Histology Of The Circulatory System PDF

Histology of the Circulatory System Georgette E. Lee, MD, FPCP, DPSN UPHR College of Medicine  Objectives Be able to distinguish successive parts of the circulatory pathway, and explain how the structure of the vessel wall meets the functional needs that...

Histology of the Circulatory System Georgette E. Lee, MD, FPCP, DPSN UPHR College of Medicine  Objectives Be able to distinguish successive parts of the circulatory pathway, and explain how the structure of the vessel wall meets the functional needs that are present in each of the parts.  Objectives Be able to describe the path of the cardiac impulse from sinoatrial node to the ventricle, and recognize the cells in the ventricle that are involved Recognize features artery, vein, capillaries, lymphatic vessels  The Circulatory system pumps and directs blood cells and substances carried in blood to all tissues of the body. It includes both the blood and lymphatic vascular systems  The Circulatory system Adult : the total length of its vessels is estimated at between 100,000 and 150,000 kilometers The internal surface of all components of the blood and lymphatic systems is lined by a simple squamous epithelium called endothelium. The Cardiovascular System Consists of the following: Heart Arteries Capillaries Veins The Cardiovascular System Heart – propels blood through the system The Cardiovascular System Arteries – series of vessels efferent from the heart that become smaller as they branch into the various organs The Cardiovascular System Arteries – Carry oxygenated blood to the tissues The Cardiovascular System Capillaries smallest vessels are the sites of O2, CO2, nutrient, and waste product exchange between blood and tissues The Cardiovascular System Capillaries form a complex network of thin, anastomosing tubules called the microvasculature or microvascular bed The Cardiovascular System Veins result from the convergence of venules into a system of larger channels they carry the deoxygenated blood The Cardiovascular System two major divisions pulmonary circulation Systemic circulation The Cardiovascular System pulmonary circulation – where blood is oxygenated in the lungs The Cardiovascular System Systemic circulation – where blood brings nutrients and removes wastes in tissues throughout the body HEART  HEART right and left ventricles propel blood to the pulmonary and systemic circulation (BLUE ARROW)  HEART right and left atria receive blood from the body and the pulmonary veins  Heart Wall : Three Major Layers Endocardium Myocardium Epicardium  Heart : Endocardium inner layer of the heart (lines the atria and ventricles and covers the heart valves) and contains blood vessels. Has 3 sublayers: – Endothelium – Smooth Muscle and Connective Tissue Layer – Subendocardial Layer  Heart : Endocardium Endothelium - innermost portion a simple squamous epithelium. Smooth Muscle and Connective Tissue - middle layer of the endocardium is mix of connective tissue and smooth muscle.  Heart : Endocardium Subendocardial Layer - outer layer of the endocardium is loose connective tissue joining the endocardium and myocardium. Heart : Endocardium  Heart : Endocardium Subendocardial layer – Where branches of the heart’s impulse- conducting system, consisting of modified cardiac muscle fibers are also located Heart : Endocardium  Heart : Myocardium Thickest layer consists mainly of cardiac muscle with its fibers arranged spirally around each heart chamber. thicker in the walls of the ventricles, particularly the left, than in the atrial walls  Cardiac skeleton Dense fibrous connective tissue of the cardiac skeleton forms part of the interventricular and interatrial septa, surrounds all valves of the heart, and extends into the valve and the chordae tendineae to which they are attached  Cardiac skeleton These regions of dense irregular connective tissue perform the following functions: – Anchoring and supporting the heart valves – Providing firm points of insertion for cardiac muscle – Helping coordinate the heartbeat by acting as electrical insulation between atria and ventricles Heart : Myocardium Heart : Myocardium  Heart : Epicardium a simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerve The epicardium corresponds to the visceral layer of the pericardium, the membrane surrounding the heart.  Heart : Epicardium During heart movements, underlying structures are cushioned by deposits of adipose tissue in the epicardium and friction within the pericardium is prevented by lubricant fluid produced by both layers of serous mesothelial cells. Heart : Epicardium Heart : Epicardium Heart : Epicardium  Conducting System of the Heart generates and propagates waves of depolarization that spread through the myocardium to stimulate rhythmic contractions. two nodes of specialized myocardial tissue in the right atrium – sinoatrial (SA) node (or pacemaker) – atrioventricular (AV) node AV bundle (of His) subendocardial conducting network  SA Node Located in the right atrial wall near the superior vena cava 6- to 7-mm3 mass of cardiac muscle cells  SA Node smaller size, fewer myofibrils, and fewer typical intercalated disks than the neighboring muscle fibers. Impulses initiated by these cells move along the myocardial fibers of both atria, stimulating their contraction  AV Node located in the floor of the right atrium near the AV valve composed of cells similar to those of the SA node, stimulate depolarization of those cells  AV Node Conducting muscle fibers from the AV node form the AV bundle, pass through an opening in the cardiac skeleton into the interventricular septum, and bifurcate into the wall of each ventricle as the left and right bundle branches.  Purkinje Fibers Impulse conducting fibers Large modified muscle cells – Cluster in groups together – 1-2 nuclei and stain pale due to fewer myofibrils Terminal branches of the AV bundle branches located in the subendocardial connective tissue Valve leaflet and cardiac skeleton.  TISSUES OF THE BLOOD VESSELS ENDOTHELIUM SMOOTH MUSCLE CELLS CONNECTIVE TISSUES  ENDOTHELIUM specialized epithelium that acts as a semipermeable barrier between two major internal compartments: – the blood and the interstitial tissue fluid. Vascular endothelial cells are squamous, polygonal, and elongated with the long axis in the direction of blood flow.  ENDOTHELIUM basal lamina is highly differentiated to mediate and actively monitor the bidirectional exchange of molecules by simple and active diffusion, receptor-mediated endocytosis, transcytosis, and other mechanisms (METABOLITE EXCHANGE)  ENDOTHELIUM: OTHER FUNCTIONS Nonthrombogenic surface – blood will not clot and actively secretes agents that control local clot formation (such as heparin, tissue plasminogen activator, and von Willebrand factor)  ENDOTHELIUM: OTHER FUNCTIONS Cells regulate local vascular tone and blood flow – secreting various factors that stimulate smooth muscle contraction (such as endothelin 1 and angiotensin converting enzyme [ACE]) or relaxation (including nitric oxide [NO] and prostacyclin.  ENDOTHELIUM: OTHER FUNCTIONS Roles in inflammation and local immune responses. – Venules endothelial cells à induce specific white blood cells to stop and undergo transendothelial migration at sites of injury or infection. – cells also secrete various factors called interleukins that affect the activity of local white blood cells during inflammation.  ENDOTHELIUM: OTHER FUNCTIONS secrete various growth factors – proteins promoting proliferation of specific white blood cell lineages and cells that make up the vascular wall. – vascular endothelial growth factor (VEGF) stimulate formation of the vascular system from embryonic mesenchyme (vasculogenesis), help maintain the vasculature in adults, and promote capillary sprouting and outgrowth from small existing vessels  Smooth muscle Smooth muscle fibers occur in the walls of all vessels larger than capillaries arranged helically in layers Arterioles and small arteries: connected by many more gap junctions and permit vasoconstriction and vasodilation which are of key importance in regulating the overall blood pressure.  Connective tissue components are present in vascular walls in variable amounts and proportions based on local functional requirements Collagen fibers found in the subendothelial layer, between the smooth muscle layers, and in the outer covering.  Connective tissue Elastic fibers provide the resiliency required for the vascular wall to expand under pressure. – Elastin is a major component in large arteries where it forms parallel lamellae, regularly distributed between the muscle layers. Three Major layers of the Blood vessel Three Major layers of the Blood vessel 1. TUNICA INTIMA – Endothelium and a thin subendothelial layer of loose connectivetissue sometimes containing smooth muscle fibers – arteries the intima includes a thin layer, the internal elastic lamina, composed of elastin, with holes allowing better diffusion of substances from blood deeper into the wall. Three Major layers of the Blood vessel 2. TUNICA MEDIA – Interposed among the muscle fibers are variable amounts of elastic fibers and elastic lamellae, reticular fibers, and proteoglycans, all of which are produced by the smooth muscle cells. Three Major layers of the Blood vessel 2. TUNICA MEDIA – Middle layer, consists chiefly of concentric layers of helically arranged smooth muscle cells – In arteries the media may also have an external elastic lamina separating it from the outermost tunic Three Major layers of the Blood vessel 3. TUNICA ADVENTITA – outer adventitia / tunica externa, – Connective tissue consisting principally of type I collagen and elastic fibers – Continuous with and bound to the stroma of the organ through which the blood vessel runs.  VASA VASORUM “vessels of the vessel” required to provide metabolites to cells in those tunics in larger vessels because the wall is too thick to be nourished solely by diffusion from the blood in the lumen. large veins commonly have more vasa vasorum than arteries – Because they carry deoxygenated blood VASCULATURE  Elastic arteries the aorta, the pulmonary artery, and their largest branches Also called conducting arteries – major role is to carry blood to smaller arteries.  Elastic arteries thick tunica media in which elastic lamellae alternate with layers of smooth muscle fibers. – adult aorta has about 50 elastic lamellae (more if the individual is hypertensive). Tunica intima is well developed – many smooth muscle cells in the subendothelial connective tissue, and often shows folds in cross section as a result of the loss of blood  Elastic arteries internal elastic lamina is more well-defined than the elastic laminae of the Media – The numerous elastic laminae of these arteries contribute to their important function of making the blood flow more uniform. – Absent in the veins except for the larger veins adventitia is much thinner AORTA VENA CAVA  Muscular Arteries Distributing arteries Distribute blood to the organs and help regulate blood pressure by contracting or relaxing the smooth muscle in the media. The intima has a thin subendothelial layer and a prominent internal elastic lamina.  Muscular Arteries The media may contain up to 40 layers of large smooth muscle cells interspersed with a variable number of elastic lamellae An external elastic lamina is present only in the larger muscular arteries. adventitial connective tissue contains lymphatic capillaries, vasa vasorum, and nerves, all of which may penetrate to the outer part of the media.  Arterioles Smallest arteries branch have only one or two smooth muscle layers less than 0.1 mm in diameter, with lumens approximately as wide as the wall is thick  Arterioles Subendothelial layer and aventitia is very thin Elastic laminae are absent Media consists of the circularly arranged smooth muscle cells  Arterioles At the ends of arterioles the smooth muscle fibers act as sphincters and produce periodic blood flow into capillaries Muscle tone normally keeps arterioles partially closed, resisting blood flow  Arterioles Almost always branch to form anastomosing networks of capillaries that surround the parenchymal cells of the organ Major determinants of systemic blood pressure  CAPILLARIES permit and regulate Average diameter : metabolic exchange varies from 4 to 10 μm between blood and individual length : not surrounding tissues more than 50 μm simple layer of make up over 90% of endothelial cells rolled the body’s vasculature, up as a tube with a total length of surrounded by more than 100,000 km basement membrane  CAPILLARIES high metabolic rates: (kidney, liver, and cardiac and skeletal muscle) – have abundant capillaries low metabolic rates: smoothmuscle and dense connective tissue – Less capillaries  CAPILLARIES Metarterioles – terminal branch of an arteriole that supplies the capillaries Act as precapillary sphincters that control blood flow into the capillaries.  CAPILLARIES These contract and relax cyclically, with 5-10 cycles per minute, causing blood to pass through capillaries in a pulsatile manner. When the sphincters are closed blood flows directly from the metarterioles and thoroughfare channels into postcapillary venules.  CAPILLARIES The cyclical opening and closing of the sphincters, most capillaries are essentially empty at any given time – with only about 5% (~300 mL in an adult) of the total blood volume Thin walls, extensive surface area, and slow, pulsatile blood flow optimize capillaries for the exchange of water and solutes between blood and tissues.  Three Histologic Types: Continuous capillaries Ultrastructural studies show numerous vesicles indicating transcytosis of macromolecules in both directions across the endothelial cell cytoplasm.  Three Histologic Types: Continuous capillaries have many tight, well- developed occluding junctions between slightly overlapping endothelial cells Found in muscle, connective tissue, lungs, exocrine glands, and nervous tissue  Three Histologic Types : Fenestrated capillaries have a sievelike structure fenestrations – small circular openings orapproximately 80 nm in diameter. – Some are covered by very thin diaphragms of proteoglycans  Three Histologic Types: Fenestrated capillaries basement membrane : continuous and covers the fenestrations. found in organs with rapid interchange of substances between tissues and the blood, such as the kidneys, intestine, choroid plexus, and endocrine glands. Three Histologic Types: Discontinuous capillaries commonly called sinusoids Endothelium – Has large perforations without diaphragms and irregular intercellular clefts, forming a discontinuous layer with spaces between and through the cells. Unlike other capillaries Three Histologic Types : Discontinuous capillaries have highly discontinuous basement membranes larger diameters, often 30- 40 μm found in the liver, spleen, some endocrine organs, and bone marrow  Venules: Postcapillary venules larger, ranging in diameter from 15 to 20 μm. the primary site at which white blood cells adhere to endothelium and leave the circulation at sites of infection or tissue damage.  Muscular Venules Size is larger , 1 mm surrounded by a recognizable tunicamedia with two or three smooth muscle layers  Small to medium veins Size: Diameter 10 mm or less a. Tunica intima - thin (1) Endothelium (2) Thin subendothelial layer (3) May be folded to form valves b. Tunica media - thin; circular smooth muscle, collagen fibers, some elastic fibers  Small to medium veins c. Tunica adventita - well developed; loose FECT with longitudinally arranged collagen and elastic fibers, bundles of longitudinal smooth muscle Function - to collect blood from smaller venous vessels Small to medium veins  Small to medium veins  Large veins - vena cavae and larger branches a. Tunica intima - thicker (1) Endothelium (2) Thin subendothelial layer b. Internal elastic lamina - usually distinguishable c. Tunica media - thin, poorly developed; mostly FECT; little smooth muscle  Large veins - vena cavae and larger branches d. Tunica adventita - very thick; moderately dense FECT with spirally arranged collagen fibers, elastic laminae, longitudinal smooth muscle Function - to collect blood from medium sized veins and return it to heart  Large veins - vena cavae and larger branches Valves The valves, which are especially numerous in veins of the legs, help keep the flow of venous blood directed toward the heart.  Large veins - vena cavae and larger branches Valves The valves, which are especially numerous in veins of the legs, help keep the flow of venous blood directed toward the heart. LYMPHATIC VASCULAR SYSTEM  Lymph Capillaries Structure - blind-ended tubules; consist only of endothelium (which lacks cell junctions); similar to post capillary venules of blood vascular system Function - to collect excess tissue fluid  Lymph Capillaries Originate locally as tubes of very thin endothelial cells which lack tight junctions and rest on a discontinuous basal lamina.  Lymph Capillaries Fine anchoring filaments of collagen extend from the basal lamina to the surrounding connective tissue, preventing collapse of the vessels.  Lymph Capillaries Interstitial fluid enters lymphatic capillaries by flowing between endothelial cells and by transcytosis. lack hemidesmosome connections to the basal lamina and extend into the lumen to form leaflets of valves facilitating fluid entry and preventing most backflow of lymph  LYMPHATIC VASCULAR SYSTEM lymph nodes - where lymph is processed by cells of the immune system Lymphatic circulation is aided by external forces (eg, contraction of surrounding skeletal muscle) with the valves keeping lymph flow unidirectional. Lymphatic vessels ultimately converge as two large trunks: – thoracic duct – right lymphatic duct  Lymphatic Vessels Thoracic Duct Right Lymphatic Duct Connects with the enters near the blood circulatory confluence of the system near the right junction of the left subclavian vein and internal jugular vein the right internal with the left jugular vein subclavian vein  Lymphatic Vessels The adventitia is relatively underdeveloped, but containsvasa vasorum and a neural network major distributor of lymphocytes, antibodies, and other immune components which are carried through many organs to and from lymph nodes and other lymphoid tissues  Clinical Correlation: Atherosclerosis Deposition of plaque within the walls of large- and medium-sized arteries, resulting in reduced blood flow within that vessel. If this condition involves the coronary arteries, the decreased blood flow to the myocardium causes coronary heart disease. The consequences of this disease may be angina pectoris, myocardial infarct, chronic ischemic cardiopathy, or sudden death.  REFERRENCES Junqueira’s Basic Histology: Text & Atlas, 14th Ed, 2016, (Mescher) Difiore’s Atlas Of Histology with Functional Correlations, 12th Ed, 2013, (Eroschenko) THANK YOU