Date post: | 18-Jan-2016 |
Category: |
Documents |
Upload: | juliana-hunter |
View: | 216 times |
Download: | 0 times |
HemodynamicsHemodynamics
ObjectivesObjectives 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.
Distribution of Cardiac Output at RestDistribution of Cardiac Output at Rest
Blood is constantly Blood is constantly reconditioned so composition reconditioned so composition remains relatively constantremains relatively constant
Reconditioning organs receive Reconditioning organs receive more blood than needed for more blood than needed for metabolic needsmetabolic needs Digestive organs, kidneys, Digestive organs, kidneys,
skinskin Adjust extra blood to Adjust extra blood to
achieve homeostasisachieve homeostasis Blood flow to other organs can Blood flow to other organs can
be adjusted according to be adjusted according to metabolic needsmetabolic needs
Brain can least tolerate Brain can least tolerate disrupted supplydisrupted supply
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)
Relations of pressure, flow and Relations of pressure, flow and resistanceresistance
Flow = Change in Pressure
ResistanceF =
PR
Flow is: Directly proportional to pressure gradientInversely proportional to resistance
( the greater the ( the greater the ∆P∆P, the greater the flow), the greater the flow)
Flow Flow ∆P ∆P
P1 = 90 mmHg P2 = 40 mmHg
∆∆P = P1 – P2 = 50 mmHgP = P1 – P2 = 50 mmHg
Pressure gradient is pressure difference between beginning and end of a vessel
Blood flows from area of higher pressure to area of lower pressure
Path of Blood Flow in the Circulatory System
Heart (left ventricle)
aorta
arteries
arterioles
capillaries
venules
veins
vena cava
Heart (right atrium)
Resistance to BF Resistance is measure of opposition of blood flow through a
vessel
Resistance arises due to interactions between the moving fluid and the stationary tube
wall interactions between molecules in the fluid (viscosity)
(the higher the R, the smaller the flow). Flow 1/R
Factors determining the resistance: Vessel length Vessel radius Blood viscosity
1. Blood vessel length1. Blood vessel length
Resistance to Flow is directly proportional to Resistance to Flow is directly proportional to the lengththe length
the longer the length the longer the length the higher the the higher the resistance resistance
e.g. Obesitye.g. Obesity
2. Blood viscosity2. Blood viscosityResistance is directly proportional to blood Resistance is directly proportional to blood viscosityviscosity
depends on:depends on: ratio of RBCs to plasma vol. ratio of RBCs to plasma vol.
conc. of proteins in plasma.conc. of proteins in plasma.
- - viscosity viscosity ( dehydration, polycythaemia) ( dehydration, polycythaemia)
- - viscosity viscosity ( ( RBCs or RBCs or Plasma prot.) Plasma prot.)
The resistance to flow is inversely proportional to the fourth power of the radius
R 1/d4 ( the smaller the diameter the
greater the resistance ------ if the diameter by ½, the
resistance 16 times)
3. Size of the Blood vessel lumen (vessel radius)
Therefore vessel radius is a major determinant of resistance to flow (happen in
arterioles-vasoconstriction and vasodilatation)
Poiseuille’s Law
} r
l
F R =8 l
r4
DIFFERENCEIN PRESSURE RADIUSVISCOSITY
(FLOW)F(FLOW)F = (P ) r
8nL
4
LENGHT
F = PR
Some Implications of Poiseuille’s Law
8 l
r4
(P)F = P
R=
If P is constant, flow is very sensitive to tube radius
r (10 - r/10)*100 Q/X [1 - (Q/Qr=10)]*100 10 0% 10,000 0%9 10% 6,561 35%5 50% 625 94%1 90% 1 99.99%
% decrease in flow% decrease in radius
What Can the Body Regulate to Alter Blood Flow and Specific Tissue Perfusion?
8 l
r4
(P)F = P
R=
P = Mean Arterial Pressure – Mean Venous Pressure
P, not subject to significant short term regulation
R = Resistance R =8 l
r4
8, , l, are not subject to significant regulation by body
r4 can be regulated especially in arterioles, resistance vessels
Arteriolar Vasoconstriction and Arteriolar Vasoconstriction and VasodilationVasodilation
SERIES AND
PARALLEL CIRCUITS
Organization in the Organization in the Circulatory SystemCirculatory System
RESISTANCE TO FLOW IN RESISTANCE TO FLOW IN SERIES SERIES VS VS IN PARALLELIN PARALLEL
Rt = R1 + R2 + R3…. SERIES RESISTANCE
1/Rt = 1/R1 + 1/R2 + 1/R3… PARALLEL RES.
SERIESR1 R2 R3
R1PARALLEL
R3R2
If: R1 = 2; R2 = 4; R3 = 6 PRU’s
Then a series arrangement gives:
RT = R1 + R2 + R3
RT = 12 PRU’s
But a parallel arrangement gives:
RT = =1.94 PRU’s
1
1 R1
1 R2
1 R3
+ +
WHAT REALLY HAPPENS IN THE CVS?
ARTERY
ARTERIOLES
CAPILLARIES
LOWER R HIGHER R LOWER R
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 flowVelocity of blood flow
CROSS SECTIONAL CROSS SECTIONAL AREA AND VELOCITYAREA AND VELOCITY
F=10ml/s
A= 2cm2 10cm2 1cm2
V= 5cm/s 1cm/s 10cm/s
V = F / A
a b c
Blood Vessel Diameter and Blood Velocity
LAMINAR VS TURBULENT LAMINAR VS TURBULENT FLOWFLOWTHE REYNOLD’S NUMBERTHE REYNOLD’S NUMBER
Nr = pDv / n
p = densityD = diameterv = velocityn = viscosity
laminar = 2000 or less
LAMINARFLOW
TURBULENTFLOW