Document aBBBBRdEg7NK1zgJaKkb148zN

156 PRANK A. PATTY but past this point the absorption curve for a gas or vapor of low reactivity bg. comes rather flat. With a relatively water-insoluble vapor such as benzene, bloo' saturation occurs so rapidly that even the venous blood may reach 70 to 80 per cenj of saturation within 30 minutes, yet relatively complete saturation may require much as 2 or 3 days. This can be explained by the fact that the fatty tissue, which has the greater affinity for benzene, removes and stores the benzene carried by the blood, but this fatty tissue has in many instances a very meager blood supply and therefore requires a longer period to attain equilibrium. Methanol is typical of vapors highly soluble in water. About 24 hours inhala' tion of this vapor is required before the blood is 70 per cent saturated (about 50 times as long as for a similar percentage saturation with benzene), yet relatively complete saturation here requires little longer than with benzene, the difference being that the fatty tissues with their smaller blood supply are not a reservoir fq methanol, and distribution throughout the body at equilibrium is directly pro', portional to the water content of each tissue. Both benzene and methanol ar examples of very slightly reactive vapors. Carbon disulfide, approximately 90 per cent of which has been found to b; metabolized, may be cited as an example of a moderately reactive, relatively water-insoluble vapor. McKee and associates9 have reported upon the blood satu> ration with carbon disulfide in inhalation exposures of dogs. From its physical an_T chemical properties, carbon disulfide would be expected to have an absorption! curve resembling that for benzene except' for the effect of a higher rate of me' lism, and McKee's data indicate that this is the case. If the points on his graph ftr the'blood'sAtWatlon^'fTtlog BreathihX50^p.p.fnTcarBbh'3isurfide are connecfe"d~Bf' an exponential curve, the data indicate a saturation of around 90 per cent at tli end of a 3%-hour exposure. The coefficient of distribution of CS2 between bibb and room air at this percentage saturation was 4.3 on a milligrams per liter basisi| If this were corrected for alveolar air temperature and humidity, the value at equii librium would be approximately 5. Desaturation was rapid and, for single : posures, more or less complete within 2 to 6 hours. This is similar to the desatur| tion curve for benzene. During accumulation or absorption the arterial 'blood.d saturated to a higher degree than venous blood, while at equilibrium or saturatidt the concentration is the same in each, and during elimination the concentration o* benzene in the arterial blood is lower. .^ .... :"The7diflCTi5ffiKjin"va'p.Ob"iroft^^^ dependent uponthe solubility or. coefficient of distribution of the particular vapiljr With a very soluble vapor the difference is negligible, while with a slightly soluble vapor, such as benzene, the difference is marked, and during accumulation., tip aFterial-bloodr-onceithe-lungs-are-fi.lled-witbvapor'by-the-first-few-inhalations?j}; equilibrated with air only slightly less in concentration of vapor than the air hi ' R. W. McKee, C. Kiper, J. H. Fountain, A. M. Riskin, and P. Drinker, J. Am. M'efc. Assoc., 122, 217 (1943). '' I 19 R. W. McKee, private communication. .^ ENTRY AND ACTION OF TOXIC MATERIALS 157 O jpri^df'In other words, the blood passing through the lungs is not sufficient to absorb "Appreciable percentage of the vapor from the alveolar air. This has an imporujj^aring on brief exposures to high concentrations of slightly soluble vapors, 'jjff^fterial blood supply to the brain is large: therefore, brief exposures to high ||||||jrations of anesthetic gases or vapors of low solubility produce rapid effects llljiphbjugh the degree of saturation of the body is very low. The administration of Ji'^Ipgood example of this point. Moderate concentrations may be given that j||nauce. unconsciousness after % hour or more and the effects will be somewhat jonged', or concentrations may be given sufficiently high to induce unconscious||ffi^ffluickly and have recovery occur shortly after removal from exposure, provid^^^^^expqsure has not been prolonged. This also has a practical bearing in de- ^S^M-'Vork where a short exposure to high concentrations of trichloroethylene, ^S^raerfeimilar solvent, may not materially raise general body concentration but ySinduce anesthesia. It is therefore important to know peak as well as average ^ra^opcentrations. '^Thc,curves in Figure 2 illustrate the differences in rate of absorption by in- 90 75 TIME.hr. feu^''illu8trating t*le absorPtion of some.representative gases and vapors into-the blood of living dogs. ^Mtevapproach to equilibrium in the circulating--blood- for different J^gpppiid vapors. Nitrogen, nonreactive and slightly soluble, but with leoifi^affinitv. approaches equilibrium rapidly (see also page 588). Carbon '%n;dderately reactive and slightly soluble, approaches saturation rapidly, 8^^ajiking on the period of complete saturation. Benzene, very slightly Soluble, reaches 70 per cent saturation rapidly "but does not equilibrium even after a period of 3 days. Methanol, a slightly sjhjg|p soluble vapor, gradually approaches saturation, requiring about ^|f?9-Per cent saturation and about 5 days for essentially complete sat- (not shown) attains saturation slightly more slowly than does 'igguse it is metabolized fairly rapidly.