In certain types of arterial disease, specifically hypertension, the subendothelial layers of blood vessels can degenerate locally. This can result in small regions of the endothelium losing their normal support. Due to the viscous drag on the arterial wall, a tear may occur between a normally supported region and an unsupported region of the endothelial lining. As a result, blood can flow from the vessel lumen through the rift in the lining and become dissected between the various layers of the artery. This condition is known as a dissecting aneurysm and is most commonly found in the proximal portions of the aorta. The high velocity of blood flow in this area, along with large shear rate values at the endothelial wall, contributes to its predilection for this site. Shear stress at the vessel wall also influences various other vascular functions such as permeability, biochemical activity of endothelial cells, integrity of blood elements, and blood coagulation. An increase in shear stress on the endothelial wall also stimulates the release of nitric oxide (NO) from vascular endothelial cells, which acts as a potent vasodilator. The viscosity of blood, which is a suspension of formed elements in blood plasma, varies as a function of the hematocrit (ratio of red blood cell volume to whole blood volume). The apparent viscosity of blood is higher than that of plasma, with the difference becoming more pronounced as hematocrit increases. Severe anemia leads to low blood viscosity, while higher erythrocyte concentrations result in a steeper slope on the curve depicting apparent viscosity. Additionally, the apparent viscosity of blood depends on the dimensions of the tube used for measurement. Smaller tube diameters, such as those found in arterioles, lead to a progressive decrease in apparent viscosity. When blood is measured in living tissues, its apparent viscosity is considerably lower compared to measurements obtained using a conventional capillary tube viscometer.
Incertaintypesofarterialdisease,particularlyhypertension,thesubendotheliallayersofvesselstendtodegeneratelocally,andsmallregionsoftheendotheliummaylosetheirnormalsupport.Theviscousdragonthearterialwallmaycauseatearbetweenanormallysupportedregionandanunsupportedregionoftheendotheliallining.Bloodmaythenflowfromthevessellumenthroughtheriftintheliningandbecomedissectedbetweenthevariouslayersoftheartery.Suchalesioniscalledadissecting aneurysm. Itoccursmostoftenintheproximalportionsoftheaortaandisextremelyserious.Onereasonforitspredilectionforthissiteisthehighvelocityofbloodflow,withassociatedlargeshearratevaluesattheendothelialwall.Shearstressatthevesselwallalsoinfluencesmanyothervascularfunctions,suchasthepermeabilityofthevesselwallsbylargemolecules,thebiochemicalactivityofendothelialcells,theintegrityoftheformedelementsinblood,andbloodcoagulation.Anincreaseinshearstressontheendothelialwallisalsoaneffectivestimulusforthereleaseofnitricoxide(NO)fromvascularendothelialcells;NOisapotentvasodilator(seethesection“ •Fig. 17.7 Therelativeviscosityofwholebloodincreasesataprogressivelygreaterrateasthehematocritratioincreases.Foranygivenhematocritratio,theapparentviscosityofbloodislowerwhenmeasuredinabiologicalviscometer(suchasahindlegbloodvessel)thaninaconventionalcapillarytubeviscometer.(RedrawnfromLevyMN,ShareL.Circ Res. 1953;1:247.)9010 1 2 3 4 5 6 7 8 7030 50 Hematocrit ratio Relative viscosity Hind leg Capillary tube viscometer
Rheologic Properties of Blood
The viscosity of a given newtonian fluid at a specified temperature stays constant over a wide range of tube dimensions and flows. However, for a nonnewtonian fluid such as blood, viscosity may vary considerably as a function of tube dimensions and flows. Therefore, the term viscosity does not have a unique meaning for blood. The term apparent viscosity is frequently used for the derived value of blood viscosity obtained under the particular conditions of measurement.
Rheologically, blood is a suspension of formed elements, principally erythrocytes, in a relatively homogeneous liquid, the blood plasma. Because blood is a suspension, the apparent viscosity of blood varies as a function of the hematocrit (ratio of the volume of red blood cells to the volume of whole blood). The viscosity of plasma is 1.2 to 1.3 times that of water. The upper curve in
Fig. 17.7 shows that the apparent viscosity of blood with a normal hematocrit ratio of 45% is 2.4 times that of plasma.
In severe anemia, blood viscosity is low. As the hematocrit increases, the slope of the curve increases progressively; it is especially steep at the upper range of erythrocyte concentrations (see
Fig. 17.7
For any given hematocrit, the apparent viscosity of blood depends on the dimensions of the tube used in estimating the viscosity.
Fig. 17.8 demonstrates that the apparent viscosity of blood diminishes progressively as tube diameter decreases to less than approximately 0.3 mm. The diameters of the blood vessels with the highest resistance, the arterioles, are considerably less than this critical value. This phenomenon
Fig. 17.7 also illustrates that the apparent viscosity of blood, when measured in living tissues, is considerably less than the apparent viscosity of the same blood measured in a conventional capillary tube viscometer.