1. The Circulatory System Microscopic structure of blood and lymph vessels Abid Ali

2. Learning objectives At the end of this session the students should be able to: Know the general organization of CVS and types of circulation Compare and contrast the histological structure of arteries, capillaries, and veins.

3. The Circulatory system consists of 1.Blood vascular system- heart, arteries, capillaries, veins 2.Lymph vascular system lymphatic capillaries, lymphatic vessels RLD on right side thoracic duct on left side −RLD and thoracic duct open into great veins of neck.

4. Arteries Blood vessels that carry blood away from the heart. −Systemic arteries carry oxygenated blood. −Pulmonary arteries carry deoxygenated blood Veins Blood vessels that carry blood towards the heart. −Systemic veins carry deoxygenated blood. −Pulmonary veins carry oxygenated blood.

5. There are two circulatory "circuits": 1.Pulmonary circulation –Pulmonary arteries carry deoxygenated blood from the “right heart" to the lungs –Pulmonary veins carry oxygen-rich blood from the lungs back to the left heart.“ 2.Systemic circulation −Blood from the ‘‘left heart’’ goes to the rest of body via aorta and its branches. −Blood from all body systems return to the “right heart” via the inferior and superior vena cavae. Functions of circulatory system Supply of oxygen Supply of nutrients Removal of waste products

6. Microvasculature The smallest blood vessels (arterioles, capillary beds and the initial venules) constitute microvasculature. These are intimately related with tissue exchange.

7. General Plane of microscopic structure of Blood Vessels: Three layers (tunics), from within out 1.Tunica intima: consists of An endothelium (present in all vessels) a subendothelial CT: its presence or absence depends on type and size of vessel. 2.Tunica media is the middle layer Concentrically-arranged smooth muscle fibres in layers. 3.Tunica adventitia is the outermost CT layer made chiefly of collagen fibres

8. Capillaries Vessels of smallest dia (7-9 µm) i.e. just wide enough to allow the passage of an RBC. branch extensively without change in calibre. The branches anastomose freely to form networks (capillary bed) The capillaries nearest to arterioles are called arterial capillaries Those near the venules are called venous capillaries

9. Central channels or preferential channels direct channels b/w arterial and venous capillaries In these the blood flow is continuous as compared to the intermittent flow in the rest of network. The immediate arterial ends of the preferential channels are called metarterioles These are surrounded by modified smooth muscle cells (pericytes) longitudinally arranged along these capillaries. They constitute the precapillary sphincters. They help regulate the blood pressure.

10. Microscopic structure a single layer of endothelial cells which rest on a basement membrane plus a few thin CT fibres and cells. 2-5 endothelial cells may form the complete circumference of a capillary. In surface view they look like a mosaic.

11. Types of capillaries Classified according to the structure of endoth cells. 1.Continuous capillaries Also k/a somatic capillaries Endothelial cells are bound to each other by tight junctions Endothelial ells have no pores in their cytoplasm. An intact basement membrane Found in nervous tissue, muscle tissue and exocrine glands.

12. 2.Fenestrated capillaries (visceral capillaries) endothelial cells have small openings (fenestrae) in their cytoplasm, about 80-100 nm in diameter. BM is continuous over fenestrae, but in the form of a thin diaphragm. Fenestrae allow greater permeability of molecules smaller than plasma proteins. Found in intestines and endocrine glands.

13. 3.Sinusoidal capillaries (Discontinuous capillaries) Have a large lumen Endothelial cells separated by wide gaps The cytoplasm of cells have fenestrae BM is broken (absent over the fenestrae) The lumen is lined with phagocytic cells Found in liver, bone marrow, spleen, endocrine glands. Provide greatest permeability.

14. Arteries Three types according to their size 1.Large sized (elastic arteries) 2.Medium sized (muscular or distributing arteries) 3.Small arteries (and arterioles) These types are all continuous with one another. A characteristic feature of arteries, regardless of size, is a well-defined lumen, rounded or oval, maintained by the muscularity of the vessel wall.

15. Large arteries (Elastic arteries, conducting arteries) The aorta and its branches (brachiocephalic, subclavian, pulmonary, beginning of common carotid and iliac arteries) TM contains large amounts of elastic fibres (and smooth muscles). This helps them to smooth out raises in blood pressure created by the pumping of heart.

16. Large arteries.......contd T. Intima Endothelium A subendothelial layer (CT; collagenous and elastic fibres) The IEM present but not conspicuous because of the abundance of elastic lamina in the tunica media.

17. Large arteries...........Contd Tunica media The thickest of the 3 tunics. A large number of smooth muscles and elastic fibers arranged in spiral lamellae around the long axis of the vessel. These lamellae are the most striking histological feature of elastic arteries, seen as wavy layers. No distinct external elastic lamina.

18. Large arteries...........Contd T. Adventitia Relatively thin DICT layer. The collagen in the adventitia prevents elastic arteries from stretching beyond their physiological limits during systole. Vasa vasorum

19. Medium-sized arteries Majority of named arteries e.g. brachial, ulnar, renal etc. Aka muscular or distributing arteries. The main constituent of the TM is smooth muscle Less elastic tissue than large arteries

20. Medium-sized arteries....contd Tunica intima consists of 3 layers ● Endothelium ● Subendothelial CT ● Internal elastic Lamina seen as a conspicuous wavy line, (not obscured by elastic lamellae as in large artery). ● This membrane separates the T.intima from T.media. In tissue preparation, the IEL of T. intima appears wavy due to postmartem contraction of the smooth muscles in the T. media. TI– area b/w arrows

21. Medium-sized arteries....contd Tunica media Several layers of circularly arranged SM fibres plus Some collagen and elastic fibers. TM – area b/w arrows

22. Medium sized arteries......contd Tunica adventitia Outermost CT coat of collagen fibres, but elastic fibres are also present. EEL separates the TM from TA EEL is not as prominent IEL. TA blends with CT of surrounding structures. TA– area b/w arrows

23. Arterioles and Small arteries T. Intima Endothelium No or little subendothelium IEM present. T. Media: consists of SMs in concentric layers.. Arteriole have 1-8 layers of SMs. Small arteries have > 8 layers; may be upto 25. T.Adventitia: Becomes thinner with decrease in size of vessel EEM disappears. The IEM persist much longer than EEM.

24. Veins, like arteries, are classified as Venule, Medium sized veins and Large veins. Venules: T. intima: only endothelium T. Media: one or two layers of smooth muscles T. Adventitia: relatively thick. Consists of collagen fibres arranged longitudinally. Medium sized veins (most named veins): T. Intima: endothelium, a thin S/E, and an IEL. T. Media-circularly arranged SMs & collagen fibrs T. Adventitia: consists of longitudinally arranged thick bundles of collagen fibres. Forms larger part of the wall of vein.

25. Large-sized veins (venae cavae, portal vein, common iliac veins) T. Intima: endothelium, thin subendothelium. IEL occasionally present. T. Media: is thin; mainly collagen fibre with few smooth muscle layers. T. Adventitia the thickest tunic and has 3 zones Next to media is a zone of dense collagen and elastic fibres running spirally. Then is a thin layer of smooth muscles arranged longitudinally. The outermost layer consists of networks of collagen and elastic fibres. Large veins contain valves.

26. characteristicArteryVein LumenNarrow, formed Wide, irregular Wall thicknessThicker than companion vein Thinner than companion artery Thickest tunicTunica mediaTunica adventitia Elastic and collagen content in tunics More than in veins Less than in artery Blood pressureMore in arteryLess in vein ValvesAbsentPresent Differences b/w artery and vein

27. The Heart The wall of each chamber of heart consists of 3 layers: Endo-, Myo- and Epicardium 1.Endocardium is a glistening inner layer Thick in atria, thin in ventricles At the arterial and venous orifices the endocardium is continuous with the intima of these vessels. Endocardium consists of 3 components: 1.an endothelium of polygonal cells with round nuclei. 2.It lies on a thin subendothelial layer of collagen fibers and smooth muscles. 3.Subendocardium: is a stouter layer of elastic tissue and smooth muscle cells. −This layer also contains, in the ventricles, specialised muscle fibres of the impulse conducting system. 2.Myocardium the middle layer. Consists of cardiac muscle

28. Epicardium is the visceral layer of serous pericardium. lined externally by simple sq epith underneath which is a thin layer of CT rich in elastic fibres. Deep to this layer is a subepicardial layer of LCT containing blood vessels and nerves.

29. Heart valves Each AV valve is attached at its base to annulus fibrosus (the skeleton of the heart). They consists of folds of endocardium coverig a central plate of dense collagenous tissue which is continuous with the CT of annulus fibrosus. The endocardium of the valve is thicker on the atrial than on the ventricular side.

30. Impulse conducting system Consists of specialized cardiac muscle fibres k/a Purkinje fibres. spindle shaped Different from ordinary cardiac muscles. Aggregations of these cells form nodes and bundles. 1.SA node the pacemaker Located in the right atrium At the junction of SVC and right atrium. 2.AV node is in the subendocardium of the median wall of right atrium.

31. Conducting system.....contd Crus commune, bundle of His Common stem arises from the AV node Is continued into the membranous septum of the ventricles Here it divides into 2 branches which go respectively to the right and left ventricles. These branches send branches to the papillary muscles and further divide to form an extensive network in the subendocardium of each ventricle. From the subendocardium branches continue into the myocardium where they further divide and finally merge with the regular cardiac muscle cells.

32. Lymph vascular system Consists of lymph capillaries, lymph vessels and lymphatic ducts Lymph is the tissue fluid that remains from the drainage by veins. Lymph circulation is one way flow Lymph capillaries begin blindly in the tissues Lymph capillaries enlarge into lymph vessels. They find lymph nodes in their path. Lymph nodes filter lymph and release lymphocytes in it. Histological aspects Lymph capillaries→ lymphatic vessels → Lymphatic ducts. 1.Lymph capillaries lined by discontinuous endothelium (gaps b/w cells). BL is interrupted. A thin layer of loose CT surrounds the endothelium.

33. 2.Lymphatic vessels The same three tunics, but not well-demarcated. T. Intima consists of endothelium A thin subendothelium contains collagen and elastic fibres. T. Media Circularly arranged SM fibres, and few elastic fibres. T. Adventitia Thick bundles, longitudinally arranged. Lymphatic Ducts Thoracic duct and right lymphatic duct resemble a vein of equal size. Large lymph vessels contain valves (like veins)