MEDICAL PHYSICS PRESSURE FLOW RESISTANCE CIRCULATION MEDICAL PHYSICS PRESSURE FLOW RESISTANCE 1

Distribution of blood to the body organs Figure 15-13: Distribution of blood in the body at rest 2

Blood vessel functions: overview Strong and elastic arteries Arterioles control blood flow and pressure Caps: thin and with large area for diffusional exchange Compliant, large, low R veins with valves assures blood return Figure 15-1: Functional model of the cardiovascular system 3

Pulse and Mean Arterial Pressures Figure 15-5: Pressure throughout the systemic circulation 4

Systolic pressure Mean pressure Diastolic pressure Large arteries Left FIGURE 10-8: Pressures throughout the systemic circulation. Left ventricular pressure swings between a low pressure of 0 mm Hg during diastole to a high pressure of 120 mm Hg during systole. Arterial blood pressure, which fluctuates between a peak systolic pressure of 120 mm Hg and a low diastolic pressure of 80 mm Hg each cardiac cycle, is of the same magnitude throughout the large arteries. Because of the arterioles’high resistance, the pressure drops precipitously and the systolic-to-diastolic swings in pressure are converted to a nonpulsatile pressure when blood flows through the arterioles. The pressure continues to decline but at a slower rate as blood flows through the capillaries and venous system. Large arteries Left ventricle Arterioles Capillaries Venules and veins Fig. 10-8, p. 283 5

Some Physics of Fluid Movement: Blood Flow Flow rate: (L/min) Flow velocity = rate/C-S area of vessel Figure 14-6: Flow rate versus velocity of flow 6

Figure 14-4 c: Pressure differences of static and flowing fluid Flow = P/R R = length · viscosity / radius4 CO = art.bp/TPR Poiseuille’s Law Flow proportional to P Resistance reduces flow Reduce vessel diameter Increase viscosity or tube length With constant flow, R can affect P Figure 14-4 c: Pressure differences of static and flowing fluid 7

Arteriolar constriction alters blood flow 8

Arterioles: large and variable R Flow = P/R R = P/Flow Parterioles and Rarterioles largest in CV system Figure 14-2 : Pressure gradient in the blood vessels 9

Capillary Blood Flow Lowest velocity Largest total cross sectional area Hydrostatic pressure drops slightly Figure 15-17: The velocity of flow depends on the total cross-sectional area 10

0.5 500 4.5 6,000 5 Velocity of flow (mm/sec) Anatomical distribution Total cross-sectional area (cm2) Blood flow rate (liters/min) 0.5 500 4.5 6,000 5 Aorta Arteries Arterioles Capillaries Venules Veins FIGURE 10-14: Comparison of blood flow rate and velocity of flow in relation to total cross-section area. The blood flow rate (red curve) is identical through all levels of the circulatory system and is equal to the cardiac output (5 liters/min at rest). The velocity of flow (purple curve) varies throughout the vascular tree and is inversely proportional to the total cross-section area (green curve) of all the vessels at a given level. Note that the velocity of flow is slowest in the capillaries, which have the largest total cross-section area. Venae cavae Fig. 10-14, p. 291 11

P R P FLOW L/min or ml/sec mm Hg ml/sec P1 > P2 P1 FLOW P2 mm Hg P = FLOW x R P R FLOW = P FLOW R = L/min or ml/sec mm Hg ml/sec Peripheral Resistance Units (PRU) 12

POISEUILLE’S LAW GOVERNING FLUID FLOW(Q) THROUGH CYLINDRIC TUBES (Pi - Po) r 4 (FLOW)Q = 8nL DIFFERENCE IN PRESSURE VISCOSITY LENGHT RADIUS 13

LAMINAR VS TURBULENT FLOW THE REYNOLD’S NUMBER p = density D = diameter v = velocity n = viscosity Nr = pDv / n laminar = 2000 or less 14

RESISTANCE AND BLOOD FLOW 2.CODUCTANCE =1/ RESISTANCE 3.DIAMETER OF THE BLOOD VESSELS 4.CRITICAL CLOSING PRESSURE 15

Total peripheral resistance Arteriolar Blood radius viscosity Number of red blood cells Concentration of plasma proteins Local (intrinsic) control Extrinsic control Myogenic responses to stretch Vasopressin Heat, cold application (therapeutic use) FIGURE 10-12: Factors affecting total peripheral resistance. The primary determinant of total peripheral resistance is the adjustable arteriolar radius. Two major categories of factors influence arteriolar radius: (1) local (intrinsic) control, which is primarily important in matching blood flow through an organ with the organ’s metabolic needs and is mediated by local factors acting on the arteriolar smooth muscle in the vicinity, and (2) extrinsic control, which is important in regulating blood pressure and is mediated primarily by sympathetic influence on arteriolar smooth muscle. Angiotensin II Histamine release (involved with injuries and allergic responses) Epinephrine and norepinephrine Local metabolic changes in O2, CO2, other metabolites Sympathetic activity (exerts generalized vasoconstrictor effect) Major factors affecting arteriolar radius Fig. 10-12, p. 290 16

1.DISTENSIBILITY=^VOL/^P x ORIGINAL VOL 2.VASCULAR COMPLIANCE(CAPACITANCE)=^VOL/^P 3.DELAY COMPLIANCE 4.VOLUME PRESSURE CURVE 5.ARTERIAL PRESSURE PULSATION 6.TRANSMISSION OF PRESSURE PULSES 7.DAMPING OF THE PRESSURE PULSES 17

CLINICAL METHOD FOR MEASURING BP 1.PALPATION METHOD 2.AUSCULTATORY METHOD(KORTKOFF SOUNDS) A.SYSTOLIC BP B.DIASTOLIC BP C.MEAN ARTERIAL BP 18

Pressure-recording device Stethoscope Inflatable cuff FIGURE 10-7: Sphygmomanometry. (a) Use of a sphygmomanometer in determining blood pressure. The pressure in the inflatable cuff can be varied to prevent or permit blood flow in the underlying brachial artery. Turbulent blood flow can be detected with a stethoscope, whereas smooth laminar flow and no flow are inaudible. Fig. 10-7a, p. 282 19

When cuff pressure is between than 120 and 80 mm Hg: Blood flow through the vessel is turbulent whenever blood pressure exceeds cuff pressure. Intermittent sounds are heard as blood pressure fluctuates throughout the cardiac cycle. FIGURE 10-7: Sphygmomanometry. (c) Blood flow through the brachial artery in relation to cuff pressure and sounds. Fig. 10-7c (middle), p. 282 20

Cuff pressure Blood pressure Fig. 10-7b, p. 282 FIGURE 10-7: Sphygmomanometry. (b) Pattern of sounds in relation to cuff pressure compared with blood pressure. The numbers on the illustration refer to the following key points during a blood pressure determination: 1 Cuff pressure exceeds blood pressure throughout the cardiac cycle. No sound is heard. 2 The first sound is heard at peak systolic pressure. 3 Intermittent sounds are heard as blood pressure cyclically exceeds cuff pressure. 4 The last sound is heard at minimum diastolic pressure. 5 Blood pressure exceeds cuff pressure throughout the cardiac cycle. No sound is heard. Fig. 10-7b, p. 282 21

Sphincters protect capillaries Effects of gravity on arterial and venous pressures. Each cm of distance produces a 0.77 mmHg change. Veins Arteries 100 mm Hg 190 mm Hg Sphincters protect capillaries VENOUS PUMP keeps PV < 25 mm Hg 22

90 mm Hg caused by gravitational effect Pressure = 0 mm Hg FIGURE 10-26: Effect of gravity on venous pressure. In an upright adult, the blood in the vessels extending between the heart and foot is equivalent to a 1.5-m column of blood. The pressure exerted by this column of blood as a result of the effect of gravity is 90 mm Hg. The pressure imparted to the blood by the heart has declined to about 10 mm Hg in the lower-leg veins because of frictional losses in preceding vessels. Together these pressures produce a venous pressure of 100 mm Hg in the ankle and foot veins. Similarly, the capillaries in the region are subjected to these same gravitational effects. Pressure = 100 mm Hg 90 mm Hg caused by gravitational effect 10 mm Hg caused by pressure imparted by cardiac contraction Pressure = 90 mm Hg Fig. 10-26, p. 301 23

VEINS AND THEIR FUNCTION 1.VENOUS PRESSURE 2.RIGHT ATRIAL PRESSURE 3.EFFECT OF GRAVITY ON VENOUS PRESSURE 4.PRESSURE REFERENCE 5.BLOOD RESERVOIRS 24