Organ Architecture 2 - Vessel Structure PDF

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CuteSanity1368

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Humanitas University

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blood vessel structure cardiology anatomy biology

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This document provides an overview of the structure and function of blood vessels, including arteries, veins, capillaries, and lymphatics. It details the three layers of blood vessels (intima, media, and adventitia), and compares elastic arteries, muscular arteries, and arterioles. It also discusses conditions like atherosclerosis and aneurysms.

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Cardiovascular system —> closed system of endothelia tubes (+ eventually smooth muscle cells/ connective tissue depending on the function of the tubes) whose purpose is that of perfusing the capillary bed (bring blood to organs through capillary beds). Major types of vessels: Arteries —> carry bl...

Cardiovascular system —> closed system of endothelia tubes (+ eventually smooth muscle cells/ connective tissue depending on the function of the tubes) whose purpose is that of perfusing the capillary bed (bring blood to organs through capillary beds). Major types of vessels: Arteries —> carry blood to the periphery Veins —> carry blood to the heart Capillaries —> places of exchange between blood and tissues (only endothelial tubes without muscles or connective tissue) Lymphatics —> one way system that carries excess interstitial fluids to the heart, it starts with blind end tubes that become larger and larger. The purpose is that of filtering lymph (through lymph nodes intercalated in the vessels). There is not a pump in the lymphatic circulation so the muscles and peripheral tissues are responsible for the moving of lymph Three layers (or tunics) of a typical vessel (medium size artery) Tunica intima (or interna) Tunica media Tunica adventitia (or externa) These three layers are more easily distinguished in arteries than in veins TUNICA INTIMA It is made of endothelium, a basement membrane (or lamina) and a lamina propria (or sub endothelial layer). The lamina propria is made of loose connective tissue with contractile cells called myointimal cells (especially in large vessels) which also have phagocytic capacities and are able to synthesise collagen and elastin (usually performed by fibroblasts). At the boundaries between the tunica intima and media there is an internal elastic lamina (sheet-like layer of fenestrated elastic material to allow oxygen and nutrients diffusion) TUNICA MEDIA Media = in the middle. It is made of smooth muscle cells and an ECM + elastic and collagen fibers. Elastic fibers are arranged in sheets and provide elasticity while collagen fibers provide a supporting framework to limit the distensibility of the vessels (especially in veins). Between the tunica media and the tuna adventitia there is another accumulation of elastic tissue called external elastic lamina (fenestrated) TUNICA ADVENTITIA It is mostly made of connective tissue (sometimes smooth muscle cells too). In larger vessels at the level of the adventitia there might be other vessels (vasa vasorum, they supply the bigger vessel with nutrients and oxygen) and nerves. Morphological/functional classification of arteries: Elastic arteries —> prevalence of elastic tissue Muscolar arteries —> prevalence of the muscolaris component Arterioles —> small arteries Moving from the elastic arteries to arterioles the elastic tissue decreases while the muscular one increases ELASTIC ARTERIES (Large-size or Conducting arteries) They have the largest size and they’re the closest to the heart. They conduct blood to other types of vessels that will then supply organs with it. Elastic arteries: aorta (the biggest vessel in our body) and its branches (brachiocephalic, common carotid, subclavian, common iliac, pulmonary). The prevalence of elastic tissue has a physiological function —> they function as pressure reservoirs during systole and they recoil during diastole to keep arterial pressure and blood moving —> elastic energy is accumulated when the heart pumps blood (the vessel dilates) so that when the pump is empty the pressure can be maintained constant thanks to the recoil (the vessel shrinks and pressure remains constant). The wall of these vessels is thin compared to the size of the lumen Tunica intima —> recognisable endothelium + basement membrane + lamina propria. The lamina propria contains elastic fibers and modified smooth muscle cells (myointimal cells) that accumulate lipids with age. Tunica media —> elastic lamellae with fenestrations in between smooth muscle cells (the elastic tissue is not concentrated in an internal and external elastic lamina but all throughout the layers of the vessels organised in sheets), there are bundles of smooth muscle cells and collagen fibers (so that it is not too elastic). No internal and external elastic laminae. prevalent MUSCULAR ARTERIES (Medium-size arteries or Distributing moresmoothmuscleinthetunicemedie arteries) They receive blood from the conductive vessels and deliver it to empower organs according to functional needs. They are less stretchy (more smooth muscle cells and less elastic fibers), more distant from the heart and they’re active in vasoconstriction Here there is still elastic tissue but the smooth muscle tissue is prevalent. The internal and external laminas are well visible and elastic fibers are concentrated there (while smooth muscle cells are concentrated in the tunica media). The tunica intima is thinner than that of elastic arteries. Tunica media —> loss of the majority of elastic lamellae, internal and external elastic laminas are clearly visible, smooth muscle cells are organised in concentric layers (low angle helix) This organisation is important to maintain a constant blood pressure (20-30 layers in big (medium size) arteries and 3-10 layers in small monewarytumice intima (medium size) arteries). Smooth muscle cells’ contraction is guided by the autonomic nervous system (ortosympathetic nervous system) Blood pressure is determined by the efficiency of the pump (heart) and by the resistance of the vessels (the muscle layer is never elasticartery musweer artery completely relaxed or contracted, it has a tone, and this tone is what inimaerestianne allows the maintenance of a constant blood pressure). Hypertensive people may have either a problem with the pumping efficiency of the heart or with the contractile activity of peripheral vessels. externa monoamine Big muscular arteries may have between 20-30 concentric layers of smooth muscle cells with a visible internal and external lamina while in small ones there are 3-10 concentric layers and the external elastic lamina disappears. ARTERIOLES (resistance vessels) Their diameter is smaller than 0.1mm (100μm). They control the amount of blood to be delivered to various organs or to districts of an organ (active in vasoconstriction and vasodilation thanks to circularly arranged smooth muscle fibers). These vessels always have a tone (partial state of contraction) and for this they arteries renee are the major determinants of systemic blood pressure. If the tone of the peripheral vessels is higher than it should (more contracted) blood pressure is higher and the patient is said to be hypertensive. Arterioles only have 1-2 layers of muscle cells. Arterioles with a diameter smaller than 75 μm are part of the micro circulation together with capillaries and venules. The wall of arterioles is relatively thicker than the lumen encircled The tunica intima is thin and there is no sub endothelial tissue, the internal elastic lamina is present The tunica media is purely muscular and it is the thickest coat (either 1-2 layers) The tunica externa is fibro-elastic, it is thinner than the media and there is no definite external elastic lamina vessels Frombiggertosmoner GGradual transition —> loss of internal elastic lamina, progressive reduction of muscle layers in the tunica media METARTERIOLES Narrow vessels arising from arterioles that give rise to capillaries. They’re among the smallest ones (10-20μm in diameter). Here the smooth muscle layer is just one and it is discontinuous. Where the smooth muscle cell is discontinuous the capillaries arise. Metarterioles are important local regulators of blood flow (they’re only in certain places, in some other places capillaries arise from small arterioles). They’re typical of the vessels in the mesentery twovesseespointogorm converging twosepanarevesseissoinea asmannonermmingbetweenthengie from which firegeevessersoniginale g Arterial anastomosis —> arteries joined to one another by anastomosis —> this way one artery can supply the territory of the other (this way if an artery is occluded blood can still be distributed to peripheral territories through anastomotic canals) ATHEROSCLEROSIS (most common form of arteriosclerosis) With aging the walls of the arteries loose elasticity and become stiffer. It is the most common cause of morbidity of the vascular diseases. It consists of an accumulation of lipids in the tunica intima of large and medium-sized arteries leading to chronic inflammation. This causes less blood to diffuse in the tissue. Aneurysm —> dilation of the vessel’s wall because of its weakening. It can be caused by atherosclerosis, aging (displacement of elastic fibers by collagen fibers), syphilis or connective tissue disorders such as Marfan syndrome and Ehlers Danlos syndrome. The vessel don’t go back to their original shape. They’re most common in arteries because blood pressure is higher. It may lead to dissection. Aortic dissection —> a passage of blood dissects a part of the wall and it causes a split —> the continuous pushing in the abnormal canal might lead to the tearing of the blood vessel MICROCIRCULATION It takes place in the micro vascular bed and it is what supplies tissues with blood (it provides nutrients and removes metabolic byproducts from all living cells in the body). The micro circulation must maximise the distribution of nutrients and oxygen by touching almost every living cell and minimise the space it occupies to allow room for Parenchymal cells, structural tissues, nerves, inflammatory and other types of cells that contribute to organ function —> complex branching pattern Components of the microcirculation: Terminal arteriole Metarteriole (might not be present) Capillary bed Postcapillary venule The microvascular bed starts at the level of a small arteriole called terminal arteriole. From the terminal arteriole there is the formation of a network of capillaries (capillary bed), and then on the other side of the capillary bed there is the postcapillary venule. Capillaries can originate from the terminal arterioles or from metarterioles, a special type of vessel (large capillary or very small arteriole) that has some characteristics of the arterioles and some characteristics of the capillaries: the smooth muscular layer is discontinuous, present only where there are the sphincteric devices that control the formation of capillaries. Otherwise, it behaves like a pure capillary. The amount of blood that flows into the capillary bed depends on the functional demands of the tissue or organ in a specific moment. In many capillary beds there is a special vessel, slightly larger than the other capillaries, called thoroughfare channel that allows continuous blood flow from the arteriole to the venule. In a sense, it bypasses the mechanism of blood flow control in the capillaries. Capillary bed —> it originates from terminal arterioles and from metarterioles. Larger capillaries (preferential or thoroughfare channels) provide a continuous blood flow while smaller capillaries (true capillaries) have an intermittent blood flow regulated by precapillary sphincters. Blood flow isn’t even throughout capillary beds because it can change according to needs (es: less blood in the periphery when it’s cold outside to avoid heat dispersion, more blood along the intestine during digestion to absorb more nutrients). The regulation of the blood distribution is not performed by capillaries (they don’t have muscle cells) but by sphinteric devices at the origin of capillaries, which are circular arrangements of smooth muscle cells that can be opened and closed (controlled by local factors released by endothelial cells/ muscle cells or by the autonomic nervous tissue). Postcapillary venules —> primary sites of leukocytes extravasation during inflammation Metarterioles and precapillary sphincters ARE NOT universal components of the microcirculation. The microcirculation is diverse in each organ Arteriovenous shunt (redirectioning) —> it bypasses the microvascular bed and it connects directly arterioles to venules and it is important for thermoregulation (they’re not capillaries, they’re just a bit smaller than arterioles) CAPILLARIES They have an overall plan but they can be of different types. They ALL have a single layer of endothelial cells (no tunica media and adventitia to allow passage of substances) and they allow only a single RBC to pass at a time. In some places capillaries may be covered by pericytes that support the wall of the vessels and have contractile properties. Functions: Gas exchange Nutrients exchange Hormones exchange Ions exchange Others Most tissues have a rich capillary supply but there are exceptions: Tendons and ligaments Cartilage Cornea Lens Epithelial tissue (supplied by the underlying lamina propria) Three types of capillaries: Continuous (least permeable, typical of lungs, brain and muscles) Fenestrated (intestine, kidney, endocrine glands) Discontinuous (also called sinusoids, most permeable, typical of spleen, liver and bone marrow) Continuous capillaries —> they’re the least permeable and they lack fenestrations. Adjacent endothelial cells are connected with tight junctions and they only allow passage of small molecules. Transport takes place mostly though caveolae and pinocytotic vesicles. They’re often sustained by pericytes. In the brain continuous capillaries constitute the BBB (blood- brain barrier, tight junctions across ALL endothelial cells). The basal lamina nude us is continuous Fenestrated capillaries —> fenestrations in the cells, not between them (large pores) to allow passage of small molecules (es: in endocrine glands to secrete hormones or in the spleen to allow the passage of erythrocytes). They’re characterised by a greater permeability in respect to continuous capillaries, they have a continuous membrane across the fenestrations. with fenestration Fenestrations are dynamic (they might become larger, smaller ecc… over diaphragm time). The can be found in the glomeruli of kidneys (filtering activity), in s Fenestration intestinal villi, in ciliary processes of eye and in endocrine glands. Fenestrations usually have a diaphragm (transmembrane proteins that form a gate that opens and closes to allow passage of big molecules) Discontinuous or sinusoidal capillaries —> they’re the most permeable (sometimes called leaky capillaries), they have a larger lumen and large fenestrations. These capillaries are characterised by intercellular clefts (holes between one cell and the other). These fenestrations don’t have diaphragms. The basal lamina is discontinuous and the endothelium might be discontinuous too. They can be found in organs where an intimate relationship between the parenchyma and the blood is necessary (liver, bone marrow, lymphoid tissue and some endocrine organs). For example the capillaries around the spleen are discontinuous to allow the passage of erythrocytes to be destroyed in the red pulp. In some of these capillaries there are modified macrophages which substitute endothelia cells or actual phagocytes closely applied to their exterior wall. Sieving plates are clusters of fenestrations With aging fenestrated capillaries undergo changes leading to a worse performance of the organ (fenestrations are diminished) liver signing Specialised capillary systems —> PORTAL SYSTEM —> an arrangement by which blood is collected by one set of capillaries and then passes through a large vessel (portal vessel) to reach another set of capillaries before being sent into the systemic circulation. Two capillary systems connected by a portal vein. Portal systems are made of: Feeding artery Primary capillary bed Portal vessel Secondary capillary bed Draining vein Portal systems can either be arterial portal systems or venous portal systems depending on what type of vessel connects the two capillary beds. Hypophysis arterialportalsystem Properties of endothelium (different functions for different vessels) They maintain a selective permeability barrier They maintain a non-thrombogenic barrier due to the production of anti thrombotic factors They produce coagulation factors to avoid excessive bleeding They deactivate some pharmacologically active substances like serotonin and prostaglandin They convert angiotensin I into angiotensin II (mainly in lungs) They modulate blood flow and vascular resistance —> vasoconstriction (endotheilin 1) and vasodilation (nitric oxide and prostacyclin) They modulate the immune response (production of cytokines, HEV to allow passage of leukocytes from lumen of venules to lymphoid organs) They are involved in lipoprotein metabolism (they break down triglycerides and cholesterol) They’re involved in angiogenesis (formation of new vessels from pre-existing vessels) Others CLINICAL DROP: Endothelial dysfunction appears to be the common denominator of multiple clinical aspects of severe COVID-19. VEINS (Capacitance or Reservoir vessels) They originate at the level of the capillary network, they bring blood from the periphery to the heart, pressure is lower than that of arteries, they have a thinner and less elastic wall, they have a thicker tunica externa, they usually have valves which can have a semilunar or sparrow nest shape (to assure blood flows in the right direction), and they form several anastomosis (easily compressible). They’re also called capacitance or reservoir vessels because most of the body blood is stored in veins (60%). They’re divided into small veins or venules (postcapillary venules, muscular venules and collecting venules), medium veins and large veins. Arterioles and large venules usually travel together. Veins are divided into receptive veins (above the diaphragm, blood flow is helped by gravity) and propulsive veins (below diaphragm, thicker wall with smooth muscles and valves to fight against gravity Veins vs. Arteries Tunica intima media and adventitia are present both in veins and arteries but are more visible in arteries. Arteries have thicker walls (and so they’re less compressible) and a thicker tunica media. Veins usually don’t retain their shape (floppy in sections). Veins have less elastic tissue and the elastic membranes are poorly developed. Postcapillary venules —> smaller venules, they’re part of the capillary post micro circulation. They have a porous endothelium and few venee pericytes. There is no smooth muscle, just a basal lamina. They allow fluids and WBC to pass to tissues, they’re the apiary arag principal site of action of vasoactive agents (like histamine, which increases permeability), they are involved in inflammation and allergic reactions. In lymphatic tissue they are crime venee are HEVs. ee arterio Muscular venules —> 1-2 layers of smooth muscle cells (tunica musaeen renee media is visible), less permeable than postcapillary venules valves Valves —> they can be found in veins larger than 2mm in diameter. They consist of delicate semilunar projections of the tunica intima of the vein wall. Typically within the core of valves there is fibroelastic tissue to sustain them. Medium size veins —> the thickest layer is the tunica adventitiathe adventitia inurgeveins and it is made of collagenous fibrous tissue whose aente une una ere e sg sue mu collagen fibers run predominantly in longitudinal direction. Large veins —> longitudinally oriented bundles of smooth muscle cells in the tunica adventitia to propel blood (for example in the venae cavae and in pulmonary veins). The tunica media is narrow and it is made of smooth muscle cells while the tunica adventitia is thick and it is made of collagen, bundles of smooth muscle fibers and elastic fibers Atypical veins —> venous sinuses are specialised veins with extremely thin walls (like coronary sinus of the heart and the dural sinus of the brain) Varicose veins —> veins can loose their ability to contain blood in the right way and become abnormally tortuous and dilated. They can be seen superficially in the lower limbs of old people. They can be caused by aging, occupational hazard and pregnancy. Varicose veins can develop in the haemorrhoidal plexus in the anal canal LYMPHATIC VESSELS They begin as blind ended tubes in the microcapillary bed (they don’t communicate with it). They’re adjuncts to blood vessels (addition), they drain excess extra cellular fluid rich in proteins. In lymphatic vessels there can be good cells (lymphocytes) or bad cells (cancer cells). The flow is unidirectional and there is a low pressure. They drain in increasingly larger vessels and finally in the venous circulation. They’re an integral component of the immune system (they conduct immune cells and lymph to lymph nodes) Lymphatic capillaries — > they’re absent in cartilage, bones, epithelia, CNS, bone marrow and placenta. They’re more permeable than blood capillaries and they’re covered by a single layer of endothelial cells which are not connected by lots of tight junctions but are rather apposed to form flap valves (or mini valves). They don’t have a continuous basal lamina and they have anchoring filaments that attach to the perivascular collagen to prevent collapsing when interstitial pressure is high (important). In bigger lymphatic vessels there are smooth muscle cells to aid the propelling of lymph (important because the lymphatic circulation doesn’t have a pump) Lymphatic capillaries → Precollecting lymphatic vessels → Collecting lymphatic vessels (smooth muscle, valves, lymphangions → Terminal lymphatic vessels → Lymphatic efferents → lymph nodes Key Points Lymphatic capillaries are slightly larger in diameter and have greater oncotic pressure than blood capillaries. When pressure is greater in the interstitial fluid than in lymph, the minivalve cells separate slightly and interstitial fluid enters the lymphatic capillary. When pressure is greater inside the lymphatic capillary, the cells of the minivalves adhere more closely, and lymph cannot flow back into interstitial fluid. Anchoring filaments attach to the minivalves to anchor the capillary to connective tissue, and also pull the capillary open to increase lymph collection when the tissue is swollen. Because lymph capillaries have a closed end, lymph is pushed forward into larger vessels as the pressure inside the capillary increases as lymph accumulates from fluid collection. Edema can occur when interstitial fluid accumulation in tissues is greater than fluid removal (acute inflammation ) or when the lymph vessels are obstructed in some way (elephantiasis). Lymphangion —> it is the functional unit of a lymph vessel that lies between two semilunar valves. Lymph progresses from one lymphngion to the other without flowing back Lacteal vessels —> modified lymphatic vessels that absorb chylomicrons (big particles that can’t be absorbed by blood vessels as they’re too big). They’re only present at the level of the intestinal villi CLINICAL DROPs: Edema —> accumulation of extra cellular fluid in interstitial spaces. Edema liquid is a plasma transudate that accumulates when movement of fluid from vascular to interstitial spaces is favoured. Localised edema is usually due to venous or lymphatic obstruction while generalised edema is usually of cardiac or renal origin. Lymphoedemas can also be hereditary. Lymph transport can be impaired due to a hypoplastic initial lymphatic capillary network, abnormal coverage of lymphatic capillaries (and so less permeability), or a lack or malfunctioning of lymphatic valves. PPP —> shock Shock is associated to change in the microcirculatory bed (es: it is too permeable, there is a production of abnormal substances by endothelial cells…) leading to its impossibility of providing nutrients to tissues Syndrome = constellation of things that come together (not necessarily at the same time). Shock is the clinical syndrome that results from inadequate tissue perfusion. Irrespective of the cause, the hypoperfusion-induced balance between the delivery of and the requirements for oxygen and substrate leads to cellular disfunction. Cellular injury due to inadequate delivery of oxygen and substrates causes the production of DAMPs and inflammatory mediators. bear aeneesea pressure in conditionmy g rammar There are different types of distributive shock Neurogenic shock: loss of vasomotor tone, vasodilation. If the central nervous system goes into shock, the autonomic nervous system does not work as it should and therefore the action that the central nervous system has on the arterioles is reduces (and arterioles always need a certain tone). This causes a vasodilation that gives rise to a situation of neurogenic shock. Anaphylactic shock: severe immune reaction to an antigen. When histamine is released, it causes a general vasodilation (histamine is a powerful vasodilator), which means that a lot of blood goes into the capillaries. The capillaries can sustain the pressure only to a certain point, so there is an increase in permeability Septic shock: occurs when an infection spreads to the rest of the body. Bacterial toxins into the bloodstream trigger vasodilation and increase capillary permeability. therefore, a lot of fluid goes from the capillary to the peripheral tissue. Control in blood flow In the capillaries, the control in flood flow is regulated by local and systemic signals. Vasodilation —> increase in blood flow and therefore and increase in pressure in the capillary bed: more fluid from inside the capillaries goes into the tissue. A sort of edema occurs. Vasoconstriction —> decrease in blood flow and in the capillary bed, so the tissue fluids are driven into the capillaries (because the pressure in the capillaries counteract less the pressure that there is in the extracellular fluid). In case of loss of blood, we may enter in a situation called hypovolemic shock, which is a decrease in blood volume. The body tries to keep blood in the body as much as possible: the first response is vasoconstriction at the periphery. Hypovolemic shock can be caused by: bleeding from cuts or wound; bleeding from blunt traumatic injuries due to accidents or seizure activity; internal bleeding from the digestive tract or due to a ruptured ectopic pregnancy. In addition to actual blood loss, the loss of body fluids can cause a decrease in blood volume. This can occur in cases of: excessive or prolonged diarrhea; severe burns; protracted and excessive vomiting excessive sweating

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