Lecture 12 Circulatory System PDF

Histology Lecture 12 Circulatory System Cardiovascular System This system composed of two circuit; pulmonary and Tunics of Vessels systemic circuit...

Histology Lecture 12 Circulatory System Cardiovascular System This system composed of two circuit; pulmonary and Tunics of Vessels systemic circuit Walls of blood vessels are composed of three layers: Consists of heart, blood vessels, lymphatic vessels tunica intima, tunica media and tunica adventitia Arteries transport blood away from heart Tunica intima is composed of a simple squamous Veins drain microvascular beds, returnining blood to heart epithelium and the subendothelial connective tissue Microvascular bed consists of arterioles, capillaries, and postcapillary venules Tunica media usually is the thickest layer of the vessel wall, is composed of helically disposed layers of smooth Arterioles regulate volume of blood flow muscle cells, and occasionally elastic membranes Capillaries have small diameter, thin walled, and are where gas/nutrient exchange take place Tunica adventitia is the outermost layer that blends into Postcapillary venules are the site of passage of blood cells surrounding connective tissue to connective tissue  Arteries There is 3 types of arteries in human body: large(elastic), medium(muscular), small arteries and arterioles All arteries have the same layers in their wall consist of: tunica intima, tunica media, and tunica adventitia Elastic Artery Aorta is a good example Special characteristics of elastic arteries are: Internal elastic lamina External elastic lamina Several elastic membranes known as fenestrated membranes alternating with smooth muscle cells located in tunica media  Tunica Intima Tunica intima is lined with simple squamous endothelial cells resting on a basal lamina The cells are joined by tight junctions and gap junctions Beneath the lining membrane is a subendothelial connective tissue and below that is a relatively dense internal elastic lamina The subendothelial layer contains collagen and a few elastin fibrils and occasional smooth muscle cells Tunica intima of elastic arteries is relatively thick Tight junctions prevent paracellular transport, thus exchange of material occurs by transendothelial pathway Tunica Media It is the thickest layer of the vessel Tunica adventitia Tunica media is composed of many fenestrated elastic Tunica adventitia is the outermost layer of the vessel lamellae, collagen and smooth muscle cells wall which merges with the connective tissue of the The wavy sheets of elastic membranes are located between body wall the smooth muscle fibers of the tunica media It consists of fibroblasts,collagen, thin elastic fibers, These elastic fibers are important in the stretching and recoil and small blood vessels; the vasa vasorum of the vessel in the movement of blood This layer is thinner than tunica media The extracellular matrix of tunica media, is composed of Prevents over-stretch of the arterial wall chondroitin sulfate, collagen, reticular and elastic fibers, is It also contains nervi vascularis synthesized and secreted by the smooth muscle cells Few fibroblasts also exist in tunica media  Muscular Arteries Muscular arteries distribute blood to the organs and tissues They have less elastic lamina and more smooth muscle cells than elastic arteries The muscle is arranged so that its contraction narrows the lumen of the vessel Internal and external elastic lamina are present Have smaller diameter than large elastic arteries Tunica Intima Tunica adventitia Endothelial lining with basal lamina Little subendothelial connective tissue Tunica adventitia is continuous with the Internal elastic lamina which is a fenestrated sheet of connective tissue of the organ which is elastic fibers supplied by this vessel Tunica media Thick connective tissue layer about equal to Thick tunica media in thickness Consists of smooth muscles, collagen type III, few High collagen content with fibroblasts elastic fibers Smooth muscle cells are arranged in spiral pattern Elastic fibers concentrated in external There is No fibroblasts elastic lamina External elastic lamina is also present  Small Arteries and Arterioles Arteriole has a diameter less than 0.1 mm and has 1-3 layers of smooth muscle in tunica media Small artery has up to 8 layers of smooth muscle cells in tunica media Small arteries and large arterioles have fenestrated internal elastic lamina, but in smaller arterioles it is absent Tunica adventitia is sparse and poorly developed Arterioles are key control points for blood flow Veins Veins are classified as large, medium and small size Large and medium veins usually accompany large and medium arteries Large and medium veins have tunica intima, tunica media and tunica adventitia Tunica media of veins are thinner than arteries of the same size Wall of a vein is much thinner than artery with the same size Some veins may have valves to prevent backflow of blood  Large Veins Vena cava is a good example of a large vein Tunica intima with endothelium, basal lamina and thin subendothelial connective tissue with internal elastic lamina Tunica media consists of 4-5 layers of smooth muscle with no distinct border, collagen and fibroblasts Tunica adventitia is thick with some smooth muscle; many collagen fibers, fibroblasts and some elastic fibers Medium veins Tunica intima: endothelium, basal lamina, subendothelial connective tissue and internal elastic lamina Tunica media: very thin, smooth muscle and some fibroblasts, reticular and elastic fibers Tunica adventitia: thicker than media, is composed of collagen bundles and a few scattered smooth muscle  Venules Capillaries Their wall consists of endothelium, basal lamina and Single layer of endothelial cells with basal lamina pericytes (contractile) in postcapillary venules Pericytes are located along the outside of the In larger venules, pericytes are replaced by smooth muscle capillaries and small venules, forming gap junctions cells with endothelial cells Venules are sensitive to histamine and serotonin, increased Site of most nutrient and gas exchange between permeability to fluid and WBC during allergic and tissue and blood inflammatory reactions Continuous (nonfenestrated) capillaries have no They have No real tunica media, just have 1-2 layers of pores in their wall, have tight junctions: CNS, PNS, muscle smooth muscle Discontinuous (fenestrated) capillaries have pores Some collagen and a few fibroblast are 80-100 nm in diameter and covered by a pore Endothelial cells of venules are cuboidal in certain diaphragm, are found in endocrine glands, intestines, lymphoid tissue and are called high-endothelial venules and in renal glomerulus that fenestrated capillaries lack diaphragms  Sinusoidal capillaries  Special thin-walled, incomplete capillaries with irregular large diameter  Discontinuous endothelial wall contain many large fenestrae without diaphragms  Found in liver, lymph nodes and hemopoietic tissues such as bone marrow and spleen Arteriovenous Anastomoses AVA are direct vascular connections between arterioles and venules that bypass the capillary bed Intermediate segment has a thickened tunica media, Its subendothelial layer is composed of plump modified polygonal smooth muscle cells Are useful in thermoregulation and are abundant in skin Glomus Vascularize nail beds and tips of fingers and toes This small organ receives an arteriole without elastic lamina and richly innervated smooth muscle cell layer,which surround the lumen, thus directly control blood flow to region before emptying into a venous plexus  Central Channel Metarterioles form the proximal portion of a central channel and its distal portion is formed by the thoroughfare channel which drain capillary bed into small venules This structure so named because it is without precapillary sphincter Heart Heart wall is composed of three layers: endocardium, myocardium and epicardium Endocardium Inner layer composed of endothelium with subendothelial fibroelastic connective tissue Middle layer is dense connective tissue with smooth muscle and elastic fibers Outer layer called subendocardium and is continuous with connective tissue of myocardium, containing small blood vessels and purkinje fibers  Myocardium Thick middle layer of the heart, is composed of cardiac muscle cells Epicardium The outermost layer, composed of mesothelial cells with underlying loose connective tissue contains coronary vessels and nerves and ganglia,and usually has adipose tissue too Homologue of the tunica adventitia of the vessels Heart Valve Endothelial lining on surface Core of dense irregular connective tissue with many elastic fibers Purkinje Fibers  Large diameter cardiac muscle cells that are pale staining, conduct electrical impulses  Lymphatic Vessels Lymphatic vessels are thin walled, unidirectional, carry excess interstitial fluid from tissues back to vascular system Lymphatic capillaries begin as blind tubes and converge into larger vessels 2 main vessels empty into vascular system at base of neck Fluid passes through lymph nodes before returning to vascular system Lymph capillaries have incomplete basal lamina and are very permeable Bundles of anchoring filaments terminate on the abluminal plasma membrane Lymphatic ducts Lymphatic ducts are similar to large veins Tunica intima composed of endothelium and several layers of elastic and collagen fibers In tunica media smooth muscle cells are oriented circularly and longitudinally Tunica adventitia contains smooth muscle and collagen fibers Department of Histology HR Mahmoudzadeh Sagheb Z Heidari MH Noori Mugahi