Biological fluid mechanics, or biofluid mechanics, is the study of both gas and liquid fluid flows in or around biological organisms. An often studied liquid biofluid problem is that of blood flow in the human cardiovascular system. Under certain mathematical circumstances, blood flow can be modeled by the Navier–Stokes equations. In vivo whole blood is assumed to be an incompressible Newtonian fluid. However, this assumption fails when considering forward flow within arterioles. At the microscopic scale, the effects of individual red blood cells become significant, and whole blood can no longer be modeled as a continuum. When the diameter of the blood vessel is just slightly larger than the diameter of the red blood cell the Fahraeus–Lindquist effect occurs and there is a decrease in wall shear stress. However, as the diameter of the blood vessel decreases further, the red blood cells have to squeeze through the vessel and often can only pass in a single file. In this case, the inverse Fahraeus–Lindquist effect occurs and the wall shear stress increases. An example of a gaseous biofluids problem is that of human respiration. Recently, respiratory systems in insects have been studied for bioinspiration for designing improved microfluidic devices. Biotribology is the study of friction, wear and lubrication of biological systems especially human joints such as hips and knees.[5] In general, these processes are studied in the context of Contact mechanics and tribology. When two surfaces rub against each other, the effect of that rubbing on either surface will depend on friction, wear and lubrication at the point of contact. For example, the femoral and tibial components of knee implants routinely rub against each other during daily activity such as walking or stair climbing. If the performance of the tibial component needs to be analyzed, the principles of contact mechanics and tribology are used to determine the wear performance of the implant and the lubrication effects of synovial fluid.