Lecture 10Lecture 10 ESS_2nd semesterESS_2nd semester

Microscopic anatomy and embryologyMicroscopic anatomy and embryology of the cardiovascular systemof the cardiovascular system

Microscopic structure of the heart, excitomotoric Microscopic structure of the heart, excitomotoric system -system -its structural peculiaritiesits structural peculiarities

Blood vessels - arteries and veins - structural Blood vessels - arteries and veins - structural differencesdifferences

Capillary bedCapillary bed

Lymph vessels and capillariesLymph vessels and capillaries

CV system distributes nutritive materials, oxygen, and hormones to all parts of the body and

removes waste products of the metabolismit consists of the heart and a series of tubular vessels:

arteriescapillariesveins

Remember: the CVS system derives from the mesenchyma and is lined by simple squamous epithelium called the endothelium

Layers of the CVS: three

the tunica intima/ endocardium - the layer found nearest to the lumenis composed of an endothelium with basement membrane and thin sheet of connective tissue

the tunica media/ myocardium - the next layer outward from the lumenconsists primarily of smooth muscle cells (or cardiomyocytes) and elastic and collagen fibres

the tunica adventitia/ epicardium - the outermost layer, in blood vessels the layer is composed of areolar connective tissue that connects the wall with the surrounding, the layer is smooth in the heart

local differences in the occurrence, thickness and composition of individual layers (e.g. there is

no tunica media in capillaries and “arteriols”, tunica media of elastic arteries contains arteries contains elastic laminae)elastic laminae)

Blood circulation

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 halfan atrium + a ventricle

valves - atrioventricularsemilunar

the wall of the heart - three 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 asit is separated by a heart septum

the endothelium and thin but continuous basement membranesubendothelial connective tissue composed of collagen, elastic fibres, solitary smooth muscle cells, small blood vessels, and nervessubendocardial layer containing the Purkinje fibres of the excitomotoric or conducting system

Cardiac valves are duplicatures of the endocardium, especially subendothelial layer

valves lack blood vessels and nervesMyocardium

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. It is also functioning as an electrical insulator with an foramen for the artrioventricular bundle.

/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 serous pericardium – mesothelium and very thin submesothelial connective tissue layer (in obese patients, it may contain an adipose tissue in considerable

amount+ cornary arteries!)

It contains two layers, both of which function in lubricating the heart to It contains two layers, both of which function in lubricating the heart to prevent friction from occurring during heart activity:prevent friction from occurring during heart activity:

The layer adherent to the fibrous pericardium is the parietal layer. The layer adherent to the fibrous pericardium is the parietal layer. The layer deep to the fibrous pericardium is the visceral layer.The layer deep to the fibrous pericardium is the visceral layer.[1] When When

this layer comes into contact with the heart (not the this layer comes into contact with the heart (not the great vessels), it is ), it is known as the known as the epicardium. .

Together these two layers form a continuous uninterrupted membrane. Together these two layers form a continuous uninterrupted membrane. Between these two layers exists a small cavity called the pericardial Between these two layers exists a small cavity called the pericardial cavity, which contains a supply of cavity, which contains a supply of serous fluid. The serous fluid that is . The serous fluid that is found in this space is known as the found in this space is known as the pericardial fluid..

The conducting system of the heart consists of non- contracting cardiomyocytes in the subendocard.layer

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 vesselsArteriesarteries conduct blood from the heart to the periphery

The wall of any arteryThe wall of any artery is is threethree--layerlayereded::

the tthe tunica intimaunica intima ( (internal layerinternal layer)) - is composed of - is composed of the the endotheliumendothelium and and subendothelialsubendothelial connective connective tissue, whose elements are predominantly oriented in a tissue, whose elements are predominantly oriented in a direction longitudinal to the vessedirection longitudinal to the vessell

the the internal elastic laminainternal elastic lamina separates the intima separates the intima from from the middle coatthe middle coat

the tthe tunica mediaunica media ( (middle layermiddle layer)) - the thickest layer - the thickest layer and its structural elements and its structural elements run run circularcircularlyly to the long axis to the long axis of vesselsof vessels

consists of consists of elastic fibres & smooth muscle cellselastic fibres & smooth muscle cells - - the the typetype of the artery depends on their mutual proportions of the artery depends on their mutual proportions

It isIt is separated separated from the outer coat from the outer coat by by the the external external elastic laminaelastic lamina

the tthe tunica adventitiaunica adventitia (external (external layer layer)) - of - of loose loose connective tissueconnective tissue with small blood vessels ( with small blood vessels (vasa vasa vasorumvasorum) and ) and nerve bundlesnerve bundles

elementselements of the external tunic of the external tunic run for the most part run for the most part longitudinallylongitudinally

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

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 foldsthe 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, brachiocephalic trunk, 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

longitudinalelastic fibres; these gradually merge into the internal elastic lamina, which is not marked off sharply from the elastic membranes of the middle coatnear 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 m

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

elasticfibres and vasa vasorum, nourish a portion of the media

Conducting artery (aorta)

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: the contraction and relaxation of smooth muscle cells in muscular arteries regulate blood pressurethey 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 cellsthe 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

ArteriolesFunction: 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 - thinconsists 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 Veinsvessels conducting blood to the heartfunction: veins serve 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 arteryan artery a a veinvein

the wall of veins is similar to arteries 3-layered

tunica intima - consists of the endothelium and very thin subendotheliallayer of connective tissue; the internal elastic membrane is delicate or

missing

tunica media - is relatively thin with exception of veins of lower extremitiesit 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 meninges 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 are distributed in organs, being

interposed between the terminal ramifications of the small arterioles and venules

a density of capillary network depends on the metabolic activity of the organ - the highest density is found in the cerebral cortex, myocardium, kidney etc.

function of capillaries: metabolic exchange between blood and surrounding tissues

the average d. of capillaries is about 8 mallowing the passage of the bloodcorpuscles in single file

total length of capillaries in the human - 90 kmthe surface area of capillary bed - 6 300 - 12 000 m2

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 havíng long processes that may partly surround the endothelial cellscells are suggested to have a contractile function because contain contractile proteins: actin, myosin and tropomyosin (called MESANGLIAL CELLS in nephron

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, theconnective tissue,and exocrine glands

fenestrated capillaries their wall consists of the same layers as that in continuous ones but

endothelial cellsare 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 pathgreatly 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)

conduct lymph to the bloodstream

they begin in the organs as blind lymphatic capillariesthey are usually collapsed in histological sectionscapillaries collect tissue (interstitial) fluid from the organs. They are called lacteals in the intestine.

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 empty the lymph into the bloodstream at the junction of the left internal jugular and left subclavian veins (angulus venosus)

Lymph vessels

Lymphatic capillaries very simple structure: their wall is composed of endothelial cells and fine reticular fibres of circular orientationthe 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