Determinants of blood viscosity Viscosity - reminder
55-60% of body mass is water 42 kg (70 kg body mass)
A
v
v
y
2/3 intracellular 28 kg
1/3 extracellular 14 kg 1/3 plasma 4-5 kg
2/3 intersticium 9-10 kg
Blood Average volume: 5 l Average viscosity: 5 mPas Average density: 1.05 g/cm3 Composition: 40-45 % corpuscular, 55-60 % plasma
Determinants of blood viscosity I. 1. Hematocrit (htc, φ):
Normal range: 0.4-0.5.
V htc = cells Vtotal
Viscosity of blood as suspension (in the physiologically relevant htc range):
lg "s = A + B# η s =suspension viscosity A, B=empirical constants
F
F
A
y
F A η v y
= = = = =
shear force area of fluid layer viscosity flow rate distance between fluid layers
F/A Δv/Δy
= shear stress (τ) = velocity gradient (D)
Units of viscosity:
F #v =" A #y "=
! Ns
1Pas = 1
m2
# D
= 10P( poise)
!
Blood is anomalous (non-newtonian) fluid: ! viscosity varies with shear stress. its
Determinants of blood viscosity II. 2. Plasma viscosity Depends on plasma proteins. In paraproteinaemias (e.g. myeloma multiplex or plasmocytoma) the concentration of immunoglobulins is high, leading to increased viscosity.
3. Plasticity of red blood cells 65% suspension of blood-cell-size particles is rock hard. In contrast, a 95% blood suspension if fluid, with viscosity of ~20 mPas! Deformation of red blood cells: droplet, parachute, arrowhead shapes.
!
1
Determinants of blood viscosity III. 4. Aggregation of red blood cells Stack or roleaux formation. More pronounced at low flow rates.
Determinants of blood viscosity IV. 5. Flow rate, velocity gradient η (mPas) 5
Roleaux 0 0
Determinants of blood viscosity V. 6. Blood vessel diameter η (mPas) 5
2 0 500 0 50 5 Diameter (µm) N.B.: -With a dicrease of vessel diameter, the anomalic (non-newtonian) behavior of blood becomes more pronounced. -Axial migration: the red blood cells line up in the axis of the vessel. In the axis the velocity gradient decreases, and near the vessel wall it increases. Increase in velocity gradient decreases apparent viscosity (Fåhraeus-Lindquist effect).
0.05
V (m/s)
CIRCULATORY SYSTEM A. Function: Maintenance of environmental parameters of cells “Steady state” Transport: Gases Metabolites Hormones, signal transmitters Immunoglobulins Heat B. Hemodynamic requirements: Slow (low flow rate) Steady (no fluctuations) Unidirectional C. The vascular system is a closed vessel system returning into itself
Fluid flow in vessel
The vascular system is a closed vessel system returning into itself
Continuity equation - reminder
A1v1 = A2v 2 = const A2v2 A1v1
A2v2
A=cross-sectional area v=flow rate
! A1v1 = A" (v ) average = const
A1v1
AΣ=total cross-sectional area A3v3
!
2
Structure and physical properties of the vascular system 100 Hgmm
2500 cm2
Blood vessels as elastic tubes Non-liear elasticity Strain is not linearly proportional to stress. Stress
Pressure
Flow rate
0.33 m/s 0.22 m/s
8 cm2 Cross-sectional area
2.5 cm2
Aorta
Arteries
0.0003 m/s
Arterioles Capillaries
Volume Veins
Pressure> pressure that sustains flow, "blood pressure". Reason of pressure drop: flow resistance; most of energy is converted to heat. Flow rate and total cross-sectional area change inversely (based on equation of continuity, Av=constant). Flow rate typically does not exceed the critical (see Reynolds number),and flow remains laminar. (But: behind aortic valve, constricted vessels, low-viscosity conditions, Korotkoff sound). Arterioles (vessels containing smooth muscle, under vegetative innervation) are pressure-regulators: "resistance vessels." Most of blood volume in veins: "capacitance vessels."
Relationship between flow intesity and pressure
Determinants of vascular elasticity: Elastin Collagen Smooth muscle Implications of vascular easticity: Storage of potential (elastic) energy Dampening of pressure pulses Constant flow rate
Auxiliary factors of circulation 1. Arterial elasticity (storage of potential energy) 2. Venous valves (Harvey's experiment) “On the Circulation of the Blood” (1628).
Flow intensity (I)
3. Muscle action Pc
Pressure (P)
N.B.: -The curves intersect the pressure axis at values greater than 0 (critical closing pressure, Pc).
4. Negative intrathoracic pressure 5. "Up-and-down" movement of atrioventricular plane
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