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
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
local differences in the occurrence, thickness and composition of individual layers (e.g. there is
no tunica media in capillaries, tunica media of elastic arteries contains elastic laminae)
Blood
circulation of the
human
systemic
pulmonary
the heart
arteries
veins
capillaries
pulmonary circulation
systemic circulation
portal circulation =
2 capillary beds link up
each other
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
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
as well as for the myocardial cells themselves - cardiac skeleton
/involves the annuli fibrosi, trigona fibrosi, and the septum membranaceum/
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
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
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
arteries are subclassified into
three types:
conducting or largesized arteries - with wall in
which elastic elements
predominate
distributing or mediumsized 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
distributing artery
conducting artery
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
Tunica media:
- elastic fibres arranged circularly as discontinuous fenestrated membranes about 2.5 mm thick
(about 50),
- circularly oriented smooth muscle interspersed between elastic membranes
Tunica adventitia:
consists of loose connective tissue containing next to the media longitudinally arranged elastic
fibres and vasa vasorum, nourish a portion of the media
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
Tunica adventitia:
is composed of the loose connective tissue, in which elastic fibres are abundant and run
longitudinally
Distributing artery
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
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
an artery
a vein
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
 veins of the pampiniform plexus of the spermatic cord and umbilical veins have
an unusually thick media
 the media is absent in the inferior vena cava and it is replaced with an abundance of
longitudinal muscle bundles in the thickened adventitia
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 mm
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
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
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 telencephalon, cerebellar
cortex), in all kinds of
muscle tissue, the
connective tissue,
and exocrine glands
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
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 mm) 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)
Lymph vessels
conduct lymph to the bloodstream,
in organs the lymph is collected by blind
lymphatic capillaries
they are often collapsed in histological
sections
capillaries 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)
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