MECHANICAL FORCES 33 



surface of the water. An extra amount of nitrogen will be dissolved in all the 

 body fluids, including the blood stream. Henry's law describes how the 

 amount of gas dissolved, w, increases linearly as pressure increases: i.e., 

 w = HP, where H is the proportionality (Henry's) constant. This expresses 

 the condition of the diver at equilibrium with his environment. If now, sud- 

 denly, he rises to the surface, the nitrogen which has diffused into the blood 

 stream is not able to diffuse out fast enough, and will come out of solution 

 in the form of small gas bubbles, which rapidly coalesce to form larger ones. 

 Under the conditions described, the bubbles so formed would be easily large 

 enough to form "air locks" and prevent the flow through the blood capil- 

 laries. This illustration simply shows the physical facts of the condition 

 known as "bends": circulation ceases, waste products of muscle activity ac- 

 cumulate, muscles cannot be reactivated; excruciating pain, paralysis, and 

 death can result. The only treatment is to increase the pressure in a pressure 

 tank in the hope that the nitrogen bubbles will redissolve. 



A second problem, and often a more important one, illustrates another 

 physical point. It is a fact that sometimes during fear the individual will 

 hold his breath tightly as he pops to the surface from a considerable depth: 

 since the opening at the epiglottis is small, only a small force by the muscles 

 is necessary to apply the considerable pressure needed to keep this valve 

 closed. Up from even 25 ft, for instance, the external pressure has dropped 

 from 30 psi to 15, and if the extra gas is not exhaled, the excess pressure 

 is a full atmosphere on the delicate walls of the lungs. Punctures, called air 

 embolism, can occur, and cause a condition not unlike pneumonia, in which 

 air-CO, exchange on the lung walls is retarded. 



The results are similar in the case of a high-flying airman if he is ejected 

 from the aircraft and is unprotected by a pressurized flying suit; or in the 

 case of a space traveller whose pressurizing equipment fails. In these cases, 

 in which the pressure is suddenly reduced from about 1 atm to (say) 0.01 

 atm, a second, more serious factor is introduced in addition to the first: the 

 body fluids boil at pressures below about 25 mm Hg at 37°C. 



Facts which the anesthetist should know about gases are expounded and 

 illustrated beautifully by Macintosh et al. 5 ; and aside from the ideal gas law, 

 Henry's law, and recollections about thermal conductivity and resistance to 

 flow through tubes — properties which are discussed briefly later — no further 

 discussions on gases are presented in this book. The reader will have erred 

 if he fails to consult Macintosh at this level of study. 



Some Important Mechanical Properties 



If a mechanical pressure (dynes cm -2 ) produces deformation, the pressure 

 is called a stress. The amount of deformation, e.g., deformed length divided 

 by the original, unstressed length, is called the strain. 



