Download VISCOSITY, HEMATOCRIT AND OXYGEN TRANSPORT

VISCOSITY, HEMATOCRIT AND OXYGEN TRANSPORT
Rajesh Kumarswami, MD (2003-2004)
Hematocrit (def.) – The percentage of blood that is cells
Normal ranges (adult): 40- 45%
Male: 42-45%
Female: 38-46%
Viscosity and flow are interrelated, that is, flow is inversely related to viscosity
In a homogenous liquid viscosity is constant
In a two phase medium (blood) viscosity depends on several factors:
1) Increased HCT- increased viscosity
- Produces increased friction between successive layers of blood
- More internal friction causes increased viscosity
If water = 1, whole blood (with hematocrit 42-45%) – 3.6 times greater
- (3-4 times more pressure required to force whole blood through a tube than water)
- At high altitude blood viscosity may be 10 times water
Viscosity of plasma 1.3 times water due to plasma proteins (not significant for hemodynamic
studies)
2) Fahreus – Lidquist (sigma effect - blood less viscous in small vessels than large)
Very prominent in capillaries (viscosity is one-half what it is in large vessels)
Red blood cell deformity (i.e., sickle cell anemia) is associated with increased viscosity
Leftward shift of hemoglobin-oxygen dissociation curve with cold and increased viscosity reduces oxygen
delivery
Summary – All of these effects are offsetting (some increase viscosity whereas others decrease viscosity).
It is unclear as to how hematocrit affects viscosity in small vessels. Current principles assume that overall
viscosity in small vessels equivalent to that in large vessels.
Editor: Dean F. Connors, MD, PhD
Author: Rajech Kumarswami, MD
(2003-2004)
VISCOSITY, HEMATOCRIT AND
OXYGEN TRANSPORT
For the Boards…
PHYSIOLOGY
Effects on oxygen transport:
Blood oxygen content:
CaO2= (Hb x 1.39 x SaO2) + (PaO 2 x .003)
Where:
Hb = Hemoglobin
SaO2 = Oxygen saturation of blood
PaO2 = Partial Pressure of oxygen in dissolved blood
Increasing hematocrit increases arterial oxygen content and oxygen transport to cells
But it increases viscosity and may increase the work associated with moving blood.
Increasing hematocrit to about 50% increases oxygen transport. Beyond that oxygen transport is
compromised.
With altitude blood volume increases and optimum hematocrit also increases . This increases oxygen
transport.
Reference:
Pharmacology and Physiology in Anesthetic Practice, 3rd edition. Robert K. Stoelting (ed). Lippincott,
Williams & Wilkins, 1999, pp. 737-739.
Rajesh Kumarswami, MD
2003-2004
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