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Anatomy and Physiology
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Blood Vessels and Hemodynamics Blood Vessels Blood vessels of the body form a closed delivery system that begins and ends at the heart Blood Vessel Structure and Function The three major types of blood vessels are arteries capillaries and veins Arterioles are the smallest branches of arteries venules are the smallest veins Arteries carry blood away from the heart veins carry blood toward the heart and only capillaries directly serve cells In the systemic circulation arteries carry oxygenated blood and veins carry oxygenpoor blood this is vice versa for the pulmonary circulation Structure of Blood Vessel WallsThe walls of all blood vessels except the very smallest have three distinct layers tunics that surround a central bloodcontaining space called the vessel lumen The innermost tunic is the tunica intima The tunica intima contains the endothelium that lines the lumen of all vessels forming a slick surface that minimizes friction In vessels larger the 1mm a subendothelial layer supports the endothelium The tunica media is mostly circularly arranged smooth muscle cells and sheets of elastin it bears chief responsibility for maintaining blood pressure and circulation Depending on the bodies needs at any given moment regulation causes either vasoconstriction diameter decreases contracted smooth muscle or vasodilation diameter increases relaxed smooth muscle The tunica externa or adventitia is composed largely of loose collagen fibers that protect and reinforce the vessel and anchor its surround structures In larger vessels the tunica externa contains tiny blood vessels the vasa vasorum that nourish the more external tissues of the blood vessel wall Arterial System Arteries can be divided into three groups 1Elastic arteries are the thickwalled arteries largest in diameter near the heartTheir large lumens make them low resistance pathways Elastic arteries contain more elastin then any other vessel type Although elastic arteries also contain a lot of smooth muscle they are relatively inactive in vasoconstrictionThey expand and recoil as the heart pumps blood which helps them maintain pressure and flow of blood 2Muscular arteries deliver blood to specific body organs Proportionately muscular arteries have the thickest tunica media of all vessels They are more active in vasoconstriction and less distensible capable of stretching There is an elastic membrane on each face of the tunica media 3Arterioles is the smallest of the arteries Arterioles diameter determines which capillary bed is flushed with blood flow which varies in response to changing neural hormonal and local chemical influences Capillaries Are the smallest blood vesselsTheir thin wall consist of just a thin tunica intima Along the surface of some capillaries are pericytes smooth musclelike cells that stabilize the capillary wall and help control permeability The diameter of a capillary is just large enough for one red blood cell to slip through in single file Capillaries role is for exchange of materials gases nutrients hormones between the blood and the interstitial fluid Types of Capillaries There are three types of capillaries1Continuous Capillaries abundant in the skin and muscles Their endothelial cells are joined together by tight junctions providing an uninterrupted lining However they leave gaps of unjoined membrane called intercellular clefts which are just large enough to allow limited passage of fluids and small solutes 2Fenestrated Capillaries the endothelial cells are riddled with oval pores or fenestrations A delicate membrane usually covers the fenestrations Fenestrated capillaries are found wherever active capillary absorption or filtrate formation occurs 3Sinusoid Capillaries are highly modified leaky capillaries found only in the liver bone marrow spleen and adrenal medulla The have large irregular lumens and are usually fenestrated Their endothelial lining has fewer tight junctions and have large intercellular clefts for passage of proteins and red blood cells Capillary Beds Capillaries form interweaving networks called capillary beds The flow of blood through a capillary bed is called microcirculation A capillary bed consists of two types of vessels 1 a vascular shunt a short vessel that directly connects the arteriole and venule at opposite ends of the bed and 2 true capillaries the actual gas exchange vessels A cuff of smooth muscle fibers called a precapillary sphincter surrounds the root of each true capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary Venous System The diameter of venous vessels increase along the route to the heart and their walls thicken as the progress from venules to larger veins Venules Capillaries unite to form venulesThe smallest venules postcapillary venules consist entirely of endothelium and are extremely porous Larger venules have one or two layers of smooth muscle cells and a thin tunica externa Veins Venules join to form veins Veins usually have three distinct tunics but their walls are thinner and lumens larger There is relatively little elastin and smooth muscle in the tunica media The tunica externa is the heaviest wall layer Veins can accommodate fairly large blood volume Veins are called capacitance vessels or blood reservoirs because they can hold up to 65 of blood at any given time The blood pressure in veins is low Two structural adaptations of veins that promote blood return are 1 their large diameter lumens offer little resistance to blood flow and 2 venous valves formed from folds of the tunica intima prevent blood from flowing backward Blood Flow Is the volume of blood flowing through a vessel an organ or the entire circulation in a given period Blood flow through individual body organs may vary widely according to their immediate needs Blood Pressure The force per unit area exerted on a vessel wall by the contained blood mm Hg Blood pressure refers to the systemic arterial blood pressure in the largest arteries near the heart The pressure gradient provides the driving force that keeps blood movingResistance
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