Histology LC 10: Cardiovascular System PDF

XXII. SUMMARY XXIII. RECITATION QUESTIONS DISCUSSION OUTLINE XXIV. REFERENCES I. INTRODUCTION TO THE CARDIOVASCULAR SYSTEM ○ Components...

XXII. SUMMARY XXIII. RECITATION QUESTIONS DISCUSSION OUTLINE XXIV. REFERENCES I. INTRODUCTION TO THE CARDIOVASCULAR SYSTEM ○ Components I. INTRODUCTION ○ Divisions II. HEART The circulatory system pumps blood (blood cells and ○ Blood Circulation Summary related substances) to all parts of the body. ○ Chambers of the Heart III. MAJOR LAYERS OF THE HEART ○ Epicardium COMPONENTS ○ Myocardium ○ Endocardium IV. MAJOR LAYERS OF BLOOD VESSELS Heart − propels blood ○ Tunica adventitia Arteries − different vessels / conducting / distributing ○ Tunica media vessels. It carries oxygenated blood and becomes ○ Tunica intima smaller as they branch into various organs. V. CARDIAC CONDUCTION SYSTEM Capillaries − smallest vessels. Site for the exchange of ○ Components ○ Nervous System Innervation oxygen, nutrients, carbon dioxide and waste products VI. FIBROUS CARDIAC MUSCLE between blood and the tissues. Capillaries together VII. MICROANATOMY OF BLOOD VESSELS with the smallest arterial and venous are collectively VIII. STRUCTURES FOUND BETWEEN THE called as microvasculature or microvascular bed. LAYERS OF THE BLOOD VESSELS Veins − from convergence of venules into larger ○ Smooth Muscle Fibers channels that continue enlarging as they approach the ○ Connective Tissue heart. It carries the deoxygenated blood to the heart. IX. LAYERS OF THE BLOOD VESSELS (ARTERIES) ○ Tunica intima DIVISIONS ○ Tunica media ○ Tunica adventitia X. VASA VASORUM Pulmonary Circulation - where blood is oxygenated in XI. CLASSIFICATION OF ARTERIES the lungs. XII. ELASTIC ARTERIES/LARGE ARTERIES ○ In the pulmonary circulation the right side of the ○ Elastic arteries heart pumps blood through pulmonary vessels, ○ Medical application: Atherosclerosis through the lungs for oxygenation, and back to the XIII. ARTERIAL SENSORY STRUCTURES left side of the heart. XIV. MUSCULAR ARTERIES XV. ARTERIOLES Systemic Circulation - where blood brings nutrients and ○ Structure of arterioles removes wastes in tissues throughout the body ○ Function of arterioles ○ Systemic circulation pumps blood from the left ○ Microvascular bed structure and side of the heart through vessels supplying either perfusion the head and arms or the lower body, and back to ○ Medical application the right side of the heart. XVI. CAPILLARIES XVII. TYPES OF CAPILLARIES When the body is at rest, 70% of blood moves through ○ Continuous capillaries ○ Fenestrated capillaries systemic circulation, 18% through pulmonary ○ Discontinuous capillaries circulation, and 12% through the heart. XVIII. PERICYTES ○ Medical application XIX. VENULES XX. VEINS ○ Small or Medium Veins ○ Large Veins XXI. VALVES HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Figure 1. Blood vascular system or the cardiovascular system Figure 2. Blood Circulation The system consisting of the heart, arteries, veins, and II. HEART micro-vascular beds is organized as the pulmonary circulation and the systemic circulation. In the pulmonary circulation the right side of the heart pumps blood through BLOOD CIRCULATION SUMMARY pulmonary vessels, through the lungs for oxygenation, and back to the left side of the heart. The larger systemic circulation pumps blood from the left side of the heart Cardiac muscle in the four chambers of the heart wall through vessels supplying either the head and arms or the contracts rhythmically, pumping the blood through the lower body, and back to the right side of the heart circulatory system ○ Blood from the body (through Superior & Inferior Vena Cava) –Right Atrium (RA) – Tricuspid Valve HEART CHAMBERS (Right Atrioventricular Valve)– Right Ventricle (RV) – Pulmonary Valve ((Pulmonary Semilunar Valve) Atria – Pulmonary Artery – Pulmonary Circulation – ○ Right and Left Atria - receive blood from the body Pulmonary Vein – Left Atrium (LA) – Bicuspid and the pulmonary veins, respectively. Valve (Mitral Valve) – Left Ventricle (LV) – Aortic Ventricles Valve (Aortic Semilunar Valve) – Aorta –Systemic ○ Right and Left Ventricle - propel blood to the Circulation. pulmonary and systemic circulation. Pulmonary artery is the only artery that carries Other Structures: deoxygenated blood. ○ Valves - flaps of connective tissue anchored in In terms of thickness, ventricles are thicker than atria. heart’s dense connective tissue (cardiac skeleton) Left ventricle is thicker than the right ventricle because ○ Chordae tendineae - cords that extend from of the high pressure in the aorta which the blood may cusps of both atrioventricular valves (AV valve) pass through going to the systemic circulation. and attach to papillary muscles, preventing the valves from turning inside-out during ventricular contraction. ○ Valves and cord covered by non thrombogenic endothelium 2 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Endocardium ○ Innermost layer ○ Near the atrial and ventricular cavity ○ Endothelial cells ○ Could be medial or lateral ○ Next to a space ○ Loose connective tissues and nerves can be seen. ○ Consists of: Thin inner lining layer of endothelium and supporting connective tissue with scattered fibers of smooth muscle. Middle myoelastic layer of smooth muscle fibers and connective tissue Deep layer of connective tissue called subendocardial layer that merges with myocardium. ○ Branches of the heart’s impulse-conducting system (Purkinje Fibers), consisting of modified cardiac muscle fibers are also located in the Figure 3. Heart subendocardial layer. As seen in the diagram, the human heart has two atria and two ventricles. The myocardium of the ventricular walls is thicker than that of the atria. The valves are basically flaps of connective tissue anchored in the heart’s dense connective tissue, or cardiac skeleton, concentrated in the regions shown in white. This fibrous tissue includes the chordae tendineae, cords which extend from the cusps of both atrioventricular valves and attach to papillary muscles, preventing the valves from turning inside-out during ventricular contraction. Valves and cords are covered by the nonthrombogenic endothelium. III. MAJOR LAYERS OF THE HEART Epicardium ○ Simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves. ○ During heart movements, underlying structures are cushioned by deposits of adipose tissue in epicardium. Figure 4. The subendocardial layer of connective tissue ○ Fat cells can also be seen (SEn) in the ventricles surrounds Purkinje fibers (P) of the ○ Friction is prevented by lubricant fluid produced by heart’s impulse conducting network. These are modified both layers of serous mesothelial cells. cardiac muscle fibers joined by intercalated disks but ○ It is divided into two layers: specialized for impulse conduction rather than contraction. Parietal epicardium: membrane surrounding Containing glycogen but relatively few organelles and the heart. peripheral myofibrils, Purkinje fibers typically are paler Visceral epicardium: It is the outermost staining than contractile muscle fibers (M). layer, it is where the large vessels enter and leave the heart. Myocardium ○ Middle layer ○ Cardiac muscle with fibers arranged spirally around each heart chamber ○ It has strong force required to pump blood through systemic and pulmonary circulations, thus thicker myocardium is found in the ventricle compared to the atria and thicker on the left side of the heart than on the right side. ○ Thicker layer than endocardium ○ Myocardial cells ○ Dense, Bright, Pink-staining ○ Blood vessels and nerve fibers can be seen 3 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag IV. MAJOR LAYERS OF BLOOD VESSELS Consists of three concentric layers (or tunica) composed of helically arranged smooth muscle and variable amounts of connective tissue. Tunica adventitia | Tunica Externa ○ Outermost layer mostly composed of Collagen Type I ○ Continuous and bound to the stroma of the heart through which blood vessels run Tunica media ○ Middle layer mostly composed of helically arranged smooth muscle ○ Has variable amounts of elastic fibers, elastic lamellae, reticular fibers, and proteoglycans Compared to the middle arterial layer which Figure 5. In the atrial walls conducting Purkinje-like fibers may have an external elastic lamina (P) often occupy most of the subendocardial layer, lying separating it from the tunica adventitia close to the endothelium (En), and merging with the Tunica intima contractile fibers of the myocardium (M), which is ○ Inner layer consisting of endothelium organized into many partially separated bundles and ○ Subendothelial layer of loose connective tissue muscles. Compared to the inner arterial layer which includes a thin layer called internal elastic lamina composed of elastin Figure 8. Walls of arteries and veins have three layers which correspond roughly to the three layers of the heart (tunica adventitia: epicardium; tunica media: myocardium; tunica intima: endocardium). An artery has a thicker media and relatively narrow lumen. A vein has a larger lumen, the Figure 6. Epicardium or Visceral Pericardium thickest layer is the adventitia, and the intima is often folded to form valves. Capillaries only have endothelium (no subendothelial layer nor other layers). V. CARDIAC CONDUCTING SYSTEM Some cardiac muscle cells are modified to specialize in the generation and conduction of depolarization waves which stimulate rhythmic contractions of myocardial fibers. COMPONENTS Figure 7. Myocardium, Epicardium and surrounding blood Sinoatrial node (pacemaker) vessels and nerve fibers ○ In the right atrial wall near the Superior Vena Cava 4 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag ○ 6-7 cubic mm mass of cardiac muscle cells with ○ Sympathetic nerve: accelerates pacemaker smaller size, fewer myofibrils, and fewer typical intercalated disks than the neighboring muscle Between fibers of myocardium afferent free nerve fibers. endings that register pain ○ Generated impulse travels to both right & left atrial ○ Discomfort (Angina pectoris) that occurs when myocardial fibers and trigger a contraction partially occluded coronary arteries cause local AV oxygen deprivation. ○ On the floor of the right atrium near the Atrioventricular valve ○ Smaller, cells similar to SA node ○ Conducting muscle fibers from the AV node form VI. FIBROUS CARDIAC MUSCLE the Atrioventricular bundle of His ○ Generated impulse travels from: AV node > bundle of His> pass through an opening in the cardiac Consists of dense irregular connective tissue, primarily skeleton into the interventricular septum > in endocardium, which anchors the valves and bifurcate into right & left bundle branches into the surrounds the AV canals, maintaining their proper walls of each ventricle shape Purkinje fibers ○ Mingle distally with contractile fibers of both ventricles and trigger waves of simultaneous contraction ○ The intercalated disks adjoining Purkinje fibers are specialized for impulse conduction rather than contraction. ○ The cytoplasm of these fibers are filled with glycogen which displace myofibrils to the periphery. This typically results in paler staining than contractile cardiac muscle fibers. Figure 10. The micrograph shows a section through a cusp of an atrioventricular valve (arrow) and attached chordae tendineae (CT). The collagen-rich connective tissue of the valves is stained pale blue here and is continuous with the fibrous ring of connective tissue at the base of the valves, which fills the endocardium between the atrium (A) and ventricle (V). The thick ventricular myocardium (M) is also shown. (X20; Masson trichrome) VII. MICROANATOMY OF BLOOD VESSELS Most larger blood vessel walls contain three major layers with sublayering. A. TUNICA INTIMA Luminal layer Figure 9. Purkinje fibers (P) are paler stained compared to Lumen is lined by an endothelium of contractile cardiac muscles (M). Purkinje fibers are below simple squamous epithelium. the endothelium (En) and the myoelastic layer, contained A subendothelial layer of loose within the subendocardial layer (SEn). fibroelastic connective tissue (FECT) is present in most medium to large vessels and may contain scattered smooth INNERVATION muscle in larger vessels. This layer is closest to the lumen Both parasympathetic and sympathetic neural components innervate the heart. − Ganglionic nerve B. INTERNAL ELASTIC LAMINA cells and nerve fibers present in regions close to the Elastica interna SA and AV nodes: Marks the boundary between the tunica ○ Affect heart rate and rhythm intima and the tunica media. ○ Parasympathetic stimulation (Vagus nerve) slows heartbeat C. TUNICA MEDIA 5 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Contains layers of either elastic ○ Nonthrombogenic surface - on which laminae/lamellae (fenestrated sheets) or blood will not clot and actively secretes FECT alternating with layers of smooth agents that control local clot formation muscle. (such as heparin, tissue plasminogen activator, and von Willebrand factor). D. EXTERNAL ELASTIC LAMINA ○ Regulate local vascular tone and Elastica externa blood flow - by secreting various factors If present, marks the boundary between that stimulate smooth muscle contraction the tunica media and the tunica (such as endothelin-1 and angiotensin adventitia. converting enzyme [ACE]) or relaxation (including nitric oxide [NO] and E. TUNICA ADVENTITIA prostacyclin). Contains loose to moderately dense ○ Inflammation and local immune FECT, +/- scattered smooth muscle cells. response Small and medium arteries and veins are present in the tunica adventitia of large arteries and veins Figure 11.Artery vs Vein Figure 13. Walls of arteries and veins with the endothelium found in the tunica intima SMOOTH MUSCLE FIBERS Occur in the walls of all vessels larger than capillaries Arranged helically in layer In arterioles and small arteries, the smooth muscle cells are connected by gap junctions and permit vasoconstriction and vasodilation that are of key importance in regulating the overall blood Figure 12. Flow of blood through the blood vessel pressure. CONNECTIVE TISSUE VIII. STRUCTURES FOUND BETWEEN THE LAYERS OF THE BLOOD Connective tissue components are present in VESSEL vascular walls in variable amounts and proportions based on local functional requirements. Collagen fibers ○ Found in the subendothelial layer ENDOTHELIUM between smooth muscle layers, and in the outer covering. Specialized epithelium that acts as a Elastic fibers semipermeable barrier between two major internal ○ Provide the resiliency for vascular wall compartments: blood plasma and interstitial tissue expansion under pressure in large fluid arteries forms parallel lamellae, regularly Vascular endothelial cells are squamous, distributed bet muscle layers. Elastin is a polygonal, and elongated with the long axis in major component in large arteries where direction of blood flow it forms parallel lamellae, regularly Endothelium with its basal lamina is highly distributed between the muscle layers. differentiated to mediate bidirectional exchange of ○ Transitions such as those from “small molecules by simple and active diffusion, receptor- arteries” to “arterioles” are not clear-cut. mediated endocytosis, transcytosis, and other However, all of these larger vessels have mechanisms. walls with three concentric layers, or Functions: tunics (L. tunica, coat) 6 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Because they carry deoxygenated blood, large veins commonly have more vasa vasorum than IX. LAYERS OF THE BLOOD VESSELS arteries Vasomoto nerves network uf unmyelinated autonomic nerve fibers, which release the TUNICA INTIMA vasoconstrictor norepinephrine. Density is greater than in veins. Innermost Consist of endothelium ○ With a thin subendothelial layer - loose connective + smooth muscle fibers Arteries and large veins ○ Includes prominent limiting later, internal elastic lamina: Elastin, with holes allowing better diffusion of substances from blood deeper into the walls. TUNICA MEDIA Middle layer Concentric layers of helically arranged smooth muscle cells Interposed among the muscle fibers are variable amounts of elastic fibers and elastic lamellae, reticular fibers, and proteoglycans, produced by Figure 15. The adventitia of the larger arteries contains a smooth muscle. supply of microvasculature to bring O2 and nutrients to Arteries: media may have thin external elastic local cells that are too far from the lumen to be nourished lamina, separating it from outermost tunic. by blood there. The following describes the figure above: arterioles (A), capillaries, and venules (V) constitute the vasa vasorum TUNICA ADVENTITIA (vessels of vessels). The adventitia of large arteries is also supplied more sparsely with small sympathetic nerves (N) for control of vasoconstriction. Above the adventitia in this Tunica externa section can be seen muscle fibers (SM) and elastic Type I collagen and elastic fibers lamellae (E) in the media (X100; H&E). Continuous with and bound to the stromal connective tissue of the organ through which the blood vessel runs XI. CLASSIFICATION OF ARTERIES ELASTIC ARTERIES/LARGE ARTERIES Conducting or Conduit Arteries ○ Major role is to carry blood to smaller arteries ○ Conduct blood from heart with elastic recoil to help move blood forward under Figure 14. Walls of arteries and veins steady pressure Closes to heart Largest, > 10 mm in diameter E.g. aorta and pulmonary, common carotid, X. VASA VASORUM subclavian, and common iliac arteries. Also known as the ‘vessels of the vessel’ Tunica media– Thick, in which elastic lamellae Arterioles, capillaries, and venules in the adventitia alternate with layers of smooth muscle fibers. and other part of the media ○ Elastic lamellae alternate with layers of Provide metabolites to cells in those tunics in smooth muscle fibers. larger vessels because wall is too thick to be ○ Adult aorta = 50 elastic lamellae (more if nourished solely by diffusion from the blood in the hypertensive) lumen 7 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag ○ Between elastic laminae - fibroblasts, Arterial blood pressure and blood velocity elastic fibers, collagen fibers, spiral (to decrease and become less variable as the circular) smooth muscle distance from the heart increases Tunica Intima– Thin, well developed, with many smooth muscle cells in the subendothelial connective tissue. Often shows folds in MEDICAL APPLICATION cross-section as a result of the loss of blood pressure and contraction of the vessel at death. ATHEROSCLEROSIS (Gr. athero, gruel or ○ Subendothelial layer- contains some porridge, and scleros, hardening) smooth muscle, elastic fibers, collagen ○ disease of elastic arteries and large fibers muscular arteries ○ Internal elastic lamina– Present ○ play a role in nearly half of all deaths in between intima and media. But not as developed parts of the world distinct as in other arteries because it is ○ initiated by damaged or dysfunctional similar to elastic laminae of next layer. endothelial cells oxidizing low- density Tunica adventitia - thin; consists mainly of lipoproteins (LDLs) in the tunica intima, collagen fibers, blood vessels, nerves; some which Induces es adhesion and intima elastic fibers, fibroblasts, macrophages may also entry of monocytes/macrophages to be present. remove the modified LDL. ○ Lipid-filled macro- phages (called foam cells) accumulate and, along with the free LDL, produce a pathologic sign of early atherosclerosis called fatty streaks ○ During disease progression these develop into fibro-fatty plaques, or atheromas, consisting of a gruel-like mix of smooth muscle cells, collagen fibers, and lymphocytes with necrotic regions of lipid, debris, and foam cells. ○ In elastic arteries atheromas produce localized destruction within the wall, weakening it and causing arterial bulges or aneurysms that can rupture. in muscular arteries such as the coronary arteries, atheromas can occlude blood flow to downstream vessels, leading to Figure 16. Comparison of the three major layers or tunics ischemic heart disease. in the largest artery and vein. The following describes the figure above: (a) Aorta, (b) vena cava. Simple squamous endothelial cells (arrows) line XII. ARTERIAL SENSORY STRUCTURES the intima (I) also have subendothelial connective tissue and in arteries are separated from the media by an internal elastic lamina (IEL), a structure absent in all but the largest CAROTID SINUSES veins. The media (M) contains many elastic lamellae and ○ slight dilations of bilateral internal carotid elastic fibers (EF) alternating with layers of smooth muscle. arteries where they branch from the The media is much thicker in large arteries than veins, with (elastic) common carotid arteries relatively more elastin. Elastic fibers are also present in the ○ baroreceptors - monitoring arterial outer tunica adventitia (A), which is relatively thicker in blood pressure large veins. Vasa vasorum (V) is seen in the adventitia of ○ media is thinner - allow greater distension the aorta. The connective tissue of the adventitia always when BP merges with the less dense connective tissue around it ○ adventitia contains sensory nerve (Both X122; Elastic stain). endings from cranial nerve IX, the glossopharyngeal nerve Ventricular contraction (systole) -> forceful ○ the brain's vasomotor centers process movement of blood through arteries -> elastin afferent impulses and adjust stretched -> distend wall within limit set by the vaso-constriction, maintaining normal wall’s collagen. blood pressure. Ventricles relaxation (diastole) -> ventricular ○ similar baroreceptors present in aortic pressure drops -> elastin rebounds passively, arch. helping to maintain arterial pressure COMPLEX CHEMORECEPTORS Aortic and pulmonary valves ○ monitor blood levels of CO2 and O2, H+ ○ prevent backflow of blood into the heart, found in carotid bodies (in walls of so the rebound continues the blood flow carotid sinus ) and aortic bodies (wall away from the heart of aortic arch ) 8 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag ○ parts of ANS called paraganglia with rich Thin subendothelial layer consisting of capillary networks scattered fine collagen and elastic fibers ○ large, neural crest-derived glomus (type and a few fibroblasts I) cells surround capillaries Internal elastic lamina ○ filled with dense-core vesicles containing ○ Prominent very distinct, usually dopamine, acetyl- choline, and other Nt , folded supported by by smaller satellite (type II) B. Tunica media cells Thick ○ respond to stimuli in the arterial blood, Contain up to 40 layers of large smooth primarily hypoxia (low O2), hypercapnia muscle cells interspersed with a variable (excess CO2), or acidosis, number of elastic lamellae (depending on ○ glossopharyngeal nerve sensory fiber the size of the vessel) forms synapses with glomus cells and Thickness decreases as diameter of signal brain centers to initiate vessel decreases cardiovascular and respiratory External elastic lamina adjustments that correct the condition. ○ present only in the larger muscular arteries (May be GLOMUS BODIES indistinct in smaller muscular ○ two small (0.5–5 mm-diameter) ganglion- arteries) like structures found near the common C. Tunica adventitia carotid arteries Thick ○ contain many large capillaries (C) Lymphatic capillaries, vasa vasorum, and intermingled with clusters of large glomus nerves are also found in the adventitia, cells (G) filled with vesicles of various and these structures may penetrate to neurotransmitters. the outer part of the media. ○ Supportive satellite cells (S) with Function: elongated nuclei ensheath each glomus ○ Distribute blood to the organs cell. and help regulate blood pressure by contracting or relaxing the smooth muscle in the media. Figure 18. LM wall of a muscular artery. In this partly constricted artery, the lumen caliber is small relative to the muscular wall thickness. A prominent internal elastic lamina Figure 17. Cells and capillaries in glomus bodies. (IEL) looks corrugated. Several layers of circular smooth (C) capillaries, (G) glomus cells, (satellite cells) muscle occupy the media (TM); loose connective tissue, the adventitia (TA). XIII. MUSCULAR ARTERIES With distance from heart, arteries have relatively less elastin and more smooth muscle Most arteries, large enough to have names, are of Medium to small arteries (also called muscular the muscular type. arteries) LAYERS OF MUSCULAR ARTERIES A. Tunica intima Thin Endothelium 9 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Figure 20. Microvasculature: (A) arterioles; (C) capillaries; and (V) venules Figure 19. A transverse section through a muscular (medium-caliber) artery shows a slightly folded intima with only sparse connective tissue between the endothelial cells (E) and internal elastic lamina (IEL). Multiple layers of smooth muscle (SM) in the media are thicker than the elastic lamellae and fibers with which they inter- sperse. Vasa vasorum (V) are seen in the adventitia. (X100; H&E) Clinical Point: Raynaud phenomenon ○ brief episodes of vasospasm in walls of small arteries and triggered by changes in temperature (cold or hot) and stress—leads to discoloration of fingers Figure 21. Arterioles (A), Tunica intima (I) consisting of or toe endothelium (E), Tunica media (M), and Tunica adventitia (Ad) XIV. ARTERIOLES STRUCTURE OF ARTERIOLES A. Tunica Intima Very thin; consisting only of endothelium Elastic lamina is absent B. Tunica Media Made up of 1-2 layers of circularly-arranged smooth muscle with some elastic fibers C. Tunica Adventitia Figure 22. Arteriole in transverse section: Smooth muscle Thin; consists of longitudinally-arranged collagen cells (SM), internal elastic lumina (IEL), endothelial cell and elastic fibers (En), and tunica adventitia (TA) FUNCTIONS OF ARTERIOLES 1) To redistribute blood flow to capillaries and to alter blood pressure by changing the peripheral resistance to blood flow 2) Arterioles can change their diameter very drastically, therefore affecting blood pressure and flow patterns 3) Arterioles are the major determinants of systemic blood pressure 4) They are referred to as peripheral resistance vessels 10 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Cyclical opening and closing of sphincters 🡪 essentially empty capillaries at any given time ○ with only about 5% (~300 mL in an adult) of the total blood volume moving through these structures. Composition: ○ Single layer of endothelial cell rolled up as a tube surrounded by basement membrane Total length: > 100,000 km Surface area: = 5000 m2 Thin walls, extensive surface area, and slow, pulsatile blood flow: - optimize capillaries for exchange of water and solutes between blood and tissues Basal lamina - helps determine which macromolecules interact with endothelial cells. Figure 23. (a) shows a well-perfused capillary bed with all - Average thickness is only 0.25 μm sphincters relaxed and open, (b) shows a capillary bed with Distinctive feature the blood shunted away by contracted sphincters - Is often Nucleus curved to accommodate the very small tubular structure MICROVASCULAR BED STRUCTURE AND The cytoplasm contains mitochondria and most other organelles, as well as a large population of PERFUSION membranous vesicles typically Along with basal lamina, junctional complexes Arterioles supplying a capillary bed typically form between the cells maintain the tubular structure, smaller branches called metarterioles in which with variable numbers of tight junctions having an the smooth muscle cells are dispersed as bands important role in capillary permeability. that act as precapillary sphincters Thoroughfare channels: distal portion of the metarteriole; lacks smooth muscle cells and CAPILLARY BED merges with the post-capillary venule. True capillaries: smallest vessels branching from The Richness (density) of the capillary network is the metarteriole and thoroughfare channel related to the metabolic activity of tissues. ○ Lacks smooth muscle cells (although Tissues with High Metabolic rate– Have pericytes may be present) abundant capillaries such as: Precapillary sphincters: sphincters regulate ○ Kidney blood flow into true capillaries ○ Liver ○ Cardiac and skeletal muscle MEDICAL APPLICATION Blood pressure (BP) depends on cardiac output (CO) and the total peripheral resistance (TPR) to blood flow Peripheral resistance: mostly due to arteriolar resistance Hypertension (HTn) or elevated BP may occur secondary to renal or endocrine problems, but more commonly essential hypertension, due to a wide variety of mechanisms that increase arteriolar constriction. Figure 24. Branching network of capillaries in the myocardium tissues such as cardiac muscle in the heart have high energy requirements so they have a dense, XVI. CAPILLARIES highly branched capillary network. The smallest vessels Permit and regulate metabolic exchange (e.g. O2, CO2, nutrient and waste product) between blood and surrounding tissues. Always function in groups called capillary beds whose size and overall shape conform to the structure supplied. Make up > 90% of the body’s vasculature 11 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag ○ Contract and relax so that blood can enter and exit ○ Humans have a lot of sphincters: esophagus, stomach (pyloric sphincters) Acts as doors to a specific organ or tissue Figure 25. Capillaries in skeletal muscle in transverse section. Tissues with Low Metabolic rate – Have few capillaries: ○ Smooth muscle ○ Dense connective tissue Supplied preferentially by one or more terminal arteriole branches called metarterioles (encircled by scattered smooth cells) Figure 27. Part a shows a well-perfused capillary bed with all the sphincters relaxed and open; part b shows a capillary bed with the blood shunted away by contracted sphincters. At any given moment, most sphincters are at least partially closed and blood enters the capillary bed in a pulsatile manner for maximally efficient exchange of nutrients, wastes, O2, and CO2 across the endothelium. Figure 26. Capillaries sectioned in transverse and longitudinal planes in a mesentery of the small intestine XVII. TYPES OF CAPILLARIES METARTERIOLES Terminal portions of arterioles CONTINUOUS CAPILLARIES Continuous with thoroughfare channels (lack muscle) connected with the post-capillary venules Consist of a single layer of smooth muscle and, by Tight, well-developed occluding junctions between vasoconstriction, control the amount of blood slightly overlapping endothelial cells continuity entering capillaries along endothelium Metarteriole muscle cells: act as precapillary Well-regulated metabolic exchange across cells. sphincters that control blood flow into the Most common capillaries Seen in muscle, connective tissue, lungs, exocrine True capillaries glands, and nervous tissue ○ branch from metarterioles Ultrastructurally numerous vesicles, transcytosis of ○ At the beginning of each true capillary, macromolecules in both directions muscle fibers act as precapillary Exchange is not via the holes sphincters that contract or relax to Vesicles can be found in the tissue which is not control blood entry incharge of the exchange via dilution since there are no holes. That’s how they exchange gasses and electrolytes PRECAPILLARY SPHINCTERS Contract 5 to 10 cycles per minute, causing blood to pass through capillaries in a pulsatile manner. When sphincters close, blood flows directly from the metarterioles and thoroughfare channels into postcapillary venules. 12 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Figure 28. Continuous capillary Figure 31. Ultrastructure of a fenestrated capillary sectioned in a transverse plane in the choroid plexus of a CNS ventricle. DISCONTINUOUS CAPILLARIES Commonly called sinusoids Permit maximal exchange of macromolecules Allow easier movement of cells between tissues and blood Individual endothelial cells have large perforations without diaphragms OkCollectively form a discontinuous layer, with Figure 29. Ultrastructure of a continuous capillary wide, irregular spaces between the cell sectioned in a transverse plane in the CNS. Highly discontinuous basal laminae and much larger diameters, often 30 to 40 μm, which slows FENESTRATED CAPILLARIES blood flow Found in the liver, spleen, some endocrine organs, and bone marrow have a sieve- like Also have tight junctions, but perforations endothelial cells penetrated by numerous small circular openings or fenestrations ∼ 80 nm diameter basal lamina is continuous and covers fenestrations. in organs with rapid interchange of substances between tissues and the blood, Seen in organs where molecular exchange with blood is important, such as kidneys, intestine, choroid plexus, and endocrine glands Figure 32. Discontinuous capillary Figure 30. Fenestrated capillary Figure 33. Sinusoidal capillary (bone marrow) 13 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Moves toward heart by contraction of smooth muscle fibers in media and by external XVIII. PERICYTES compressions from surrounding muscles and other organs mesenchymal cells along continuous capillaries Valves project from the tunica intima to prevent and postcapillary venules backflow of blood long cytoplasmic processes partly surrounding endothelial layer VENULES produce own basal lamina, which may fuse with endothelial cells With well-developed networks of myosin, actin, Size varies from 10 microns (post-capillary and tropomyosin venules) to 1 mm (muscular venules) ○ contractile function to facilitate flow of They have larger diameters than capillaries; blood cells. consist of endothelium surrounded by pericytes After tissue injuries, pericytes proliferate and Functions: differentiate to form smooth muscle and other cells ○ Collect blood from capillaries in new vessels as the microvasculature is ○ Respond to vasoactive agents (e.g., reestablished. histamine, serotonin) by altering Supporting cells in the capillaries. They could permeability differentiate and become endothelium or smooth ○ A site of material exchange between muscle. tissue fluid and blood ○ Site of exit for white blood cells (WBCs) from blood into the tissue Figure 35. Sections of the three types of blood vessels Figure 34. Capillaries are normally associated with Three types of venules: small, medium, and perivascular contractile cells called pericytes (P) which large have a variety of functions. The more flattened nuclei All arteries have a corresponding vein beside each belong to endothelial cells. one Medical Application: Transition from capillaries to venules occurs hyperglycemia in diabetes diabetic gradually microangiopathy Immediate postcapillary venules are similar to ○ diffuse thickening of capillary basal capillaries with pericytes, but their diameters range laminae from 15-20 μm ○ decrease in metabolic exchange, Primary site where WBCs adhere to the particularly in kidneys, retina, skeletal endothelium and leave the circulation at sites of muscle, and skin. infection or tissue damage ○ Very common disease that affects the Postcapillary venules converge into larger capillaries, the reason for many collecting venules that have more contractile complication cells ○ Impaired exchange of sugar, oxygen and With greater size, venules become surrounded by nutrients tunica media with 2-3 smooth muscle layers: ○ Capillaries are everywhere, i.e. kidney muscular venules disease, change in skin color, increased Characteristic feature of all venules is the large stroke risk, increased risk for heart attack diameters of their lumen compared to the overall in diabetes, prone to cardiovascular thinness of the wall diseases XX. VEINS Carry blood back to the heart from microvasculature all over the body Blood under very low pressure 14 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag Figure 38. Veins usually travel as companions to arteries and are classified as small, medium, or large based on size and development of the tunics. 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. Figure 36. (a) Compared to arterioles (A), postcapillary venules (V) have large lumens with some pericytes (P); (b) Larger collecting venules (V) have bigger diameters than arterioles (A) Figure 39. Small or medium veins LARGE VEINS big venous trunks, paired with elastic arteries Figure 37. Light-microscopy of a venule and close to heart arteriole in transverse section. The venule has a ○ Tunica intima: well developed thin wall and a relatively larger lumen than the ○ Tunica media: thin with alternating layers arteriole. The lumen of each vessel holds many of smooth muscle and connective tissue. erythrocytes, but the venule lumen also has many ○ Tunica adventitia: thicker than media in white blood cells, a feature often seen in sections large veins and frequently contains of venules. Venules have thin walls and are thus longitudinal bundles of smooth muscle. the main site of migration of leukocytes from the Both the media and adventitia contain elastic bloodstream to tissues. Via contraction, smooth fibers, but internal and external elastic laminae like muscle in arterioles regulates pressure in the those of arteries are not present arterial system. Function: to collect blood from medium sized veins and return it to heart SMALL OR MEDIUM VEINS Diameters of 10mm or less Located close and parallel to corresponding muscular arteries A. Tunica intima: thin subendothelial layer B. Tunica media: small bundles of smooth muscle cells + reticular fibers + delicate network of elastic fibers C. Tunica adventitia: well-developed collagenous layer Function: to collect blood from smaller venous vessels Figure 40. Wall of large vein with valve. muscular media layer (M); adventitia (A) of dense irregular connective tissue; intima (I) projecting into the lumen as a valve (V) 15 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag XXI. VALVES paired folds of the intima projecting across the lumen in medium and large veins rich in elastic fibers lined on both sides by endothelium numerous in veins of the legs ○ help keep the flow of venous blood directed toward the heart Figure 43. Size ranges, major features, and important roles of major blood vessel types (zoomed in) XXII. SUMMARY Table 1. A summary of the differences among the types of blood vessels Arteries Veins Capillaries They have thick They have thin They do not layers of layers of have muscles muscles and muscles and and elastic fibers elastic fibers elastic fibers They have a They have a They have a narrow lumen wide lumen very small lumen They have a They have a Wall is only one thick outer wall thin outer wall cell thick No semilunar Have semilunar No semilunar valves valves to prevent valves backflow of blood Arteries are Veins are Capillaries are situated deep to situated near found inside all Figure 41. Size ranges, major features, and important roles body surface the body tissues of major blood vessel types (Table 11-1, Pge 224 surface Junqueira’s Basic Histo) Table 2. Summary of different blood vessels Arteries & Capillaries Veins & Arterioles Venules Function: carry Function: allow Function: carry blood from the exchange of blood from heart to the materials tissues to the tissues between the heart blood and the tissues Thick walls with Very thin, Thin walls, Figure 42. Size ranges, major features, and important roles elastic layers to permeable walls, mainly collagen, of major blood vessel types (zoomed in) resist high only one cell since blood at pressure and thick to allow low pressure smooth muscle exchange of layer materials Small lumen Very small Large lumen to lumen; blood reduce cells must distort resistance to to pass through flow 16 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag c. Bundle of His No valves No valves Many valves to d. Purkinje Fiber (except in heart) prevent backflow 8. Which of the following is/are incorrectly paired. a. Bathmotropy: degree of excitability Blood at high Blood pressure Blood at low b. Chronotropy: rate of heartbeat pressure falls in capillaries pressure c. Inotropy: the strength of contractility d. Dromotropy: rate of chemical Blood usually Blood changes Blood usually impluses in the heart oxygenated from deoxygenated 9. Which of the following is/are true about heart (except in oxygenated to (except in a. It beats normally 100,000 times a day. pulmonary deoxygenated pulmonary b. Pumps 1.5 gallon of blood every minute. circulation) (except in circulation) c. Has its own electrical impulse and will pulmonary continue to pump to beat when separated circulation) from your body for period of time. d. Every cell in the body gets blood from the heart. 10. These are contractile fibers joined by intercalated XXIII. RECITATION QUESTIONS disks specialized for impulse conduction a. Purkinje fibers b. Collagen fibers 1. T/F. The myocardium is thicker within the atria, as c. Nerve fibers this muscle must pump all blood returning to the d. Elastic fibers heart into the lungs for oxygenation. 11. Smallest vessels which are the sites of oxygen, FALSE (Ventricular myocardium is thicker) carbon dioxide, nutrient and waste products 2. It is reflected back as a parietal layer lining the exchange between blood and tissues? pericardium. a. Endocardium ❖ Answer: Capillaries b. Myocardium 12. At rest, how many percent of blood is in the c. Epicardium pulmonary circulation? d. Pericardium ❖ Answer: 18% 3. Fat cells can also be seen in the: 13. How many percent is in the systemic circulation? a. Epicardium ❖ Answer: 70% b. Myocardium ❖ the rest (12%) is inside your heart c. Endocardium d. Pericardium 14. Normal pathway for circulation from the body 4. What is the effect of the parasympathetic going back to systemic circulation? stimulation on heartbeat? ❖ Answer: Right atrium - Right ventricle a. Rapid heartbeat - Pulmonary artery - Pulmonary b. Slows heartbeat circulation - Pulmonary vein - Left c. No effect atrium - Left Ventricle - Systemic d. Irregular beating 5. True about of AV node except; Circulation a. Found on the floor of the right atrium near 15. How many atria and ventricles do we have? AV valve ❖ Answer: 2 each b. Conduct fibers from AV node, pass ❖ Atria as receiving chambers & ventricles through an opening in the cardiac as pumping chambers skeleton into interventricular septum and 16. Cross sectional view of the heart: what are the 3 bifurcate into RBB and LBB into wall of layers of the heart? each ventricle c. 6 to 7 mm3 mass of cardiac muscle ❖ Answers: cells with smaller size, fewer Innermost layer: Endocardium myofibrils, and fewer typical Middle layer: Myocardium intercalated disks than the Outermost layer: Epicardium/ neighboring muscle fibers. Pericardium d. NOTA 17. What are the 3 layers of the blood vessels? 6. At rest, the percentage of blood in the systemic ❖ Answer: circulation is a. 12% Innermost layer: Tunica intima b. 18% Middle layer: Tunica media c. 70% Outermost layer: Tunica adventitia d. 60% / Tunica externa 7. 6- to 7-mm3 mass of cardiac muscle cells with 18. What do you call the fluid in between the smaller size, fewer myofibrils, and fewer typical pericardium? intercalated discs found in the wall of the right atrium ❖ Answer: Pericardial fluid a. Atrioventricular node 19. What are the two pericardium present in your b. Sinoatrial node heart? 17 HISTOLOGY LC 10: Cardiovascular System - Dr. Pebble Narita Agdamag ❖ Answer: Visceral and Parietal 26. Fibrous cardiac skeleton’s main function is to pericardium anchor the valves, how many valves do we have? ❖ Directly surrounding your heart is the ❖ Answer: 4 valves visceral; parietal is the one that is further ❖ Usually these valves are found in your away from heart endocardium (innermost layer) mostly 20. Endocardium consists of 3 layers? made up of connective tissue ❖ Answer: Inner layer thin endothelium Middle Myoelastic (smooth XXIV. REFERENCE muscle fibers) & the connective tissues Mescher, A.L. (2018). Junqueira’s Basic Histology Atlas of Deep Subendocardial layer Histology. 15th edition. US: McGraw-Hill Education. 21. Endocardium is special because of these special fibers which run just beneath the endocardium and part of the conduction system of the heart: ❖ Answer: Purkinje fibers ❖ At electrical activity- SA - AV - Bundle branch- Common bundle branch - Right and Left - Purkinje fibers which goes on top of your atria into your ventricles 22. Main difference between atria and ventricle? ❖ Answer: Atria: thinner myocardium; receives blood Ventricles: thicker myocardium; propels blood 23. Difference between the right ventricle & left ventricle? ❖ Answer: Left ventricle is thicker because it is a higher pressure chamber compared to the right ventricle. ❖ Notes: ○ Both ventricles have afterload (which is the pressure by which your heart pumps against/ generally known as the blood pressure) ○ Diastolic bp: 80mmHg ○ Systolic bp: 120mmHg (also your systemic blood pressure) ○ Pulmonary pressure: 40mmHg or less ○ Right ventricle will pump against a 40mmHg pressure while the left ventricle will pump against 120mmHg. Since the right ventricle will pump against lower pressure, its myocardium doesn’t have to be very thick. 24. Innervations of the heart: between your sympathetic and parasympathetic, which one will slow down your heart rate? ❖ Answer: Parasympathetic ❖ Sympathetic: accelerates heart rate 25. When you have occluded coronary arteries (ex. on stable coronary heart disease), what do you call the type of pain that you have? ❖ Answer: Angina pectoris 18