L— PHYSIOLOGY. 1G1 



should at all times be furnished with the right amount of blood to satisfy 

 their needs, however much these needs may vary. The general circulation 

 rate and the calibre of the blood vessels must be accurately adjusted lest 

 in satisfying the needs- of some organs others should be starved, and until 

 we can express this adjustment in quantitative terms we cannot hope to 

 assess correctly the relative importance to be attributed to physico- 

 chemical, nervous or hormonal factors in ensuring the necessary co- 

 ordination. We devote enough attention in all conscience to the measure- 

 ment of blood pressure, and no doubt this has materially aided us to 

 appreciate the cruder and more elementary phenomena associated with 

 the circulation, but when we look at the question from the point of view 

 of the tissues and are brought face to face with the true functional aspect 

 of the circulation, blood pressure as such becomes a more or less irrelevant 

 detail. The failure lies in the fact that we cannot construe our measurements 

 into what is of true functional significance, namely, changes in the rate 

 of blood flow. We cannot of course expect an adequate supply of blood 

 without an adequate driving force or blood pressure, but the requisite 

 force at any moment must depend on the precise setting of the calibre of 

 the blood vessels, and in regard to this we must confess that we have 

 still got a vast amount to learn. The adaptation and accommodation 

 of the circulation to meet the requirements of the body is the real essential 

 that we have to study, and it is not until we have gained an insight into 

 the quantitative changes of the local and general circulation, and the 

 factors on which these depend, in the normal and functionally intact 

 animal, that we shall be able to claim to understand the circulation of 

 the blood. 



We meet with exactly the same difficulties in the case of the other 

 functions of the body when we try to translate potentialities into actuali- 

 ties. Take the case of the kidney, for example. Controversy has raged 

 for years as to whether the cells in the different regions of the kidney are 

 to be regarded as playing an active or a passive role in the formation of 

 urine. Observations on the anaesthetized animal or the isolated kidney 

 are slowly solving some of the difficulties, but even when we have reached 

 agreement on the vexed question as to the degree to which the properties 

 of filtration, active re-absorption and specific secretion can be ascribed 

 to the kidney epithelium we are only at the beginning of our troubles, 

 for we have still to ascertain how these different potentialities may be 

 brought into play to account for the normal behaviour of the kidney. 

 Observations on man have already done much to show the surprising 

 delicacy with which the kidney responds to alterations in the composition 

 of the blood, and to throw suspicion on the justice of conclusions based on 

 experimental alterations of so gross a character that they could have no 

 counterpart in normal life. The kidney is not acting as a simple drain ; 

 it is playing a perfectly definite part in helping to maintain the com- 

 position of the blood normal, and experiments on human subjects have 

 thrown into strong relief the interdependence of the kidney and other 

 organs in maintaining this normality. 



The physiological regulation of the hydrogen ion concentration of the 

 blood and tissues, a problem to which so much attention has been directed 

 in recent years, affords an instance of the interaction of the organs in 



1927 M 



