Blood Vessels and Hemodynamics PDF

Chapter 21 The Cardiovascular System: Blood Vessels and Hemodynamics 1  Structure and function of blood vessels 5 main types Arteries – carry blood AWAY from the heart Arterioles Capillaries – site of exchange Venules Veins – c...

Chapter 21 The Cardiovascular System: Blood Vessels and Hemodynamics 1  Structure and function of blood vessels 5 main types Arteries – carry blood AWAY from the heart Arterioles Capillaries – site of exchange Venules Veins – carry blood TO the heart 2  Basic structure 3 layers or tunics 1. Tunica interna (intima) 2. Tunica media 3. Tunica externa Modifications account for 5 types of blood vessels. 3 4  Elastic Arteries Largest-diameter arteries have lot of elastic fibers in tunica media Help propel blood onward despite ventricular relaxation (stretch and recoil ) 5  Muscular Arteries Medium-sized arteries with more muscle than elastic fibers in tunica media Capable of greater vasoconstriction and vasodilation to adjust rate of flow called distributing arteries because they direct blood flow 6  Arterioles Small arteries delivering blood to capillaries tunica media containing few layers of muscle Metarterioles form branches into capillary bed to bypass capillary bed, precapillary sphincters close & blood flows out of bed in thoroughfare channel vasomotion is intermittent contraction & relaxation of sphincters that allow filling of capillary bed 5-10 times/minute 7 8 Types of Capillaries Continuous capillaries intercellular clefts are gaps between neighboring cells Fenestrated capillaries plasma membranes have many holes Sinusoids very large fenestrations 9 Relationship between Velocity of Blood Flow and Total Cross-sectioned area in Different Types of Blood Vessels 10  Capillary Exchange Movement of materials in & out of a capillary Diffusion. Transcytosis. Bulkflow. 11 Diffusion Most important method Substances move down their concentration gradient O2 and nutrients from blood to interstitial fluid to body cells CO2 and wastes move from body cells to interstitial fluid to blood Can cross capillary wall through intracellular clefts, fenestrations or through endothelial cells Most plasma proteins cannot cross Except in sinusoids – proteins and even blood cells leave Blood-brain barrier – tight junctions limit diffusion 12 Transcytosis Small quantity of material Substances in blood plasma become enclosed within pinocytotic vessicles that enter endothelial cells by endocytosis and leave by exocytosis Important mainly for large, lipid-insoluble molecules that cannot cross capillary walls any other way 13 Bulk Flow: Filtration & Reabsorption Passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction Based on pressure gradient Diffusion is more important for solute exchange Bulk flow more important for regulation of relative volumes of blood and interstitial fluid Filtration – from capillaries into interstitial fluid Reabsorption – from interstitial fluid into capillaries 14  Dynamics of Capillary Exchange 10 9 Starling’s law of the capillaries is that the volume of fluid & solutes reabsorbed is almost as large as the volume filtered 15 Net Filtration Pressure net outward pressure of 10 mm Hg at arterial end of a capillary bed net inward pressure of 9 mm Hg at venous end of a capillary bed About 85% of the filtered fluid is returned to the capillary escaping fluid and plasma proteins are collected by lymphatic capillaries (3 liters/day) 16  Edema An abnormal increase in interstitial fluid if filtration exceeds reabsorption result of excess filtration increased blood pressure (hypertension) increased permeability of capillaries allows plasma proteins to escape result of inadequate reabsorption decreased concentration of plasma proteins lowers blood colloid osmotic pressure inadequate synthesis or loss from liver disease, burns, malnutrition or kidney disease 17  Hemodynamics: Factors affecting blood flow Blood flow – volume of blood that flows through any blood vessel in a given period of time (in mL/min) P 1- P 2 Flow (F) = R Constant vL d: the diameter of blood vessel Resistance (R) = Π d4 v: viscosity, measure the resistance R α 1/d4 of a liquid to flow. R αv Π (P1- P2) d4 Flow= Constant vL 18 19 20  Plasma = 55% of whole blood Blood viscosity is determined primarily by the hematocrit Platelets “Buffy coat”