Chapter 8: Blood Rheology

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Chapter 8 Blood Rheology

• Christina Kolyva

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Blood Composition

• Whole blood consists of formed elements and
  plasma
• Formed elements Red blood cells (RBCs) or
  erhythrocytes (99.9)
• White blood cells (WBCs) or leukocytes
• Platelets
• Plasma consists of Water (92)
• Plasma proteins (7)
• Other solutes (1)
• Hematocrit (H) is the percentage of whole blood
• occupied by cellular elements

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Red Blood Cells

• In adult males 1 µl of whole blood contains
  4.5-6.3 billion RBCs
• Shape Biconcave disk-thin central region and
  thick outer margin. Why?
• Composition Only organelles related to
  transport of respiratory gases
• Hemoglobin (Hb) accounts for 95 of the cells
  intracellular proteins
• Function
• Production No nuclei or ribosomes, so they
  cannot divide or produce their own proteins.
  Life span 120 days
• RBC formation (erythropoiesis) occurs in red
  bone marrow

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White Blood Cells

• In adults 1 µl of whole blood contains 6-9
  thousand WBCs
• Shape Divided to granulocytes and agranulocytes
• Composition They do have a nucleus
• They contain vesicles and lysosomes
• Function Defend the body against invasion by
  pathogens
• Remove toxins, waste, abnormal or damaged cells
• Production They survive from days (N) to months
  or years (L)
• Produced in the bone marrow
• Ls also produced in lymphoid tissues

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Platelets

• In adults 1 µl of whole blood contains 150-500
  thousand platelets
• Shape Flattened disks, round when viewed from
  above
• Composition They do not have a nucleus
• They carry enzymes and other substances
  important for the process of blood clotting
• Function Transport chemicals for initiation and
  control of clotting
• Form temporary platelet plug in the walls of
  injured blood vessels
• Actively contract when the clot has been formed
• Production They live for 9-12 days
• Produced in the bone marrow by magakaryocytes

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Plasma

• Composition Contains significant quantities of
  dissolved proteins
• Albumins (60) Important for the transport
  of fatty acids, thyroid hormones and steroid
  hormones. Also major contributors to the
  osmotic pressure of plasma
• Globulins (35) Antibodies and transport
  proteins
• Fibrinogen Important for blood clotting.Fit
  forms fibrin, which is the network for a
  blood clot
• Also contains regulatory proteins,
  electrolytes, organic nutrients and organic
  waste

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Viscosity

• Viscosity µ
• Units cP ( )

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Newtonian, Non-Newtonian behaviour
Rheological curves shear stress-shear rate
curves

• Bingham fluids (2)
• Casson fluids (3)
• Pseudoplastics (4, 5)

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Apparent viscosity

• For non-newtonian fluids apparent viscosity µa
  is defined as the slope of the rheological curve
  at a specific shear rate
• Relative apparent viscosity is the ratio of the
  apparent viscosity of a solution divided by the
  apparent viscosity of the solvent

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Viscometers
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Blood viscosity

• Blood is a non-Newtonian fluid
• Apparent blood viscosity depends on shear rate
• Low shear rategt Rouleaux formations and
  sedimentationgthigh apparent viscosity
• High shear rategt the stacks break downgt
  newtonian behaviour

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Blood viscosity

• The blood has yield stress

• Yield stress depends on H and also on the
  fibrinogen concentration in plasma
• Empirical relation

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Blood viscosity

• Relative viscosity depends also on H and on the
  flexibility of the RBCs

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Blood viscosity

• The dependence on H is non-linear for tube sizes
  down to 9 µm. For smaller tubes the relation is
  linear

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Blood viscosity

• Blood viscosity depends on plasma viscosity .
  The latter depends on the protein concentration
  of plasma
• Protein concentration of plasma also affects the
  flexibility of the RBCs and the interactions
  between them (adhesiveness, aggregation)

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Blood viscosity

• Blood viscosity also depends on temperature, on
  the presence of platelets (thrombi formation)
  and on the presence of WBCs (but only at
  pathological conditions)
• Conclusion? The parameters that determine plasma
  viscosity affect also each other. It is
  difficult to study each one separately

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Model

• Blood is modeled as a Casson fluid
• When tgtgtt0 k µa and blood behaves like a
  newtonian fluid
• At high shear rates µa can be calculated as

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Fahraeus-Lindqvist effect

• The apparent viscosity of blood depends on the
  geometry of the instrument in which it is
  measured

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Fahraeus effect

• Reduction in tube hematocrit in microvessels
  relative to the supply hematocrit

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Blood rheology in the circulation

• High shear rates, therefore blood can be
  considered newtonian
• In the capillaries though, the
  Fahraeus-Lindqvist effect must be taken into
  account

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Blood rheology in the circulation

• Isolated rat hearts-blood apparent viscosity was
  changes by adding albumin
• Minimal resistance remained constant despite the
  changes in apparent viscosity

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Blood rheology in the circulation

• Surface of endothelial cells is lined with
  glycocalyx

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Blood rheology in the circulation

• Consists of membrane-bound molecules
  glycoproteins, glycolipids, proteoglycans and
  proteins

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Blood rheology in the circulation

• Implications of glycocalyx in blood rheology
• Decrease in H larger than predicted by the
  anatomical diameter
• Increased resistance to flow
• Shear stress on the endothelial surface is
  small-transmitted via the glycocalyx
• Regulation of blood flow via changing the shape
  of the layer