PSIO 202, Lecture 10 - PSIO 202 Human Anatomy and...

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Unformatted text preview: PSIO 202 Human Anatomy and Physiology Lecture 10 Cardiovascular System: Hemodynamics; Pressure, Flow, and Resistance Objectives and Reading Assignment Reading: Tortora, pages 772-775 Objectives Describe the relation between cross sectional area of the vascular system, and blood flow velocity Compare and contrast systolic, diastolic and mean blood pressures in the systemic and pulmonary circulations Describe the principal components of vascular resistance, and explain why vessel diameter has such a large impact on resistance Calculate systemic vascular resistance given values for cardiac output and mean aortic, and mean right atrial pressure Hemodynamics hemodynamics is the study of fluid flow in the vascular system fluids always flow from regions of high pressure to regions of lower pressure resistance to fluid flow is caused by friction between the molecules in the fluid and the walls of the tube frictional resistance always reduces flow Blood flow and cross sectional area of the vascular system Vessel total cross sectional area flow velocity Aorta Vena cavae Capillaries 3 cm2 14 cm2 5,000 cm2 40 cm/sec 5-20 cm/sec 0.1 cm/sec Relation Between Total CSA and Flow Velocity Determinants of Blood Pressure Blood pressures are not equal throughout the cardiovascular system If they were equal, blood would not flow, because flow requires a pressure difference (or “driving pressure”) Blood pressure is pulsatile, and is in phase with the heartbeat It peaks during systole, and falls during diastole Arterial Blood Pressure Arterial BP reflects two factors of the arteries close to the heart Their elasticity (compliance, or distensibility) The amount of blood forced into them at any given time Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls) Pressures in the Vascular System Arterial Compliance The large arteries are very compliant, meaning that the volume change for any change in pressure is relatively large C = dV / dP High compliance (and elasticity) allows the flow to be propelled throughout the cardiac cycle, owing to the alternating expansion and recoil of the arteries after each contraction of the left ventricle This generates a pressure wave called the “pulse” Mean Arterial Blood Pressure SBP MBP DBP Estimating Mean Blood Pressure MBP = DBP + 1/3 (SBP – DBP) Example: SBP = 120 mmHg; DBP = 80 mmHg MBP = 80 mmHg + 1/3 (120 – 80 mmHg) = 93 mmHg Pulse Pressure PP = SBP - DBP The PP reflects the stroke volume if C is constant (C = dV / dP) dV = C x dPP If Stroke Volume is constant, a high PP reflects a reduction in compliance (“stiff arteries”) Pressures in the Pulmonary Circulation Pressures in the pulmonary circuit are much lower than those in the systemic circuit The pulmonary circulation is very compliant Mean pressure in the pulmonary artery is about 14 mmHg at rest (SBP = 24 mmHg; DBP = 9 mmHg) The mean left atrial pressure is about 5 mmHg; thus, the pressure gradient for left atrial filling is about 9 mmHg (14 - 5). Vascular Resistance Blood flow is proportional to the driving pressure, and inversely proportional to the “resistance” to flow Flow = driving pressure / Resistance Resistance is calculated after flow and pressure are measured Resistance is the sum of all forces that retard flow Components of Resistance R = (viscosity) (vessel length) / diameter or radius to the 4th power Radius = 1.0 Radius = 1/2 4 = 1/16; thus, a halving of the radius or 14 = 1; diameter results in a 16-fold increase in resistance!!!!! ...
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This note was uploaded on 01/30/2010 for the course PSIO 202 taught by Professor Staff during the Spring '08 term at University of Arizona- Tucson.

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