Histology: The Microcirculation and Lymphatic System PDF

HISTOLOGY THE MICROCIRCULATION AND LYMPHATIC SYSTEM Prepared by: Isa, Rash, Zen, Julius , Mhyr DR. F BASTERO September 20, 2024 adventitia (called vasa vasorum-literally, BLOOD VESSELS...

HISTOLOGY THE MICROCIRCULATION AND LYMPHATIC SYSTEM Prepared by: Isa, Rash, Zen, Julius , Mhyr DR. F BASTERO September 20, 2024 adventitia (called vasa vasorum-literally, BLOOD VESSELS "vessels of the vessels") perfuse the outer The basic constituents of the walls of blood half to two-thirds of the media. vessels are endothelial cells (ECs) TYPES OF BLOOD VESSELS smooth muscle cells (SMCs) ARTERIES extracellular matrix (ECM) including are divided into three types based on their elastin, collagen, and glycosaminoglycans. size and structural features: (1) large elastic arteries (conducting LAYERS OF BLOOD VESSELS arteries) including the aorta, the major INTIMA branches of the aorta (innominate, The intima normally consists of a single layer subclavian, common carotid, and iliac), and of ECs attached to a basement membrane pulmonary arteries; with a thin underlying layer of ECM; the (2) medium-sized muscular arteries intima is demarcated from the media by the (distributing arteries) comprising smaller internal elastic lamina branches of the aorta (e.g., coronary and renal arteries); and MEDIA (3) small arteries (≤2 mm in diameter) The media of elastic arteries (e.g., the and arterioles (20 to 100 um in diameter) aorta) are arranged in layers of lamellar within tissues and organs. units of elastin fibers and SMCs akin to tree rings. This high elastin content allows these vessels to expand during systole and recoil during diastole-functionally propelling blood toward the tissues. In muscular arteries, the media is composed predominantly of circumferentially oriented SMCS. Arterioles are the principal points of physiologic resistance to blood flow. Since the resistance to fluid flow is inversely proportional to the fourth power of the diameter (i.e., halving the diameter increases resistance 16-fold), small decreases in the lumen size of arterioles caused by structural changes or vasoconstriction can have profound effects on blood pressure ADVENTITIA The adventitia lies external to the media and in many arteries is separated from the media by a well-defined external elastic lamina. The adventitia consists of loose connective tissue and can also contain nerve fibers. Diffusion of oxygen and nutrients from the lumen is adequate to sustain thin-walled vessels and the innermost SMCs of all vessels. In large- and medium-sized vessels, however, small arterioles within the HISTOLOGY The Microcirculation and Lymphatic System September 20, 2024 HISTOLOGY THE MICROCIRCULATION AND LYMPHATIC SYSTEM Prepared by: Isa, Rash, Zen, Julius , Mhyr DR. F BASTERO September 20, 2024 LYMPHATICS Lymphatics are thin-walled, endothelium-lined channels that drain lymph (water, electrolytes, glucose, fat, proteins, CAPILLARIES and inflammatory cells) from the interstitium Capillaries are slightly smaller (5 um) than of tissues, eventually reconnecting with the the diameter of a red blood cell (7 to 8 um); blood stream via the thoracic duct. they have an EC lining but no media, although variable numbers of pericytes, cells Lymphatics transport interstitial fluid and that resemble SMCs, typically lie just deep to inflammatory cells from the periphery to the endothelium. lymph nodes, thereby facilitating antigen presentation and cell activation in the nodal Capillaries have a huge cross-sectional tissues-and enabling continuous monitoring area and also have a relatively low flow rate. of peripheral tissues for infection. This can The combination of thin walls and slow flow be a double-edged sword, however, as these makes capillaries ideally suited for the channels can also disseminate disease by exchange of diffusible substances between transporting microbes or tumor cells to blood and tissues. distant sites. VEINS ELASTIC ARTERIES Relative to arteries at the same level of The large elastic arteries, such as the branching, veins have larger diameters, pulmonary and aorta, brachiocephalic, larger lumens, and thinner and less subclavian, common carotid, and common well-organized walls. iliac have walls containing many fenestrated These structural features augment the layers of elastinin their tunica media. capacitance of the venous side of the circulation, which on average contains about two-thirds of total blood volume. Reverse flow (due to gravity) is prevented in the extremities by venous valves HISTOLOGY The Microcirculation and Lymphatic System September 20, 2024 HISTOLOGY THE MICROCIRCULATION AND LYMPHATIC SYSTEM Prepared by: Isa, Rash, Zen, Julius , Mhyr DR. F BASTERO September 20, 2024 ELASTIC ARTERIES: the successive lamellae are joined by slender Tunica Intima interconnecting strands of elastin The tunica intima of these arteries consists of the endothelium, a thin squamous ELASTIC ARTERIES: epithelium, separated from the elastica Tunica Adventitia interna by loose connective tissue The tunica adventitia of elastic arteries is containing a few fibroblasts, occasional relatively thin and consists of fibroblasts, smooth muscle cells, and thin collagen longitudinal bundles of collagen fibers, and a fibers. The endothelium provides a smooth loose network of thin elastic fibers. Small lining layer for the vessel and a partially blood vessels called vasa vasorum ramify selective diffusion barrier between the blood over the surface of the vessel to form a and the outer tunics of the vessel wall. Its network in the adventitia from which cells are polygonal in outline, 10-15 um wide capillaries penetrate into the media. Blood is and 25-50 um long, with their long axis returned via small veins that are confluent oriented longitudinally. Adjacent endothelial with nearby larger veins. cells are attached by simple occluding junctions and occasional gap junctions. MUSCULAR ARTERIES Rod-like cytoplasmic inclusions are observed in electron micrographs of arterial MUSCULAR ARTERIES endothelium called Weibel-Palade bodies. Tunica Intima They are membrane-bounded strucures The intima is thinner than that of the about 0.1 um in diameter and up to 3 um elastic arteries but otherwise similar in its long. They are sites of storage of von organization. Willebrand factor, a very large The thickness of the media varies from glycoprotein synthesized by endothelial cells three or four layers of smooth muscle cells in throughout the vascular system but stored in small arteries to as many as 40 in large Weibel-Palade bodies only in arteries. This arteries. factor is believed to be secreted continuously into the blood plasma. It is a major MUSCULAR ARTERIES: participant in blood platelet aggregation Tunica Adventitia and adhesion to form a clot at sites of The tunica adventitia of muscular arteries injury to the vessel wall. Its congenital may be thicker than the media. It consists of absence results in von Willebrand disease, fibroblasts, elastic and collagen fibers, characterized by prolonged blood coagulation oriented, for the most part, longitudinally. time and excessive bleeding after injury. The loose organization and prevailing longitudinal orientation of its components ELASTIC ARTERIES: impose little restraint on changes in the Tunica Media diameter of the vessel in vasoconstriction The tunica media of elastic arteries is made and vasodilatation. up of multiple concentric, fenestrated lamellae of elastin alternating with thin ARTERIOLES layers consisting of circularly oriented Small arteries and arterioles are smooth muscle cells, and fibers of collagen physiologically important segments of the and elastin in a proteoglycan extracellular circulation because they constitute the matrix. The elastic lamellae and other principal component of the peripheral extracellular components are apparently resistance to flow that regulates blood secreted by the smooth muscle cells. The pressure elastic lamellae have relatively large 200um down to about 40um fenestrations of varying size and shape and HISTOLOGY The Microcirculation and Lymphatic System September 20, 2024 HISTOLOGY THE MICROCIRCULATION AND LYMPHATIC SYSTEM Prepared by: Isa, Rash, Zen, Julius , Mhyr DR. F BASTERO September 20, 2024 ARTERIOLES smooth muscle cellsin the walls of arterioles Tunica intima and venules. Consists of continuous endothelium and a very thin subendothelial layer consisting of The caliber of the capillaries in reticular and elastic fibers. A very thin and different regions of the body varies, fenestrated elastica interna is present in the averaging from 9 to 12 um in larger arterioles but absent in terminal diameter arterioles. 25% of the total capillary bed of the ARTERIOLES body is patent but with increased Tunica media physiological activity, the narrowed Larger arterioles. It consists of two vessels open and flow through them is layers of smooth muscle cells restored to meet an increased need Smallest arterioles, there is a single for the exchange of metabolites layer, individual cells completely With small capillaries, the lumen may encircle in the endothelium. be encircled by a single endothelial cell CAPILLARIES In larger capillaries, the wall may be The capillary wall consists of extremely made up of portions of two or three attenuated endothelial cells, with their basal cells lamina supported by a sparse network of reticular fibers. Scattered along the outside CONTINUOUS CAPILLARIES of the capillaries are cells called pericytes Muscle-type capillaries An uninterrupted layer around the lumen of the capillary Found muscle, nervous tissue, and the connective tissues FENESTRATED CAPILLARIES The peripheral portions of the endothelial cells are interrupted by circular fenestrations or pores 60-70nm in diameter, each closed by a very thin pore diagphragm PERICYTES Found in the pancreas, intestinal tract, Have a long primary process deployed kidney cortex, and endocrine glands longitudinally along the capillary wall and secondary processes extending from the primary processes, circumferentially around the continuous vessel lamina with the basal lamina of the endothelium. It has the usual organelles (Golgi complex, mitochondria, and a few meandering tubules and cisternae of the ER. Pericytes are contractile because they contain tropomyosin, isomyosin of the smooth muscle type, and a protein kinase that is involved in the contraction of muscle. Thus it is involved in the control of blood VEINS flow through the microvasculature. There is Blood is carried back to the heart in the evidence that suggests, pericytes may veins from the capillaries. Veins are more undergo further differentiation to become numerous than the arteries and have a larger lumen, the venous system has a much HISTOLOGY The Microcirculation and Lymphatic System September 20, 2024 HISTOLOGY THE MICROCIRCULATION AND LYMPHATIC SYSTEM Prepared by: Isa, Rash, Zen, Julius , Mhyr DR. F BASTERO September 20, 2024 greater capacity than the arterial system. Their tunica intima consists of The walls of veins are thinner, more supple, endothelium, its basal lamina, and and less elastic than those of arteries. Thus associated reticular fibers. It is in histological sections, veins are usually sometimes bounded externally by a collapsed and have a slit-like lumen, unless a moderately dense network of elastic special effort has been made to fix the fibers, but there is no true elastica distension interna The media consists of a layer of circular smooth muscle but this is thinner and more loosely organized than in arteries. The tunica adventitia is usually their thickest layer consists of bundles of collagen and networks of elastic fibers. LARGE VEINS Large veins include the inferior vena cava, the portal, the splenic superior mesenteric, the external iliac, and the azygos veins. Tunica intima has much the same structures in medium-sized veins, but in these larger trunks, the subendothelial connective tissue may be considerably thicker. Smoot muscle is a prominent component of the veins in the gravid uterus, and the pulmonary veins have a well-developed media containing circular smooth muscle Tunica media is lacking in large veins Thick tunica adventitia makes up the greater part of the thickness of the wall. It is rich in elastic fibers and bundles of collagen that are oriented, for the most part, longitudinally. In VENULES AND SMALL VEINS the inferior vena cava, the collagen Capillaries converge to form postcapillary fibers are reported to have a spiral venules of slightly larger size (15-20/um. course that is believed to facilitate the The ultrastructure of the wall of these slight lengthening and shortening that vessels is not significantly different from that the vessel undergoes in the ascent of capillaries. It consists of very thin and descent of the diaphragm. The endothelium surrounded by reticular fibers inferior vena cava is also exceptional and pericytes. in that is adventitia contains scattered longitudinal bundles of smooth VEINS OF MEDIUM SIZE muscle. Where the pulmonary veins Veins in the size range of 2-9 m include the and the vena cava enter the heart, cutaneous and deeper veins of the cardiac muscles extend a short extremities distal to the brachial and distance in their adventitia. popliteal, the veins of the head, and many of those of the viscera. HISTOLOGY The Microcirculation and Lymphatic System September 20, 2024 HISTOLOGY THE MICROCIRCULATION AND LYMPHATIC SYSTEM Prepared by: Isa, Rash, Zen, Julius , Mhyr DR. F BASTERO September 20, 2024 The principal functions of the lymph vascular VASA VASORUM system are. Also called minute vessels, they penetrate the walls of both large arteries and veins to 1. To return to the blood, fluid, and supply oxygen to their tissues. These are plasma protein that escape from the more numerous and extend more deeply into circulation the walls of veins than they do in arteries. 2. To return lymphocytes of the recirculating pool to the blood LYMPHATIC VESSELS 3. To add the blood immune globulins The lymph vascular system is a drainage (antibodies) produced in the lymph system. Its terminal branches, the lymphatic nodes. capillaries, end blindly and transport a clear fluid called lymph. It is found in all tissues except the central nervous system, cartilage, bone and bone marrow, thymus, teeth, and placenta. Along the course of lymphatic vessels, there are encapsulated aggregations of lymphoid tissue called lymph nodes. Lymph is essentially an ultrafiltrate of the blood plasma by continual seepage of fluid constituents of the blood across capillary walls and into the surrounding interstitial spaces. It consists of water, electrolytes and 2-4% protein depending on the site and conditions of its formation. HISTOLOGY The Microcirculation and Lymphatic System September 20, 2024