SCPA602 Cardiovascular System

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1 SCPA602 Cardiovascular System Associate Professor Dr. Wannee Jiraungkoorskul Department of Pathobiology, Faculty of Science, Mahidol University Tel: , 1

2 Objectives Describe the anatomy and histology of cardiovascular system Describe the interrelationships and functions of the different parts of the cardiovascular system. 2

3 HEART:- FUNCTION Distribution of O 2 & nutrients (glucose, amino acids) to tissue Transportation of CO 2 and waste products from tissue Distribution of water, electrolytes and hormones through the body Supporting the host defense system Thermoregulation 3

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5 HEART:- CIRCUIT Pulmonary Circulation The right ventricle contracts forcing blood through the pulmonary valve into the pulmonary artery Blood traverses the lung capillaries and enters the pulmonary vein and finally the left atrium The heart itself receives its blood via the coronary circulation Systemic Circulation Blood from left ventricle, enters aorta Blood flows to arterioles to capillaries then to venules and converge at superior and inferior vena cava enters right atrium and flows through the tricuspid valve into the right ventricle 5

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8 HEART 8

9 HEART:- PERICARDIUM Pericardium composed of two layers separated by a space called the pericardial cavity Outer layer - the parietal pericardium Inner layer - the visceral pericardium 9

10 HEART:- PERICARDIUM Pericardial cavity between visceral and parietal pericardium filled with pericardial fluid about 25 to 35 ml lubricates and reduces friction between heart and sac Pericarditis inflammation of pericardium; causes dryness of membrane and painful friction of the heart as it beats 10

11 HEART:- PERICARDIUM The parietal pericardium consists of Outer layer - thick, fibrous connective tissue, attached to the diaphragm forms a strong protective sac for the heart Inner layer -serous, consisting largely of mesothelium, small amount of connective tissue, forms a simple squamous epithelium and secretes a small amount of fluid. 11

12 HEART:- PERICARDIUM The visceral pericardium (epicardium) forms the outer covering of the heart and has an external layer of flat mesothelial cells M. These cells lie on a stroma of fibrocollagenous F support tissue, which contains elastic fibres. A = adipose tissue 12

13 Epicardium or visceral pericardium The external tunic of the heart, the epicardium, is the site of the coronary vessels and contains considerable adipose tissue. This section of atrium shows part of the myocardium (M) and epicardium (Ep). The epicardium consists of loose connective tissue (CT) containing both autonomic nerves (N) and fat (F). The epicardium is the visceral layer of the pericardium and is covered by the simple squamous-to-cuboidal epithelium (arrows) that also lines the pericardial space. These mesothelial cells secrete a lubricate fluid that prevents friction as the beating heart contacts the parietal pericardium on the other side of the pericardial cavity. X100. H&E. 13

14 Epicardium or visceral pericardium 14

15 HEART:- MYOCARDIUM Myocardium - contractile element composed of specialised striated muscle fibres called cardiac muscle. The amount of myocardium and the diameter of muscle fibres in the chambers of the heart varies according to the workload of the chamber. The left and right atria push blood into empty ventricles against minimal resistance during diastole and therefore have a thin wall composed of cells of small diameter. 15

16 HEART:- MYOCARDIUM The right ventricle pushes blood through the pulmonary valve and through the lungs, to enter the left atrium. It therefore has a moderately thick muscle layer composed of fibres intermediate in diameter between atrial and left ventricular muscle cells. The left ventricle pumps blood through the highpressure systemic arterial system and therefore has the thickest myocardium with the largest diameter muscle fibres. 16

17 HEART:- MYOCARDIUM A section through the left and right ventricles showed the difference in thickness between the wall and their different shapes. 17

18 HEART:- ENDOCARDIUM Endocardium - composed of three layers 1. Outmost layer - irregularly arranged collagen fibres, Purkinje fibres (conducting system) 2. Middle layer - thickest endocardial layer, regularly arranged collagen fibres, variable numbers of elastic fibres 3. Inner layer - flat endothelial cells, which are continuous with the endothelial cells lining the vessels entering and emerging from the heart. 18

19 Endocardium & subendocardial conducting network The endocardium (En) is a thin layer of connective tissue lined by simple squamous endothelium. Between the endocardium and myocardium is a layer of variable thickness called the subendocardial layer (SEn) containing small nerves and in the ventricles the conducting (Purkinje) fibers (P) of the subendocardial conducting network. These fibers are cardiac muscle cells joined by intercalated disks but specialized for impulse conduction rather than contraction. Purkinje fibers are usually larger than contractile cardiac muscle fibers with large amounts of lightly stained glycogen filling most of the cytoplasm and displacing sparse myofibrils to the periphery. (a): Purkinje fibers running separately within the subendocardial layer. 19

20 Endocardium & subendocardial conducting network (b): Purkinje fibers intermingling with contractile fibers within the myocardium (M). Along with the nodes of specialized cardiac muscle in the right atrium which generate the electrical impulse, the network of conducting fibers comprises the conducting system of the heart. Both X200. H&E. 20

21 HEART:- VALVES Ensure one-way blood flow Atrioventricular (AV) valves (2) right AV valve has 3 cusps (tricuspid valve) left AV valve has 2 cusps (mitral, bicuspid valve) chordae tendineae - cords connect AV valves to papillary muscles (on floor of ventricles) Semilunar valves - control flow into great arteries Pulmonary semilunar valve: from right ventricle into pulmonary trunk Aortic semilunar valve: from left ventricle into aorta 21

22 aortic valves mitral valves pulmonary valves tricuspid valves 22

23 HEART:- VALVES Consist of leaflets of collagenous tissue, the surfaces being invested with a thin endothelial layer continuous with that of the heart chambers and great vessels. The base of a valve arising from the myocardium M. The tough central fibrous sheet, the lamina fibrosa F, represents a merging of the fibro-elastic supporting layers S beneath the endothelium E. Burkitt, Young, Heath (1993) Wheater's Functional Histology 3rd Ed. Fig. 8.5, p

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25 ARTERIES 3 types: large/elastic medium/muscular small arteries/arterioles All 3 have same layers in wall: tunica interna (intima) tunica media tunica externa (adventitia) 25

26 Tunica intima The tunica intima has one layer of endothelial cells supported by a thin subendothelial layer of loose connective tissue with occasional smooth muscle cells. In arteries, the intima is separated from the media by an internal elastic lamina, the most external component of the intima. This lamina, composed of elastin, has holes (fenestrae) that allow the diffusion of substances to nourish cells deep in the vessel wall. As a result of the loss of blood pressure and contraction of the vessel at death, the tunica intima of arteries may have a slightly folded appearance in tissue sections. 26

27 Tunica media The tunica media, the middle layer, consists chiefly of concentric layers of helically arranged smooth muscle cells. Interposed among the smooth muscle cells are variable amounts of elastic fibers and lamellae, reticular fibers of collagen type III, proteoglycans, and glycoproteins, all of which is produced by these cells. In arteries, the media has a thinner external elastic lamina, which separates it from the tunica adventitia. 27

28 Tunica adventitia The tunica adventitia or tunica externa consists principally of type I collagen and elastic fibers. This adventitial layer is gradually continuous with the stromal connective tissue of the organ through which the blood vessel runs. 28

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30 ARTERIES:- WALL 30

31 Tunics of the vascular wall Comparison of the three major layers or tunics in the largest artery and vein. (a): aorta (b): vena cava. Simple squamous endothelial cells (arrows) line the tunica intima (I) which has subendothelial loose connective tissue and is separated from the tunica media by the internal elastic lamina (IEL), a prominent sheet of elastin. The media (M) contains elastic lamellae and fibers (EF) and multiple layers of smooth muscle not seen well here. The tunica media is much thicker in large arteries than veins, with relatively more elastin. Elastic fibers are also present in the outer tunica adventitia (A), which is relatively thicker in large veins. Vasa vasorum (V) are seen in the adventitia of the aorta. The connective tissue of the adventitia always merges with the less dense connective tissue around it. Both X122. Elastic. 31

32 Tunics of the vascular wall 32

33 Arterioles (a): Arterioles are microvessels with a tunica intima (I) that consists only of the endothelium (E), in which the cells may have rounded nuclei. They have tunica media (M) with only one or two layers of smooth muscle, and usually thin, inconspicuous adventitia (Ad). X350. Masson trichrome. 33

34 Arterioles (b): Three arterioles of various sizes are shown here and a capillary. X400. H&E. 34

35 Arterioles (c): A large mesenteric arteriole is cut obliquely and longitudinally and clearly shows the endothelial cells (arrow heads) and one or two layers of smooth muscle cells (M) cut transversely. Adventitia merges imperceptibly with neighboring connective tissue. X300. PT. 35

36 VEINS Classified as large, medium and small Large and medium veins usually accompany large and medium arteries Veins have lower blood pressure: avg. 10mmHg with little fluctuation thinner walls, less muscular and elastic tissue expand easily, have high capacitance one-way venous valves aid skeletal muscles in upward blood flow 36

37 Vein Veins usually travel near arteries and are classified as small, medium, or large based on size and development of the tunics. (a): Micrograph of small vein (V) shows a relatively large lumen compared to the small muscular artery (A) with its thick media (M) and adventitia (Ad). The wall of a small vein is very thin, containing only two or three layers of smooth muscle. X200. H&E. 37

38 Vein (b): Micrograph of a convergence between two small veins showing valves (arrow). Valves are thin folds of tunica intima projecting well into the lumen which act to prevent backflow of blood. X200. H&E. 38

39 Vein (c): Micrograph of a medium vein (MV) showing a thicker wall, but still less prominent than that of the accompanying muscular artery (MA). Both the media and adventitia are better developed, but the wall is often folded around the relatively large lumen. X100. H&E. 39

40 (d): Micrograph of a medium vein containing blood and showing valve folds (arrows). X200. Masson trichrome. Vein 40

41 Wall of large vein with valve. Large veins have a muscular tunica media (TM) that is very thin compared to the tunica adventitia (TA) composed of dense irregular connective tissue. The wall is often folded as shown here. The tunica intima here projects into the lumen as a valve (V), composed of the subendothelial connective tissue with endothelium on both sides. X100, PT. 41

42 Vessels of the microvasculature. Arterioles (A), small capillaries (C) and venules (V) make up the microvasculature where, in almost every organ, exchange takes place between blood and the interstitial fluid of the tissues. X200. Masson trichrome. 42

43 References Kierszenbaum AL, Tres L. Histology and Cell Biology: An Introduction to Pathology. 4 th ed., Sounders. 2015, 752pp. Moore KL, Agur AMR and Dalley AF Essential Clinical Anatomy. 5 nd Edition. Wolters Kluwer. 686pp. 43