212 SPEEDS OF SOME PROCESSES IN BIOLOGICAL SYSTEMS 



brane is 1 mole per liter, the values come out to 1000 to 50,000 ohms/cm 2 , 

 in general agreement with values found by direct measurement for living 

 membranes. The values determined depend on the permeability, discussed 

 later. 



Osmosis 



Following the foregoing discussion, very little needs to be said about 

 osmosis. It is simply the diffusion of water from the place of higher water 

 concentration to the place of lower water concentration. More properly, it 

 is the diffusion of water down an activity (effective concentration) gradient. 

 The speed of the process is described by Fick's laws — the first for the steady 

 state of constant concentrations, and the second for the unsteady state of 

 changing concentrations. 



Osmotic pressure and water balance, both properties of the equilibrium 

 state, were discussed in Chapter 2. 



As an anatomist, Fick naturally had an interest in these important proc- 

 esses; but this interest must have been accompanied by a remarkable insight. 



ON FLUID FLOW; BLOOD 



Poiseuille's Law 



Holding a special place among the kinetic processes of importance in 

 biology is the transport of fluids, both gases and liquids, along tubes and in 

 and out of storage chambers. One need mention only the circulation of 

 blood and the respiration of air as examples. 



The first striking fact is the flow itself: it takes place (almost) no matter 

 how small the applied mechanical force; and the rate of flow increases line- 

 arly with increasing driving force. Flow is opposed by frictional forces or 

 "internal barriers" which the moving fluid must surmount — the smaller the 

 internal barriers the faster the flow resulting from a given applied force. 



Ideally at least, as was first stated by the French physicist, J. L. Poiseuille, 

 in 1884, a liquid moves in a tube by the sliding of one imaginary layer of 

 liquid over another. The surface layer moves very slowly, if at all, relative 

 to the speed of layers far removed from the surface. The presumed velocity 

 distribution is indicated by the lengths of the arrows in Figure 8-10. 



Figure 8-10. The Gliding Layers in Nonturbulent Fluid Flow. Length of the arrow is pro- 

 portional to speed. 



