(~10 years) indicate that the viscosity of granite and glass under ambient conditions are on the order of 1020 poises. Physiology Physiology includes the study of many bodily fluids that have complex structure and composition, and thus exhibit a wide range of viscoelastic flow characteristics. In particular there is a specialist study of blood flow called hemorheology. This is the study of flow properties of blood and its elements (plasma and formed elements, including red blood cells, white blood cells and platelets). Blood viscosity is determined by plasma viscosity, hematocrit (volume fraction of red blood cell, which constitute 99.9% of the cellular elements) and mechanical behaviour of red blood cells. Therefore, red blood cell mechanics is the major determinant of flow properties of blood. The leading characteristic for hemorheology has been shear thinning in steady shear flow. Other non-Newtonian rheological characteristics that blood can demonstrate includes pseudoplasticity, viscoelasticity, and thixotropy. Red Blood Cell Aggregation There are two current major hypotheses to explain blood flow predictions and shear thinning responses. The two models also attempt to demonstrate the drive for reversible red blood cell aggregation, although the mechanism is still being debated. There is a direct effect of red blood cell aggregation on blood viscosity and circulation. The foundation of hemorheology can also provide information for modeling of other biofluids. The bridging or "cross-bridging" hypothesis suggests that macromolecules physically crosslink adjacent red blood cells into rouleaux structures. This occurs through adsorption of macromolecules onto the red blood cell surfaces. The depletion layer hypothesis suggests the opposite mechanism. The surfaces of the red blood cells are bound together by an osmotic pressure gradient that is created by depletion layers overlapping. The effect of rouleaux aggregation tendency can be explained by hematocrit and fibrinogen concentration in whole blood rheology. Some techniques researchers use are optical trapping and microfluidics to measure cell interaction in vitro. Disease and Diagnostics Changes to viscosity has been shown to be linked with diseases like hyperviscosity, hypertension, sickle cell anemia, and diabetes. Hemorheological measurements and genomic testing technologies that act as preventative measures and diagnostic tools. Hemorheology has also been correlated with aging effects, especially with impaired blood fluidity, and studies have shown that physical activity may improve the thickening of blood rheology. Zoology Many animals make use of rheological phenomena, for example sandfish that exploit the granular rheology of dry sand to "swim" in it or land gastropods