The fluid mechanics of blood: equilibrium and sedimentation

A continuum physics theory of blood in equilibrium and sedimentation is presented, based on the observed behaviour of blood using high-speed photography. Present interpretation of the rheological behaviour of blood is based on particulate suspension theory. When the observed cellular behaviour of blood (by high-speed photography) is considered carefully it is found that none of the initial assumptions made in that theory are realistic. Thus although such a theory provides sophisticated mathematics it really gives no insight into the true fluid mechanics of blood. The theory developed in this paper highlights the importance of time scales as well as length scales in understanding the physics of blood. It emphasises that blood should be considered (for macroscopic rheological purposes) as a mixture of two continuous fluids. The average behaviour of plasma molecules gives a continuous plasma fluid while the average behaviour of the erythrocytes gives a continuous erythrocyte fluid. Blood is simply a mixture of these two fluids. The fluid mechanics of blood in equilibrium and during sedimentation is developed using these concepts. Confirmation of the theoretical predictions is provided by simple laboratory experiments.