Document Xznv74jvvnnEGGbKV9rB1D1Oy

148 FRANK A. PATTY Then during expiration, alveolar air containing 5 to 6 per cent carbon dioxide and 12 to 14 per cent oxygen, is forced from the alveoli through the dead space, leaving this dead space filled with alveolar air. The first part of the exhalation is essentially atmospheric air and the last part alveolar air, with the mixture growing richer in alveolar aiT throughout expiration. Undiluted alveolar air can be obtained successfully only from air forcibly exhaled at the end of an ordinary^ respiration. Failure to consider this well-established fact can lead to serious errors in sampling expired air, as for instance in the determination of the radon content of the alveolar air. The maximum rate of physical activity that can be maintained by an individual is probably more dependent upon his maximum breathing capacity, minute ventilation, than upon lung volume. This minute ventilation is partially conditioned by frictional resistance along the air passages due to impediments, such as constrictions and abrupt changes in direction, or air. passages merging at too great an angle. C. REGULATION AND CONTROL OF RESPIRATION The respiratory movements are regulated and controlled chemically as well as by the voluntary and involuntary nervous systems. Carbon dioxide acts as a stimulus to the respiratory center, and the C02 tension in the blood is by far the most significant factor of control. It is thought to act by raising the hydrogenion concentration of the respiratory center. Lactic acid resulting from strenuous exercise is'thought to have a similar effect, Oxygen--or, rather, lack of it--is likewise.a stimulus,-but-under normaLconditions-does.not-come-into-play-.becaus1 the partial pressure of oxygen in the alveolar air must fall markedly before the oxygen tension of the arterial blood is significantly affected. This is easily under- ; stood upon examination of the oxygen dissociation curve for blood, as is illustrated in Figure 1. Men with long experience in testing atmospheres in the holds Figure 1. Dissociation curve of oxyhemoglobin for human blood expressed as the normal range; of percentage of saturation with oxygen. (After Barcroft.) ENTRY AND ACTION OF TOXIC MATERIALS 149 ._________tiffs say that they can recognize atmospheres deficient in oxygen by this ^^^^atiSulatihg effect, but this is not recommended practice even for the well !ualclkbecause the margin between irrespirable atmospheres and those causing ^5?r(tS|tory distress in the form of hyperpnea and dyspnea is too narrow. The ness of the onset of weakness and unconsciousness is so very marked that ftenan enter an oxygen-deficient atmosphere by means of a ladder, the 'Sfva of his being able to remount the ladder unassisted, after becoming MjPdistress, is not favorable. AtfMVCT Pi Pme'iitension or partial pressure of oxygen in normal air is 20.95 per cent of an ire, which equals 159.2 mm. Hg at sea level. Significant symptoms of [|do not occur until the percentage falls below 16 per cent, while un- |riess may occur at concentrations below 11 per cent, and breathing soon fSit?he oxygen falls below 6 per cent. tfr D. FUNCTION OF HEMOGLOBIN |Wtransportation of oxygen from the lungs to the cellular tissues is ed largely by means of the hemoglobin in the red cells of the blood, e carbon dioxide removed from the tissues is transported chiefly by the ,. Each 100 ml. of arterial blood contains about 15 g. of hemoglobin, |mbines with 19 or 20 ml. of oxygen. In blood that is in equilibrium with atmospheric pressure, about 1 per cent of the total oxygen is in n the plasma. Normal venous blood contains 12 to 14 ml. of oxygen " blood, and about 1 per cent of this is dissolved in the plasma. For to the lungs the venous blood carries 55 to 60 volume per cent Ijloxide and deposits about 10 per cent of this as it takes up its oxygen, |He amount of carbon dioxide in simple solution is about 3 volume in the venous blood and 2.5 volume per cent in the arterial blood. jjSJbod in making its circuit flows through the capillaries of the lungs tettately 1 second,8 and through active tissue'in a similar time, it is Ipitiere solution of oxygen and carbon dioxide'cannot account for the 'Miese gases, even though the equilibration in'the lungs is highly effi- m3n*5^'J$^peed of exchange is due to the light combination of these gases with oxyhemoglobin and carbhemoglobfii'; respectively, as well faijmiie'iifi-Sence of catalysts in the red cells that pteywrimportant p&rt in the M dioxide exchange. Carbonic anhydrase is said toy Best8 lo'increase in either direction the reversible chemical reaction H2COa while oxidation and reduction in the tissues are greatly acceler- ygen-activating catalysts or by dehydrogenases. ______ _ ... , .......... Me |E. CIRCULATION AS A FACTOR, AND ITS REGULATION I jb oxygen in the lungs cannot take part in the metabolic process until T Rj(nKestt annnrdl N. RB. 'TI'an.y?llon^r, TTh1.ea Phy..s'aio7nlog.ic.a1 l DB--asis .otf MX/eAdical Practice. m6tLh e- dj., tWttmil Wmit#csmk , Smc-s, Baltimore,' 1956. i