The study of the flow and deformation of biological materials. As all living things flow and deform, biorheology is involved in the study of every aspect of biology. The behavior and fitness of living organisms depend partly on the mechanical properties of their structural materials. Humans are healthy when the rheological properties of their tissues are normal; deviations lead to illness. For example, a thinner than normal fluid in the knee joint results in pain and arthrosis, and softened or weakened bones result in fractures.