RESPIRATION 125 



dissociated. Below 14 mm. the remaining half is given off 

 rapidly and suddenly. 



Arterial blood enters the capillaries with 20 volumes of oxygen 

 per cent, at a partial pressure of 100 mm. It leaves the tissues 

 a second later with only 12 volumes of oxygen, having yielded up 

 8 volumes, or 40 per cent., of its original amount, in consequence 

 of the fact that the partial pressure of oxygen in the tissues is 

 practically nil. Dissociation of the haemoglobin in the tissues 

 has therefore occurred, and there is evidence to show that this 

 is assisted by the presence of carbon dioxide. 



Taking advantage of the fact that only a trifling percentage 

 of the oxygen in blood is dissolved, while the bulk is chemi- 

 cally combined, Haldane was able to show that the oxygen 

 capacity of the blood varied with the colouring power, and thus 

 established a method of clinically estimating the haemoglobin 

 in any sample. 



Carbon Dioxide in Blood. — This gas is held in blood both in 

 simple solution and in chemical combination, the latter account- 

 ing for the larger part of the contained gas. Two-thirds exist 

 in the plasma, and one-third in the corpuscles. In a gas-pump 

 the blood gives up all the carbon dioxide it contains ; but if, 

 instead of using blood, serum be so extracted, it is found 

 impossible to obtain the whole of the carbon dioxdie from the 

 latter without the use of an acid. This notable difference 

 between the blood and serum suggests that the haemoglobin 

 of the red corpuscles acts like an acid, and breaks up the 

 compound containing the carbon dioxide. It appears accord- 

 ing to the most recent views that a portion of the carbon dioxide 

 is held as sodium bicarbonate, and another portion in an organic 

 combination. The latter is probably furnished by the protein 

 of the plasma on the one hand, and by combination with 

 haemoglobin on the other, for haemoglobin has been shown to 

 be capable of forming a weak compound with carbon dioxide. 



It is believed that as the organic combination of carbon 

 dioxide is more readily dissociated than the other, it is 

 this part which does the main work in collecting the C0 2 in the 

 tissues and liberating it at the lungs. The carbon dioxide 

 carried by haemoglobin is believed to be united with the protein 

 portion of the molecules, thus leaving unaffected the iron moiety 

 by which the oxygen is carried. 



Circumst' aces influencing Respiratory Exchange. — Respiratory 

 exchange is influenced by food, being not merely increased as the 

 result of the considerable muscular effort involved during mas- 

 tication, but directly excited by the nature of the food. Thus 

 maize causes in horses a greater excretion of carbon dioxide and 

 absorption of oxygen than an equal weight of oats. A protein 



