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

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1 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

2 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)

3 Blood circulation of the human systemic pulmonary the heart arteries veins capillaries

4 pulmonary circulation systemic circulation portal circulation = 2 capillary beds link up each other

5 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

6 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/

7 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

8 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

9 Microscopic structure of blood vessels Arteries arteries conduct blood from the heart to the periphery wall of any artery shows 3-layereded 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

10 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

11 distributing artery conducting artery

12 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 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 elastic fibres and vasa vasorum, nourish a portion of the media

13 the all medium-sized arteries Distributing arteries or arteries of muscular type 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

14 Distributing artery

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16 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

17 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

18 an artery a vein

19 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

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21 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 m 2 function of capillaries: metabolic exchange between blood and surrounding tissues

22 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

23 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

24 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 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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26 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)

27 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)

28 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