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Chapter 19 – Blood Vessels and Circulation Types of blood vessels • Arteries – Carry blood away from the heart – Branch into smaller vessels called arterioles • Capillaries – Smallest vessels – Form beds in tissues/organs – Where diffusion occurs • Veins – Carry blood back to heart – Venules – small veins from capillary beds • Converge to form larger veins Vessel walls • With the exception of capillaries, vessels have 3 layers/tunics surrounding the central lumen • Tunica intima/interna – Endothelium – simple squamous epithelium continuous with the endocardium of the heart – Tends to be convoluted in large arteries • Tunica media – Smooth muscle and elastin fibers • Can alter lumen size – Vasodilation – muscle relaxes/lumen enlarges – Vasoconstriction – muscle contracts/lumen decreases – Thicker in arteries • Tunica externa or adventitia – Loose connective tissue and collagen – Large vessels have small vessels within externa called vasa vasorum Vessel walls Arterial system • Elastic/conducting arteries – Largest in diameter • Aorta and major branches – Large amount of elastic fibers • Expand and recoil • Muscular arteries – Relative to size, largest tunic media – Ability to vasocontrict • Arterioles – Few elastic fibers – Diameter controls amount of blood delivered to capillary beds Capillaries • Consist only of tunica intima and basement membrane – Continuous – endothelium is complete • Prevents loss of blood cells and proteins – Fenestrated – contains pores • Allows for movement of small proteins • Choroid plexi, GI tract for absorption, kidneys, endocrine system • Forms beds – network of capillaries Capillary microcirculation • Blood from arteriole to venule • Terminal arteriole → metarteriole → true capillaries → thoroughfare channel → post capillary venule • Precapillary sphincter – Band of smooth muscle at metarteriole/capillary junction – When contracted, blood bypasses capillary bed – Blood goes directly from metarteriole to thoroughfare channel Venous system • Venules – Very porous, like capillaries • Veins – Walls are thinner and lumens are larger as compared to same size artery – Relatively little smooth muscle • Large veins tend to collapse in histological preparation – Externa is the largest layer Venous system cont • Since veins are exposed to relatively low pressure: – Veins contain valves • Prevents backflow of blood due to gravity – Skeletal muscles “milk” veins • Contraction of muscles close to veins help move blood through the vessel – Abdominal pressure differences due to breathing • Abnormalities – Varicose veins • Poor circulation causes valves to become leaky • Veins stretch out and become floppy – Usually superficial veins that have little support from underlying tissues • Hemorrhoids – varicose veins of anal veins Vascular anastomoses • Where multiple vessels unit • Arterial anastomoses – Arteries that supply same area merge – Provides alternate blood supply • Venous anatomoses – More common than aterial Disorders • Atherosclerosis – Lipid deposits on arterial walls – Can cause restriction or blockage of blood flow • Arteriosclerosis – Decreased elasticity “hardening of arteries” – Affects proper blood flow Circulation physiology • Blood flow – Volume of blood that passes through a specific point of a vessel in a specific time • ml/min • Blood pressure – Force per unit exerted by blood on vessel wall • mm Hg – Usually refers to arterial pressure – Pressure gradient – pressure difference is required for blood to flow Circulation physiology cont • Resistance – Opposition to flow; measures friction the blood encounters – Influenced by: • Blood viscosity • Blood vessel length – Longer the vessel = greater resistance • Blood vessel diameter – Blood next to the wall flows more slowly • F = ∆P/R – F = flow; ∆P = pressure difference; R = resistance – As pressure increases, flow increases – As pressure decreases, flow decreases Systemic blood pressure • Vessel pressure is highest in the aorta and decreases to zero in right atrium – Blood flows from region of high pressure to low • Blood pressure is measured in arteries – exposed to highest pressure • Systole – ventricular contraction – ~120mm Hg • Diastole – ventricular relaxation – ~80mm Hg Maintaining blood pressure • Hormonal control (discussed during endocrine system) • Baroreceptors – detect pressure changes – Located in aortic arch, carotid arteries, and large head/neck arteries – When relaxed, the medulla oblongata sends signals to vessels for vasoconstriction, which increases pressure – When stretched, medulla doesn’t send signal – causes vasodilation, which decreases pressure Baroreceptors Maintaining blood pressure cont • Chemoreceptors – detect changes in respiratory gases – Increases in carbon dioxide is detected by aortic arch and carotid sinuses • Vasocontriction causes an increase of blood pressure, which increases blood flow – Gets to respiratory system to unload carbon dioxide more quickly – Decrease of carbon dioxide causes vasodilation – causes slower flow Chemoreceptors Pulse • Surges of pressure in an artery • Pulse rate should equal the heart rate (beats per minute) – Apical pulse – actual heartbeat count – BIG difference = pulse deficit • Felt with fingers (do not use thumb) Blood pressure • Pressure from blood against the vessel walls (arteries) – Reported as diastolic pressure over systolic pressure • Measured with a sphygmomanometer – Inflation blocks bloodflow through brachial artery – Pressure of cuff gradually reduced to allow partial flow • Sounds of Korotkoff – tapping sound of blood flowing back into artery – First appears at systole • Blood spurting into artery – Sound disappears when artery is no longer compressed • Diastole Blood pressure alteration • Hypertension – >140 systole; >90 diastole – To pump against increased pressure, heart works harder • Myocardium increases (especially LV), heart ultimately weakens • Hypotension – <100 systole Circulatory shock • Blood vessels inadequately filled; abnormal blood flow • Hypovolemic – Significant blood loss • Vascular – Poor circulation due to extreme vasodilation • Anaphylactic – allergies; septic – infection • Cardiogenic – Heart not pumping (myocardial infarction) Arteries Arteries Arteries Veins Veins Veins