Lecture 10General med_2nd semester

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Lecture 10 General med_2nd semester

• Microscopic anatomy and embryology
• of cardiovascular system
• Microscopic structure of the heart, excitomotoric
  system -
• its structural peculiarities
• Blood vessels - arteries and veins - structural
  differences
• Capillary bed
• Lymph vessels and capillaries

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• CV system distributes nutritive materials,
  oxygen, and hormones to all parts of body and
• removes waste products of metabolism
• it consists of the heart and a series of tubular
  vessels
• arteries
• capillaries
• veins
• Remember the CVS system is deriving from the
  mesenchyma and is lined by simple
• squamous epithelium called the endothelium
• Layers of the CVS
• the tunica intima/ endocardium - the layer
  borders the lumen and consists of endothelium
  with basement membrane and thin sheet of
  connective tissue
• the tunica media/ myocardium - lies outward from
  the lumen
• consists primarily of smooth muscle cells (or
  cardiomyocytes) and elastic and collagen
• fibres
• the tunica adventitia/ epicardium - the outermost
  layer, it is composed of areolar connective
  tissue that connects vessel with its surrounding,
  but in the heart is smooth

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Blood circulation of the human systemic pulm
onary
the heart arteries veins capillaries
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pulmonary circulation
systemic circulation
portal circulation 2 capillary beds link up
each other
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The heart functions as a pump the right and the
left half the atrium the ventricle valves -
atrioventricular semilunar
the wall of the heart - 3 layers the
endocardium (tunica interna) - is in contact with
blood the myocardium (tunica media) -
intermediate solid layer of cardiac muscle
tissue the epicardium (tunica externa) -smooth
external covering layer visceral layer of
pericardium
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• Endocardium
• is continuous with the tunica intima of the large
  vessels entering and leaving the heart
• the endocardium of the left half of the heart is
  not continuous with the one on the right half as
• it is separated by a heart septum
• the endothelium and thin but continuous basement
  membrane
• subendothelial connective tissue composed of
  collagen, elastic fibres, solitary smooth muscle
  cells, small blood vessels, and nerves
• subendocardial layer containing the Purkinje
  fibres of the excitomotoric or conducting system
• Myocardium
• varies in thickness in different parts, being
  thickest in the left ventricle and thinnest in
  the atria
• has rich blood supply (many capillaries are seen
  in histological sections)
• cardiomyocytes have no regenerative capacity - if
  they were damaged, then degenerate
• and are substituted with connective tissue
• a connective tissue mass in the myocardium that
  serves as insertion site for the fibres in valves

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• Epicardium
• is a smooth serous covering of the heart
  corresponding to the visceral layer of
• pericardium - mesothelium and very thin
  submesothelial connective tissue layer
• (in obese patients, it may contain an adipose
  tissue in considerable amount)
• cardiac valves are duplicatures of the
  endocardium, especially subendothelial layer
• valves lack blood vessels and nerves

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• Conducting system of the heart
• consists of non-contracting cardiomyocytes
• the sinoatrial node (node of Keith-Flack)
• it lies on the medial wall of the right atrium
  near the entrance of the superior vena cava
• the atrioventricular node (node of Tawara)
• it runs on the right side of the interatrial
  septum
• the atrioventricular (AV) bundle (bundle of
  Hiss)
• it divides into 2 branches (for the left and
  right ventricles)
• the Purkinje fibres - terminal ramifications of
  the AV bundle

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• Microscopic structure of blood vessels
• Arteries
• arteries conduct blood from the heart to the
  periphery

• wall of any artery shows 3-layered organization
• the tunica intima (internal layer) - is composed
  of the endothelium and subendothelial connective
  tissue, whose elements are predominantly oriented
  in a direction longitudinal to the vessel
• the internal elastic lamina separates the intima
  from the middle coat
• the tunica media (middle layer) - the thickest
  layer and its structural elements run circular to
  the long axis of vessels
• consists of elastic fibres and smooth muscle
  cells - the type of the artery depends on their
  mutual proportions
• from the outer coat is separated by the external
  elastic lamina
• the tunica adventitia (external layer) - of
  loose connective tissue with small blood vessels
  (vasa vasorum) and nerve bundles
• elements of the external tunic run for the most
  part longitudinally

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• arteries are subclassified into three types
• conducting or large-sized arteries - with wall
  in which elastic elements predominate
• distributing or medium-sized arteries -
• with a predominance of smooth muscle cells in the
  media
• arterioles - small arteries that immediately
  control the
• supply of blood to the capillary bed

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distributing artery conducting artery
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• Notice In cross-sections of fixed preparations
  (in which smooth muscle cells are contracted),
  the arteries
• show the distinct scalloped line of the internal
  elastic lamina with the characteristic
  corrugation of the intima
• coat, as the elastic membranes are unable to
  contract and are thrown into longitudinal folds
• the endothelial nuclei consequently tend to bulge
  into lumen
• Conducting arteries arteries of elastic type
  
• have resistant, elastic and not thick wall
  (relative to the size of the lumen)
• the aorta, carotids, subclavian, axillary and
  iliacs
• Function elastic arteries absorb and store the
  contractile energy of the left ventricle and
• transform the pulsatile flow of blood in smooth
  out
• Tunica intima
• - endothelium (its cells are elongated in the
  direction of the long axis),
• - subendothelial layer consists of loose
  connective tissue containing many fine
  longitudinal
• elastic fibres these gradually merge into the
  internal elastic lamina, which is not marked off
• sharply from the elastic membranes of the middle
  coat
• near the boundary of two coats the longitudinally
  running smooth muscle cells are found

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Conducting artery (aorta)
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Conducting artery (aorta)
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• Distributing arteries or arteries of muscular
  type
• the all medium-sized arteries
• they have thicker wall relative to the size of
  the lumen compared with elastic arteries
• Function arteries regulate blood pressure by
  contraction and dilatation of smooth muscle
  cells in thei wall
• they also regulate the perfusion of different
  parts of the body under physiologic conditions
• Tunica intima
• - endothelium
• - subendothelial layer diminishes in thickness
  with decreasing size of the artery
• it consists of cellular connective tissue with
  very fine elastic fibres and a few smooth muscle
  cells
• the internal elastic membrane is well-developed
  (in later life it tends to split into several
  layers)
• Tunica media
• - smooth muscle cells are prevalent they are
  arranged circularly and form 3 to 40 layers,
• - elastic network is fine and interlaced between
  leiomyocytes (muscle cells)
• the external elastic lamina is always present and
  sharply demarcates this layer from the external
  coat

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Distributing artery
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• Arterioles
• Function they regulate the flow of blood through
  capillary bed
• Tunica intima - consists of the endothelium and
  the internal
• elastic membrane
• (the subendothelial layer is mostly missing)
• Tunica media - thin
• consists of 2- 4 layers of smooth muscle cells
  wrapped
• round the intima
• Tunica adventitia - is reduced to a thin sheath
  of collagen
• fibres

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• Variations in the structure of arteries
• - cerebral arteries resemble veins in having a
  thin wall but contain a prominent internal
• elastic membrane
• - coronary arteries have thick wall with
  considerable elastic component
• - arteries of the penis contain longitudinal
  muscle fibres in the thickened intima (cushions)
• - umbilical arteries have an inner longitudinal
  and an outer circular layer of smooth muscle in
• the media

Veins vessels that conduct blood from organs to
the heart function veins function as a blood
reservoir the wall of veins shows 3-layered
organization but is much thinner in proportion to
the size of the lumen than is that of the
arteries the wall, although thin, is however very
strong because the connective tissue components
are greatly developed (elastic and muscular
elements are inconspicuous) after death the wall
of the veins tends to collapse in some body
regions, in particular the lower limbs, the veins
over 2 mm in a diameter are provided with vein
valves that prevent the blood in flowing back
from the heart valves are usually arranged in
pairs opposite to one another histologically,
they are duplicatures of the tunica intima
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an artery a vein
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• the wall of veins is similar to arteries
  3-layered
• tunica intima - consists of the endothelium and
  very thin subendothelial
• layer of connective tissue the internal elastic
  membrane is delicate or missing
• tunica media - is relatively thin with exception
  of veins of lower
• extremities
• it contains a considerably amount of collagen
  fibres and a little elastic
• fibres and smooth muscle cells
• tunica adventitia - is well-developed, being much
  thicker than the middle coat
• it contains collagen and elastic fibres and
  smooth muscle cells grouped into
• small bundles that run chiefly longitudinal
• robust vasa vasorum sometimes penetrate even the
  intima
• Variations of structure in veins
• veins of the brain and menings lack valves and
  have no media
• veins of bones, retina, placenta and trabecular
  veins of the spleen show similar structure

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• Capillaries
• capillaries are the smallest branches of the CVS
  that penetrate most organs, being
• interposed between the terminal ramifications of
  the small arterioles and venules
• a density of capillary network depends on the
  metabolic activity of organ
• - the highest density is found in the cerebral
  cortex, myocardium, kidney etc.
• the average d. of capillaries is cca 8 ?m
• allowing the passage of bloodcorpuscles in single
  file
• total length of capillaries in the human - 90 km
• the surface area of capillary bed -
• 6 300 - 12 000 m2
• function of capillaries metabolic exchange
  between
• blood and surrounding tissues

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• the capillary wall is very simple in structure
  and consists of
• the endothelium - endothelial cells are held
  together by zonulae occludentes, and an
  occasional desmosomes and gap junctions
• the basal lamina
• a delicate envelope of reticular fibres, in which
  fibroblasts, macrophages and pericytes occur
• pericytes are non-differentiated mesenchymal
  cells having long processes that may partly
  surround the endothelial cells
• cells are suggested to have a contractile
  function because contain contractile proteins
  actin, myosin and tropomyosin

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• by electron microscopy, the capillaries are
  grouped into 3 types
• continuous, or somatic capillaries
• fenestrated, or visceral capillaries
• sinusoidal capillaries, or sinusoids
  (discontinuous capillaries)

continuous capillaries have all layers good
developed capillaries of this type occur in
the central nervous system (cortex of the
telen-cephalon, cerebellar cortex), in all kinds
of muscle tissue, the connective tissue, and
exocrine glands
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• fenestrated capillaries
• their wall consists of the same layers as that in
  continuous ones but endothelial cells
• are provided by circular pores (fenestrae)
• the fenestrae are 60 - 80 nm in d. and are closed
  by a diaphragm that is thinner
• than a cell membrane and does not show trilaminar
  structure of a biomembrane

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fenestrated capillaries occur in organs with
rapid interchange of substances between cells and
blood, i.e. intestinal villi, pancreas, choroid
plexus, ciliary body Remember modified
fenestrated capillaries are contained in the
renal glomeruli the fenestrae are larger (70 - 90
nm) and more numerous than in the fenestrated
capillaries of other organs, no diaphragms are
present in the fenestrae sinusoidal
capillaries, or sinusoids (discontinuous
capillaries) are characterized by a tortuous
path greatly enlarged diameter (30 - 40 ?m) and
discontinuous basal lamina and absence of
pericytes the endothelial cells are separated
each other by numerous and large gaps that
facilitate the transport of substances between
blood and cells sinusoids occur mainly in the
liver, some hematopoietic organs (such as the
bone marrow), some endocrine glands
(adenohypophysis, islets of Langerhans)
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conduct lymph to the bloodstream, in organs
the lymph is collected by blind lymphatic
capillariesthey are often collapsed in
histological sectionscapillaries unite each
other to form small and medium-sized lymphatic
vessels (lymphatics)contain the lymph and have
valves in their lumina main ducts ductus
thoracicus - thoracic duct right lympatic duct
- truncus lymphaceus dx. both ducts empty the
lymph into the bloodstream at the junction of the
left internal jugular and left subclavian veins
(angulus venosus)
Lymph vessels
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Lymphatic capillaries very simple structure
their wall is composed of endothelial cells and
fine reticular fibres of circular orientation the
basal lamina is not developed
Lymphatic vessels and ducts are thin walled tubes
their walls resemble the walls of veins it
consists of 3 layers an intima, a media and an
adventitia