Useful to describe gas transport across the alveoli or the placenta. See also Fick's law. The rate of diffusion of a gas across a barrier is proportional to the concentration gradient for the gas. Applicable to gas flows across lung and placental membranes. See also Graham's Law.
Flow is pressure difference divided by resistance.
In laminar flow systems the resistance is constant, independent of flow. When turbulence occurs, the resistance increases with flow. The gas flow rate through a straight tube of uniform bore is proportional to the pressure gradient and the fourth power of the radius, and is related inversely to the viscosity of the gas and the length of the tube. For tubes which are very long compared to their diameter, the flow is laminar when the Reynold's number is below 2,000. Flow will be turbulent at Reynold's numbers as low as 280 for short tubes.