Document J35G4O0kmRjg4O86zrQnnJaze

586 HEINZ SPECIIT for depressions. On the other hand, the more generally practical use of incref' pressures is in connection with the displacement of water while diving atp water and solids (such as sand and mud) in caisson work. In these case|_ weight of the air becomes a constant and usually negligible factor upon wf is superimposed the pressure exerted by the column of fluid above the free sujpif in such an enclosed system. In diving a uniform, or arithmetic, incremesff pressure per unit depth is added on descent. Due to the relatively mod|!|f depths that can be withstood in diving, and the nearly incompressible natjT` fluids, this increment is practically constant. In sea water it is about for every 33 feet, or roughly V2 p.s.i. per foot. ^ 4. Partial Pressures The pressure changes described above are shared by the constituent g. the atmosphere in the ratio of their occurrence (Dalton's law) in air, or as modi in any gas space, such as the alveoli of the lungs. This situation followssfro the kinetic concept of gases, since each species of molecule exerts its pr|| throughout any definable space as if it were there alone. Thus, P(air) = p"02 + p'"H20 + p"" inert gas, etc., where P(air) is the barometric p>rg, ` and p', p", and so forth, are the partial pressures of the several gases of tfr (Certain deviations from this simple relation may be expected under high fto, pressures due to interference of different molecular species with each othekf change in' the sum of these component pressures must be shared proportion^, by`'the'cOffSfituentsr'Thus, "a'cm-veTiescribing-the-partial-pressure-of-ox; ^ air at different altitudes would be the line drawn about one fifth the di? from the ordinate axis for air pressure at any altitude in Figure 1. It wff seen later that the partial pressure of gases is most important in the of gaseous exchange and actually forms the nucleus of all rational approapf the problems of the effects of abnormal atmospheric pressures upon diyt organisms. ; i! The pressure-volume relationship of gases at constant temperature^^E altitude is determined from Boyle's law (PV = constant) by the relation P,|p P2F2. By suitable arrangement the volume assumed by wet air at con; temperature at any altitude is calculable from the formula10,11 P1 --pH20 V2(wet) = Vi(wet) -pH20 where the symbols have the following significance: V, = the initial volu ground-level7-Fi-'=-JtKe-re8ulting-volume-at-altitude'-of-the-sameiweiglfE;cff. contained in Vi, Pi == the initial pressure at ground level, P2 = the re " C. H. Fugitt, V. S. Naval Air Training Bases, Pensacola, Fla., Project X.-^ No. 1 (1945). u Aero MedicalLaboratory Staff, AA F Manual No. 25--2 (March, 1945). Y EFFECTS OF ABNORMAL ATMOSPHERIC PRESSURE 587 . altitude, and pH20 = the vapor pressure of water at the specified 5. Solubility or Absorption ^greater or lesser extent all gases are soluble in other substances and, jSojThe moment chemical reaction with the solvent, the concentration of f$Md is proportional to its pressure (partial pressure) above the interface llftpblvent (Henry's law). The rate of solution depends on the difference frequency at which the gas molecules. impinge on the surface and vhich the gas in solution tends to escape from solution, and on the nature lik -af sji|||cte are equally true of gases dissolved in body substances. The living T4s|jfj|xists in a state of dynamic equilibrium with the gases that impinge IpThe diffusion of gases through the outer skin of the body is of a low .^asfpown by Behnke and Willmon.18 As far as practical considerations Mp|. gaseous exchange of the body is through the lung surface, where |isflffected readily. I^'een shown by Haldane14 and others that the volume of oxygen passed lung-blood interface per mm. Hg differential pressure (across the pries from 25 to 56 ml. per minute, depending on the state of exercise, i^clioxide the value is at least 20 to 30 times as great. This ratio is ^skg&qual but unrelated to that of their solubilities in water at equilibrium, |Sldepends-on-faGtors-affeGting-diffusivityA5 .greater solubility in water of carbon dioxide, as compared with oxygen, ^/Jniairily to its tendency to form a hydrate with water and to ionize in ' InVTh'.s tendency acts to decrease the partial pressure of carbon dioxide in Jja|hr:and thus to maintain a greater difference in concentration across the can be effected with inert gases. 3m|lied above, the solution of gases in the body 'i| primarily a solution 'r;,,ai)i'after this initial action further distributiomdepends on the diffusion, MiOn^gartition, and chemical combination, if any, in other body substances, gaiirthe blood and watery fluids the most important gas reservoir of the s'noe ^ oocurs in a relatively large Quantity. Gas dissolved in ^&?W-n"by--`Behnk'--Ge^h''T 'ana ^ers -to-be a major -source of pjKirt^Sson disease. Table 2 shows the solubilities of several atmospheric ptjer/and olive oil and the ratio of these solubilities, indicating that at Jr.. R. T. Dillon,.and D. D. Van Slyke, J. Biol. Chem., 165' 597 (1934). .. iMke^and T. -L: -Willmon. Am. J, Physiol.. 131. 627 (1940-414__________ ^^^^Samahe and J. G. Priestley, Respiration. Yale TJniv. Press, New Haven, 1935. raQtf, Science, 121, 216 (1955). BIBehnke, Bull. New York Acad. Med., 18, 561 (1942). jitei^3h"and. G. E. Hawkinson, Naval Med. Research Inst., Research Project X-8/f., biWJfri-P** l..ciou), t lilllsi