1062 PHYSIOLOGY 



carbon dioxide. It must be remembered that 40 mm. carbon dioxide repre- 

 sents approximately the normal carbon dioxide tension in the blood. It is 

 true that at 150 mm. oxygen pressure the blood is practically saturated 

 with oxygen whatever (within physiological limits) the pressure of the carbon 

 dioxide. At lower pressures of oxygen, however, the pressure of carbon 

 dioxide makes a considerable difference. Thus at an oxygen pressure of 

 20 mm. Hg. the amount of oxyhsemoglobin formed is 67-5 per cent, at a 

 carbon dioxide pressure of 5 mm., whereas at a pressure of carbon dioxide 

 of 40 mm. the amount of oxyhaemoglobin is only 29-5 per cent. In con- 



FIG. 498. Dissociation curve of sheep's blood. 



1, normal blood ; 2, blood containing 0-04 per cent, added lactic acid ; 

 3, blood containing 0-08 per cent, added lactic acid. 



sequence of this fact, in the tissues, where the carbon dioxide tension is high, 

 the oxyhsemoglobin will be dissociated with greater ease, so that oxygen 

 will be set free where it is most wanted. 



We are now in a position to understand how the oxygen is taken up 

 by the blood as it circulates round the pulmonary alveoli. Arterial blood, 

 such as that which fills the pulmonary veins and the systemic arteries, is 

 very nearly (i.e. about 90 per cent.) saturated with oxygen, and will only 

 take up about 2 volumes per cent, more on shaking it with air at the body 

 temperature. Venous blood requires 8 to 10 volumes per cent, of oxygen to 

 saturate it ; but we have already mentioned that, at a tension of 30 mm. 

 oxygen, the blood becomes nearly saturated. The tension of oxygen in 

 the alveoli is considerably above this. In the trachea the tension of oxygen 

 is about % of an atmosphere (since the air here contains 16 volumes per 

 cent.), and the tension in the alveoli will be a little lower than this. 

 If we take the oxygen tension in the alveoli at i of an atmosphere,* it will 

 still be something over 100 mm. Hence the venous blood brought to the 

 alveoli by the pulmonary artery will, on there coming into intimate contact 



* The oxygen tension in the alveoli has been reckoned at about 12-6 per cent, to 

 13 '5 per cent, of an atmosphere. 



