Overview
A gradient is the change of a quantity (temperature, concentration, velocity) with distance, and it drives transfer processes: heat conduction, diffusion and internal friction. This topic describes these processes and their medical applications, such as dialysis, and the exchange of heat between the body and its surroundings.
Gradients and transfer processes
A gradient is the rate at which a quantity changes in space. Where a gradient exists, a flux appears that tends to remove it, always from the higher to the lower value. The flux is proportional to the gradient (J = −k · grad X).
Diffusion
Diffusion is the movement of particles from a region of higher to one of lower concentration caused by their random thermal motion. Fick's law gives the flux; the diffusion coefficient D rises with temperature and falls as the particles become larger. Diffusion carries oxygen, nutrients and drugs across tissues and membranes.
Internal friction
Internal friction (viscosity) is the transfer of momentum between adjacent fluid layers moving at different velocities. The force between layers is F = η·S·(dv/dx), where η is the dynamic viscosity (Pa·s). Blood is a non-Newtonian fluid whose viscosity depends on the haematocrit and on the shear rate.