98 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



poison by combining with the haemoglobin of the blood, and thus 

 interfering with normal respiratory processes. The colour of the 

 blood and its resistance to reducing agents are in such cases charac- 

 teristic. 



Nitric Oxide Hsemog^lobin. — When ammonia is added to blood, and 

 then a stream of nitric oxide passed through it, this compound is 

 formed. It may be obtained in crystals isomorphous with oxy- and 

 CO-hsemoglobin. It also has a similar spectrum. It is even more 

 stable than CO -haemoglobin ; it has little practical interest, but is of 

 theoretical importance as completing the series. 



Bohr has advanced a theory that hsemoglobin forms a compound with 

 carbonic dioxide, and that there are numerous oxyhaemoglobins containing 

 different amounts of oxygen, but his views have not been accepted. 



Very dilute solutions of haemoglobin and its derivatives show an absorption 

 band in the ultra-violet region in addition to those just described in the visible 

 regions of the spectrum (see more fully Advanced Course). 



CHEMISTRY OF RESPIRATION 



The consideration of the blood, and especially of its pigment, is sO' 

 closely associated with respiration that a brief account of that process 

 follows conveniently here. 



The lungs consist essentially of numerous little hollow sacs in the 

 walls of which is a close plexus of capillary bloodvessels. These air 

 sacs, or alveoli, communicate with the external air by the trachea, 

 bronchi, and bronchial tubes. Inspiration is due to a muscular effort 

 that enlarges the thorax, the closed cavity in which the lungs are 

 situated. Owing to the atmospheric pressure the lungs become dis- 

 tended. The atmospheric air does not, however, actually penetrate 

 beyond the bronchial tubes ; the gases which get into the smallest 

 tubes and air sacs do so mainly by diffusion. Expiration is ordinarily 

 brought about by the elastic rebound of the lungs and chest walls, and 

 is only a muscular effort when forced : but even the most vigorous 

 expiratory effort is unable to expel the alveolar air. This air and the 

 blood in the capillaries are only separated by the thin capillary and 

 alveolar walls. The blood parts with its excess of carbonic acid 

 and watery vapour to the alveolar air ; the blood at the same time 

 receives from the alveolar air the oxygen which renders it arterial. 



The intake of oxygen is the commencement, and the output of 

 carbonic acid the end, of the series of changes known as respiration. 

 The intermediate steps take place all over the body, and constitute 

 what is known as internal or tissiie respiration. The exchange of 

 gases which.occurs in Ifc^ lungs is sometimes called in contradistinc- 



