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Chapter 19 Physiology of Circulation.doc

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University of Ottawa
Anatomy and Physiology
Jacqueline Carnegie

Chapter 19 Physiology of Circulation Blood Flowthe volume of blood flowing through a vessel organ or the entire circulation in a given period in terms of mLminblood flow is equivalent to cardiac output COrelatively constant under resting conditionsblood flow through individual body organs vary widely and is related to their immediate needsBlood Pressurethe force per unit area exerted on a vessel wall by the contained bloodexpressed in millimetres of mercury mm Hgblood pressure refers to the systemic arterial blood pressure in the largest arteries near the heart the pressure gradient the differences in blood pressure within the vascular system provides the driving force that keeps blood moving from an area of higher pressure to lower pressure in the body Resistancethe opposition to flow and is a measure of the amount of friction that blood encounters as it passes through the vesselsmost friction is encountered in the peripheral systemic circulation away from the heart so it is called peripheral resistance there are 3 sources of resistance1 Blood Viscositythe internal resistance to flow that exists in all fluids is known as viscosityit is related to the thickness of a fluidthe greater the viscosity the less easily molecules slide past one another and the more difficult it is to get and keep fluid movingblood is more viscous than waterit contains formed elements and plasma proteinsblood flows more slowly under the same conditionsblood viscosity is fairly constantpolycythemia excessive RBCs can increase blood viscosity and peripheral resistance will increase anemia low RBC count can decrease blood viscosity and peripheral resistance will decrease2 Total Blood Vessel Lengththe longer the vessel the greater the resistancean infants blood vessels lengthen as they grow to adulthood and peripheral resistance and blood pressure increase 3 Blood Vessel Diameterchanges in blood vessel diameter are frequentfluid close to the wall of a tube is slowed by friction as it passes along the wall and fluid in the centre of the tube flows more freely and fasterthe relative speed and position of fluid in the different regions of the tubes cross section remain constantthis is known as laminar flow or streamliningthe smaller the tube the greater the friction since more of the fluid contacts the tube wallresistance varies inversely with the 4th power of the vessel radiusif the radius of a vessel is doubled the resistance drops to 116 of its original value large arteries close to the heart do not change dramatically in diameter and contribute little to peripheral resistancesmall diameter arterioles that can enlarge or constrict that can enlarge or constrict in response to neural or chemical controls are the major determinants of peripheral resistance when blood encounters an abrupt change in tube size or rough protruding areas of the tube wall ie fatty plaques the smooth laminar blood flow is replaced by turbulent flow which is an irregular fluid motion where blood from different laminae mixesturbulence increases resistance Relationship Between Flow Pressure and Resistance blood flow F is directly proportional to the difference in blood pressure P between 2 points in circulation blood pressure or hydrostatic pressure gradientwhen P increases blood flow speeds upwhen P decreases blood flow declinesblood flow F is inversely proportional to the peripheral resistance R in the systemic circulationif R increases F decreasesFDPRof the 2 factors R is more important than DP in influencing blood flow because R can easily be changed by altering blood vessel diameterwhen the arterioles serving a particular tissue dilate ie decreasing the resistance blood flow to that tissue increasesSystemic Blood Pressureblood flows through the blood vessels along a pressure gradient from a higher to lower pressure areathe pumping action of the heart generates blood flow and pressure results when flow is opposed by resistance systemic blood pressure is highest in the aorta and declines throughout the pathway to finally reach 0 mm Hg in the right atriumthe steepest drop in blood pressure occurs in the arterioles which have the greatest resistance to blood flow
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