46 Professor W. M. Bayliss [Feb. 18, 



WEEKLY EVENING MEETING, 



Friday, February 13, 1920. 



Sir James Reid, Bart., G.C.V.O. K.C.B. M.D. LL.D., 

 Vice-President, in the Chair. 



Professor W. M. Bayliss. M.A. D.Sc. F.R.S. 



The Volume of the Blood and its Significance. 



The system of vessels in which the blood is contained must be 

 conceived of as a closed system. But the walls are distensible and 

 elastic ; they can therefore stretch and collapse to accommodate 

 varying amounts of liquid. This is possible, however, only to a 

 limited extent. Although the veins have thinner walls than the 

 arteries, and appear to be less supported by surrounding structures 

 than are the capillaries, it is remarkable that they oppose a greater 

 resistance to a bursting pressure than do the arteries. Veins, 

 moreover, have a muscular coat which is in a more or less contracted 

 state during life. Hence the introduction of more fluid into the 

 system must encounter a certain resistance and raise the internal 

 pressure, unless the muscular coat actively relaxes to accommodate 

 the fluid introduced. 



This closed system contains, under normal conditions, about four 

 litres of blood in man. It consists, as is generally known, of the 

 heart, of branching tubes (arteries), leading from the heart to the 

 tissues, where they break up into a network of much finer tubes, the 

 capillaries, which unite again to form veins, and so lead the blood 

 back to the heart. Consider the distribution of the blood at the 

 time when the heart is at rest. The amount present in each part, 

 including the heart itself, is obviously in proportion to the capacity 

 of each part. 



The heart, however, works as a pump. The way in which the 

 blood is circulated was first clearly propounded by Harvey in 1616, 

 although Leonardo da Vinci came very near to the discovery more 

 than a century before. Harvey saw the blood sent out from the 

 heart, propelled to the tissues in the arteries, and returned to the 

 heart by the veins. The course of the blood from one to the other 

 through the minute capillaries could not be seen until the invention 

 of the microscope by Leeuwenhoek, who made use of it in 1686 to 

 observe the blood traversing the capillaries in the tail of the tadpole. 



The heart, then, when it contracts, drives out the blood which is 

 contained in its cavities, or nearlv the whole of it. This same 



