Hemodynamics 1

Objectives Define resistance and understand the effects of adding resistance in series vs.in parallel in total resistance and flow. Describe the relationship between pressure, flow and resistance in the vasculature. Explain how Poiseuille’s law influences resistance to flow and define the factors that determine resistance. Describe the change in pressure along vascular tree and explain how flow to any organ is altered by change in resistance to that organ. Explain types of flow, laminar versus turbulent and the transition between them; Reynold’s number. 2

Hemodynamics: Factors affecting blood flow How much blood flow and what determines how much? Blood Flow: Volume of blood flowing through any tissue in a given time period (mL/min) 3

Relations of pressure, flow and resistance Flow = Change in Pressure Resistance F = PP R Flow is: Directly proportional to pressure gradient Inversely proportional to resistance 4

Pressure differences or gradient ∆P ( the greater the ∆P, the greater the flow) Flow  ∆P Flow  ∆P P1 = 90 mmHg P2 = 40 mmHg ∆P = P1 – P2 = 50 mmHg 5

Resistance to BF   (the higher the R, the smaller the flow). Flow  1/R   Resistance arises due to   interactions between the moving fluid and the stationary tube wall   interactions between molecules in the fluid (viscosity)   Factors determining the resistance:   Vessel length   Vessel radius   Blood viscosity 6

1. Blood vessel length Resistance to Flow is directly proportional to the length the longer the length  the higher the resistance the longer the length  the higher the resistance e.g. Obesity e.g. Obesity 7

2. Blood viscosity Resistance is directly proportional to blood viscosity depends on:  ratio of RBCs to plasma vol.  conc. of proteins in plasma.  conc. of proteins in plasma. -  viscosity ( dehydration, polycythaemia) -  viscosity ( dehydration, polycythaemia) -  viscosity (  RBCs or  Plasma prot.) 8

3. Size of the Blood vessel lumen (vessel radius) The resistance to flow is inversely proportional to the fourth power of the radius R  1/d 4 ( the smaller the diameter the greater the resistance if the diameter  by ½, the resistance  16 times) Therefore vessel radius is a major determinant of resistance to flow (happen in arterioles- vasoconstriction and vasodilatation) 9

Poiseuille’s Law } r l F R = 8  l  r 4 DIFFERENCE IN PRESSURE RADIUS VISCOSITY (FLOW)F (FLOW)F = (  P ) r 8nL 4 LENGHT F = PP R  10

Some Implications of Poiseuille’s Law 8  l  r 4 (  P) F = PP R = If  P is constant, flow is very sensitive to tube radius r(10 - r/10)*100 Q/X[1 - (Q/Q r=10 )]* %10,000 0% 910% 6,561 35% 550% % 190% % % decrease in flow % decrease in radius 11

What Can the Body Regulate to Alter Blood Flow and Specific Tissue Perfusion? 8  l  r 4 (  P) F = PP R =  P = Mean Arterial Pressure – Mean Venous Pressure  P, not subject to significant short term regulation R = Resistance R = 8  l  r4 r4 8, , l,  are not subject to significant regulation by body r 4 can be regulated especially in arterioles, resistance vessels 12

Path of Blood Flow in the Circulatory System Heart (left ventricle) aorta arteries arterioles capillaries venules veins vena cava Heart (right atrium) 13

Flow is a measure of volume per unit time Velocity is a measure of distance per unit time Velocity = Flow/Cross sectional area Velocity of blood flow 14

CROSS SECTIONAL AREA AND VELOCITY F=10ml/s A= 2cm 2 10cm 2 1cm 2 V= 5cm/s 1cm/s 10cm/s V = F / A abc 15

Blood Vessel Diameter and Blood Velocity 16

SERIES AND PARALLEL CIRCUITS Organization in the Circulatory System 17

RESISTANCE TO FLOW IN SERIES VS IN PARALLEL Rt = R1 + R2 + R3…. SERIES RESISTANCE 1/Rt = 1/R1 + 1/R2 + 1/R3… PARALLEL RES. SERIES R1R2R3 R1 PARALLEL R3 R2 18

If: R 1 = 2; R 2 = 4; R 3 = 6 PRU’s Then a series arrangement gives: R T = R 1 + R 2 + R 3 R T = 12 PRU’s But a parallel arrangement gives: R T = =1.94 PRU’s 1 1 R 1 1 R 2 1 R

WHAT REALLY HAPPENS IN THE CVS? ARTERY ARTERIOLES CAPILLARIES LOWER RHIGHER R LOWER R 20

LAMINAR VS TURBULENT FLOW THE REYNOLD’S NUMBER Nr = pDv / n p = density D = diameter v = velocity n = viscosity laminar = 2000 or less LAMINAR FLOW TURBULENT FLOW 21