In the tiniest blood vessels, red cells must squeeze through single file
Engineers usually treat blood as a smooth, uniform liquid, and for large vessels that works. Shrink the pipe to barely wider than a red blood cell and the rules shift: friction against the vessel wall actually drops. Narrow it further and the cells must squeeze through one at a time, pushing that wall stress back up.
Biomechanics applies the methods of mechanics to living things, at every scale from whole bodies down to organs, cells, organelles and single proteins. It is a branch of biophysics, and its name joins the Greek words for life and mechanics. Blood flow is a classic problem. At ordinary scales it can be modelled with the Navier-Stokes equations, treating blood as an incompressible fluid, but that assumption breaks down in the finest vessels, where individual cells matter and two opposing effects named after Fahraeus and Lindquist take over.
Materials in the body are organised in layers of structure, from molecules up to tissues and organs, and their behaviour depends on all of them. Researchers split them into hard and soft tissues. Wood, shell and bone deform only slightly and can be handled with the theory of linear elasticity. Skin, tendon, muscle and cartilage stretch far more, so they demand finite strain theory and computer simulation. Biotribology studies friction, wear and lubrication in joints such as hips and knees, including tests of lab-grown cartilage.
Comparing species is a field of its own. Movement and feeding get special attention because they tie closely to survival and place heavy mechanical loads on the body, whether an animal runs, jumps or flies. Insect breathing systems have even inspired better microfluidic devices, part of a wider effort called biomimetics that borrows nature's solutions for engineering problems. Plant biomechanics looks at how crops withstand environmental stress.
Computers have transformed the discipline. The finite element method lets researchers simulate how tissues respond to forces without ethical limits on experiments, and it now supports surgical planning and training; some projects, such as BioSpine, are open source. Sports scientists use strain gauges, force platforms and muscle-activity recordings to analyse technique with a cricket bat, hockey stick or javelin, aiming to improve performance and reduce injury.
Source: Biomechanics
