346 RESPIRATION. 



substances present have a deleterious action, for an atmosphere containing 

 simply 1 per cent, of carbonic acid (with a corresponding diminution of 

 oxygen) has very little effect on the animal economy, whereas an atmos- 

 phere in which the carbonic acid has been raised to 1 per cent, by breath- 

 ing is highly injurious. In fact, air rendered so far impure by breathing 

 that the carbonic acid amounts to 0.08 per cent, is distinctly unwholesome, 

 not so much on account of the carbonic acid as of the accompanying impuri- 

 ties. Since these impurities are of unknown nature and cannot be estimated, 

 the easily determined carbonic acid is usually taken as an indirect measure 

 of their presence. We have seen that the average man loads at each breath 

 500 c.c. of air with carbonic acid to the extent of 4 per cent. He will 

 accordingly at each breath load 2 litres to the extent of 1 per cent. ; and 

 in one hour, if he breathe 17 times a minute, will load rather more than 

 2000 litres to the same extent. At the very least, then, a man ought to be 

 supplied with this quantity of air hourly, and if the air is to be kept fairly 

 wholesome, that is with the carbonic acid reduced below 0.01 per cent., he 

 should have even more than ten times as much. 



THE RESPIRATORY CHANGES IN THE BLOOD. 



285. While the air in passing in and out of the lungs is thus robbed 

 of a portion of its oxygen and loaded with a certain quantity of carbonic 

 acid, the blood as it streams along the pulmonary capillaries undergoes 

 important correlative changes. As it leaves the right ventricle it is venous 

 blood of a dark purple or maroon color ; when it falls into the left auricle 

 it is arterial blood of a bright scarlet hue. In passing through the capil- 

 laries of the body from the left to the right side of the heart it is again 

 changed from the arterial to the venous condition. We have to inquire, 

 What are the essential differences between arterial and venous blood, by 

 what means is the venous blood changed into arterial in the lungs, and the 

 arterial into venous in the rest of the body, and what relations do these 

 changes in the blood bear to the changes in the air which we have already 

 studied ? 



The facts that venous blood at once becomes arterial in appearance on 

 being exposed to or shaken up with air or oxygen, and that arterial blood 

 becomes venous in appearance when kept for some little time in a closed 

 vessel, or when submitted to a current of some indifferent gas such as 

 nitrogen or hydrogen, prepare us for the statement that the fundamental 

 difference between venous and arterial blood is in the relative proportion 

 of the oxygen and carbonic acid gases contained in each. From both 

 a certain quantity of gas can be extracted by means which do not other- 

 wise materially alter the constitution of the blood ; and this gas when 

 obtained from arterial blood is found to contain more oxygen and less 

 carbonic acid than that obtained from venous blood. This is the real 

 differential character in the two bloods; all other differences are either, 

 as we shall see to be the case with the color, dependent on this, or are 

 unimportant and fluctuating. 



If the quantity of gas which can be extracted by the mercurial air-pump 

 from 100 volumes of blood be measured at C. and a pressure of 760 mm., 

 it is found to amount in round numbers to 60 volumes. 



The vacuum produced by the ordinary mechanical air-pump is insufficient to 

 extract all the gas from blood. Hence it becomes necessary to use a mercury pump 

 capable of producing a large Torricellian vacuum. In the form of mercurial pump 

 which bears Ludwig's name (Fig. 92), two large globes of glass, one fixed and the 

 other movable, are connected by a flexible tube : the fixed globe is made to com- 



