Blood Vessel Types and Characteristics Lecture PDF

Types of blood vessels: ◦The aorta branches out to form arteries ◦Resistance increases when you get to the arterioles (resistance vessels) ◦The body can sense a stimulus and change the diameter of arterioles to control blood flow. ◦Waste is collected by Venules ◦Vei...

Types of blood vessels: ◦The aorta branches out to form arteries ◦Resistance increases when you get to the arterioles (resistance vessels) ◦The body can sense a stimulus and change the diameter of arterioles to control blood flow. ◦Waste is collected by Venules ◦Veins and veins hold blood (known as capacitance vessels) - most of the time blood is kept in veins and Venules Histological organisation of blood vessels: ◦Tunica interna (intima): ‣ Innermost lining of the blood vessel ‣ Lined by endothelium ‣ In large arteries, the internal elastic membrane marks the outer boundary of the tunica interna ‣ IEL - inner elastic lamina (arteries) - looks like a wavy pattern on a micrograph ◦Tunica media: ‣ Middle layer (thick) ‣ Smooth muscle cells (SMC) supported by varying amounts of connective tissue ‣ SMC contraction controlled by ANS (Nervi vasorum - nerves), hormones, local chemicals ‣ Arteries have external elastic membrane ◦Tunica externa (adventitia): ‣ Outermost layer ‣ Blends into surrounding tissue and stabilises blood vessels (keeps vessels in position) ‣ In thick walled vessels: blood supply -> vasa vasorum Distinguishing arteries and veins: ◦Walls of arteries thicker than the veins ◦In cross sections, arteries appear rounder than veins ◦The endothelial lining of constricted arteries is thrown into folds and is pleated (due to IEL)  The arterial system: ◦Large elastic conducting arteries ◦Medium muscular (distributing) arteries ◦Arterioles Large elastic conducting arteries: ◦Largest, closest to the heart ◦Aorta, pulmonary arteries ◦Tunica media contains high density of elastic fibres Medium muscular (distributing arteries): ◦Distribute blood to skeletal muscles and internal organs Arterioles - resistance vessels: ◦ Part of microcirculation ◦Tunica externa absent or very thin ◦Tunica media: SMC ◦Control blood flow between capillaries and arteries ◦Called as resistance vessels ◦Site of largest drop in blood pressure Capillaries: ◦One layer of endothelium and its basement membrane ◦Diameter of one RBC ◦For nutrient and gaseous exchange ◦Three types based on the size of the gaps between the endothelial cells: ‣ Continuous - majority of capillaries are continuous ‣ Fenestrated -fenestrations in the endothelial layer (filtering in kidneys etc) ‣ Sinusoid - huge gaps between endothelium to allow movement of cells (bone marrow etc) The venous system: ◦Collect blood from tissues, returning it to the heart ◦Pressure much less than the arteries ◦Walls are thinner and less elastic ◦Capacitance vessels (70% of total blood volume) ◦Valves: infolding of tunica intima only present in veins and not Venules  Venules: ◦Collect blood from capillaries ◦Resemble capillaries ◦Endothelium, thin middle layer with few muscle cells and elastic fibres ◦No smooth muscles present ◦Very thin tunica externa made of connective tissue fibres ◦Venules and capillaries are the primary sites of emigration of white blood cells to enter the tissue fluid Medium veins: ◦Have all three layers ◦Thin tunica media with few SMC ◦Thick tunica externa Large veins: ◦Have diameters greater than 10mm ◦Have all three layers ◦Thicker tunica externa ◦Include superior vena cava and inferior vena cava