Blood is known to be a non-Newtonian fluid with a non-linear stress-strain rate relationship. As the red blood cells (RBCs) constitute more than 99% of the particulate matter in blood, they govern the flow properties of blood. The white blood cells and platelets usually occupy only less than 1% of the total volume of blood cells in normal human blood and therefore, they exert little influence on the bulk rheology of blood. The volume fraction of RBCs in blood is called the 'hematocrit'; the normal value is 0.47 in adult human males and 0.42 in adult females.
The viscosity of RBC suspensions is strongly influenced by the following three factors:
(a) volume fraction of RBC - with the increase in the volume fraction of RBC, the suspension viscosity increases;
(b) cell deformability - the suspension viscosity increases with the decrease in RBC deformability;
(c) aggregation of RBCs - RBCs tend to pile up into rod-shaped aggregates, like a pile of coins, when at rest.
Such piles of cells are called 'rouleaux'. Rouleaux formation is believed to be due to bridging between the cells by plasma proteins such as fibrinogen. Aggregation of RBCs results in an increase in the viscosity of blood.
This project aims to review current models of blood and propose a more accurate model which incorporates the distribution of RBCs in the blood.
Figure: Blood velocity solution during a sinusoidal applied pressure using the Casson approximation for stress tensor.