Chapter 8: Blood Rheology

1. Christina Kolyva Chapter 8: Blood Rheology

2. 0.1% 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

3. 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 cell’s 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

4. Lymphocyte (20-30%) Basophil (<1%) Eosinophil (2-4%) Monocyte (2-8%) Neutrophil (50-70%) 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

5. 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

6. 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

7. Viscosity • Viscosity μ: • Units: cP ( = )

8. Newtonian, Non-Newtonian behaviour Rheological curves = shear stress-shear rate curves • Bingham fluids (2): • Casson fluids (3): • Pseudoplastics (4, 5):

9. Apparent viscosity • For non-newtonian fluids apparent viscosityμαis 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

10. Viscometers

11. Blood viscosity • Blood is a non-Newtonian fluid • Apparent blood viscosity depends on shear rate • Low shear rate=> Rouleaux formations and sedimentation=>high apparent viscosity • High shear rate=> the stacks break down=> newtonian behaviour

12. Blood viscosity • The blood has yield stress • Yield stress depends on H and also on the fibrinogen concentration in plasma • Empirical relation:

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

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

15. 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)

16. 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

17. Model • Blood is modeled as a Casson fluid: • When τ>>τ0 k= μα and blood behaves like a newtonian fluid • At high shear rates μα can be calculated as:

18. Fahraeus-Lindqvist effect • The apparent viscosity of blood depends on the geometry of the instrument in which it is measured

19. Fahraeus effect • Reduction in tube hematocrit in microvessels relative to the supply hematocrit

20. 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

21. 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

22. Blood rheology in the circulation • Surface of endothelial cells is lined with glycocalyx

23. Blood rheology in the circulation • Consists of membrane-bound molecules: glycoproteins, glycolipids, proteoglycans and proteins

24. 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