered arteries
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nerve supply Acetylcholine: act on endothelium
produce nitric oxide diffuse to smooth muscle cells
muscle relax lumen dilated
Large Elastic Arteries
Help to stabilize blood flow Include: aorta & its large
branches Fleshly: yellowish color from elastin in media Intima:
thicker than muscular arteries
internal elastic lamina: present may not easily discerned
Media: elastic fibers & concentrically
Adventitia: relatively underdeveloped 23
Control blood flow organs Intima: very thin subendothelial
layer
prominent internal elastic lamina Media: up to 40 layers of
more
prominent smooth muscle cells intermingled with elastic lamellae,
reticular
fibers, proteoglycans external elastic lamina:
only in larger muscular arteries Adventitia: connective
tissue
contain lymphatic capillaries, vasa vasorum, nerves
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Arterioles
Muscular arteries branch repeatedly smaller and smaller 2 or 3
medial layers of muscle
Arterioles: smallest arteries 1 or 2 layers of smooth muscle layers
generally less than 0.5 mm in diameter lumen as wide as wall is
thick subendothelial layer: very thin elastic laminae: absent
media: circularly arranged smooth
muscle cells adventitia: very thin & inconspicuous 25
Capillaries
Composed of : a single layer of endothelial cells
Endothelial cells: form a tube Capillaries:
5-10 μm average diameter length: 50 μm comprise over 90% of all
blood vessels total length: 96000km total diameter: 800 times
larger than aorta
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Velocity of blood: in aorta: 320 mm/s in capillaries: 0.3 mm/s
because their thin wall & slow flow
a favorable place for exchange water, solutes, macromolecules
In general: endothelial cells: polygonal & elongated in the
direction of blood flow nucleus: bulge into lumen junctions of
tight zonula occludentes type present
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Pericytes: enclosed by a basal lamina
may fuse with that of endothelial cells well-developed networks of
myosin, actin,
tropomyosin : contractile function After tissue injuries:
pericytes proliferate & differentiate form tunica media &
cells with various
other functions in re-establishing microvasculature & its
ECM
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Capillary: structural variations 3 types 1. continuous (tight)
capillaries
allow regular exchange of material most common type found in
muscles, connective tissue,
exocrine glands & nervous tissue pinocytotic vesicles :
luminal
& basal surface transport of material
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2.Fenestrated capillaries: allow more extensive molecular exchange
presence of small circular fenestrae
covered by a very thin diaphragm containing heparan proteoglycans
no lipid bilayer
basal lamina: continuous voering fenestrae found in kidney,
intestine, choroid plexus,
endocrine glands
3. Sinusoid (Discontinuous) capillaries : permit maximal exchange
endothelial cells:
large fenestrae without diaphragm form a discontinuous layer
separate from one another:wide space basal lamina: also
discontinuous
irregularly shaped diameter: 30-40μm found in liver, spleen, some
endocrine organs,
bone marrow
Postcapillary venules: similar structurally to capillaries with
pricytes but diameter: from 15-20μm participate in exchange
converge larger collecting venules
2 or 3 smooth muscle layers muscular venules
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Veins
Blood entering vein: very low pressure move forward heart:
contraction of tunica media external compression from
surrounding
muscles or organs Valve:
projection from tunica intima to prevent back-flow of blood
Most veins: small or medium veins diameter less than 1 cm located
in parallel with
muscular arteries 33
Intima thin subendothelial layer
Media small bundles of smooth muscle cells intermixed with
reticular fibers & a delicate
network of elastic fibers Adventitia
collagenous well-developed
Well developed intima Media: relative thin
few layers of smooth muscle cells abundant connective tissue
Adventitia: thick in large veins frequently contain longitudinal
bundles of
smooth muscle Both media & adventitia:
contain elastic fibers not like those of elastic arteries
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Collect excess interstitial fluid from tissue space return to
blood
Fluid: lymph Flow in only one direction: toward the heart Lymphatic
capillaries
originate in various tissue thin, closed-ended vessels a single
layer of endothelium +
an incomplete basal lamina held open by bundles of anchoring
filaments
of elastic fiber system also bind vessels to connective
tissue
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Lymphatic capillaries larger lymphatic vessels Interposed in path :
lymph nodes Lymphatic: found in almost all organs
except CNS & bone marrow Larger lymphatics:
similar to veins thinner walls lack a clear-cut separation between
tunics more numerous internal valves
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Lymphatic vessels: dilated assumed a nodular or beaded, between
valves
Lymphatic circulation: aided by external forces
contraction of skeletal muscles unidirectional flow: many valves
contraction of smooth muscles of larger
lymphatic vessels: also help to propel End up as 2 large
trunk:
thoracic duct & right lymphatic trunk empty lymph veins
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Structure of lymphatic ducts: similar to that of large veins
reinforced smooth muscle in middle layer
muscle bundles: longitudinally & circularly arranged former
predominating
adventitia: relatively undeveloped contain vasa vasorum &
neural network
Lymphatic vessels: also a major distributor of lymphocytes,
antibodies, immune components