RESPIRATION 613 



previously (page 472). Further work is required on the question, especially in 

 connection with the great sensibility of the higher centres to deprivation of oxygen, 

 although it has been stated that the actual consumption of oxygen is not great. 



The Blood Itself. We have already seen that the nucleated blood corpuscles 

 have a fairly considerable oxidation metabolism. Morawitz (1909) showed that the 

 blood of rabbits made anaemic by the injection of phenyl-hydrazine has also a fairly 

 considerable metabolism, and that this is due to the young non-nucleated red cells 

 which are present in such conditions in considerable numbers. In contrast with 

 this, the actual metabolism in the normal blood is extraordinarily small. 



Technique. For the methods used in the various experiments referred to 

 in the preceding paragraphs, the original papers must be consulted. There is a 

 possible criticism to be brought against these methods, in which the rate of the 

 blood flow is measured by the time taken to fill a certain volume of a graduated 

 pipette inserted into the vein. This value is obviously of great importance in 

 the determination of the oxygen consumed in a given time. When vascular 

 dilatation occurs, as is usual in an active organ, the time taken to fill the tube 

 is very short, and is only a small part of the total duration of an observation, 

 so that the assumption must be made, that the rate of flow and consumption of 

 oxygen continues to be the same as that during the small sample of the total 

 effect of a stimulation which is actually measured. For this reason, it seems 

 desirable that further observations should be made, in which the whole blood 

 passing through an organ in a considerable time should be collected, and its 

 oxygen and carbon dioxide contents compared with that of the arterial blood 

 entering. This criticism is not intended to cast doubt on the results given above, 

 but it seems to me that it may be quite easy to overestimate the oxygen 

 consumption when vaso-dilatation occurs, since the measurement only applies 

 to so short a period. The application of this consideration to the question of 

 the nature of vaso-dilatation will be clear later, when we have to discuss the 

 regulation of the blood supply in Chapter XXIII. 



It is evident that the amount of oxygen required by active organs is far larger 

 than the blood could carry merely in the ordinary state of solution in liquids. 

 We have, therefore, to consider in the next place the extraordinary substance, 

 haemoglobin, contained in the red blood corpuscles, by whose agency oxygen in 

 adequate amount is conveyed to the tissues. As far as difficulty of understanding 

 is concerned, this mechanism, as we shall see, is similar to that of its near 

 relative, chlorophyll. 



HAEMOGLOBIN 



Although it is this substance which is contained in the red blood corpuscles 

 of the vertebrates, and is responsible for the taking up of oxygen, and the giving 

 it off again when required, it is not to be supposed that there are no other 

 similar substances. In fact, in the blood of molluscs and Crustacea there is 

 a pigment, haemocyanin, which serves the same purpose. This pigment contains 

 copper, whereas, as we shall see, haemoglobin contains iron. As yet we know com- 

 paratively little about haemocyanin, especially with regard to its relation to oxygen. 

 It is a matter which would well repay investigation to determine whether it has 

 the remarkable properties which haemoglobin has in this respect, properties which 

 are at present unique. The work of Alsberg and Clark (1914), to be given pre- 

 sently, indicates that haemocyanin has not the peculiar properties of haemoglobin. 



Let us proceed to examine these properties. 



Haemoglobin, as is well known, is a compound of a protein with a complex 

 acid substance, containing iron and pyrrol derivatives, as we have seen (page 560). 

 We will leave, for the present, further remarks as to its chemical constitution, 

 merely stating what is necessary for the immediate question. This is, that 

 it exists in two forms, oxyhaemoglobin, which is regarded as a compound of 

 the other form, haemoglobin, or " reduced haemoglobin," with oxygen. This oxygen 

 can be removed by exposure to a vacuum, so that it is stated to be " loosely 

 combined," and haemoglobin remains. 



