1905.] Blood Pressure in Man. /^^^ C>f[ / '^ 



WEEKLY EVENING MEETl-^;. ^^^ j'^Lj 

 Friday, Februuiy 3, 1905. ^\!^r^^^'^^^ 



Sir Williazvi Crookes, D.Sc. F.R.S., Honorary Secretary and 

 Vice-President, in the Chair. 



Professor T. Clifford Allbutt, M.A. M.D. LL.D. D.Sc. F.R.S. 



Elood Pressure in 3Ian. 



The lecturer began by contrasting Galen's conception of the oscillation 

 of the blood, about the liver as a centre, with the cardiac circulation 

 of Harvey. The pulmonary circulation — for the purposes of this 

 lecture — was omitted, and attention directed exclusively to that in the 

 systemic arteries. 



The physical characters of the flow of fluids were briefly described 

 by the example of water in an open stream. A stream might well up 

 from a spring in a flat country, and swim with very low pressure to 

 its mouth ; or, falling from a mountain, might have pressure enough 

 to carry men and horses off their legs. If the volume were also great, 

 as in the sea, it might exercise a pressure of many tons to the square 

 yard, and smash great bulwarks to pieces. But in the higher animals 

 the blood flows in closed channels, so that in such a scheme as theirs 

 the dimensions of the channels assume a very important value. More- 

 over, in mammaha the circulating fluid is not water, but a thicker 

 fluid — the blood — which (in man) has at least four times the viscosity 

 of water. The enormous value of friction in the circulation was then 

 considered, and it was shown that in this factor the kind of vessel 

 wall does not signify much, as the waU is Hned by a practically 

 stationary layer of the fluid ; friction, therefore, which uses up -^-^ of 

 the heart's power, depends on the factor of viscosity together with 

 that of the dimension of the channels, or closed bed. It may be said 

 that the blood pressures — that is, the arterial pressures — in man 

 depend on viscosity and dimension of stream bed. 



Now so far the closed tubes had been regarded as rigid. But if 

 in animals the tubes were rigid, the circulation would be carried on 

 under great difficulties. For instance, there would be no accommoda- 

 tion ; only so much blood could be driven into the system as issued at 

 the periphery ; the stream, too, would be quite intermittent, with very 

 high maximum and very low minimum pressures, which would not 

 serve for continuous nutrition, and by its extremes of pressures would 

 soon wear down the arteries. For instance, in the bagpipes, were it 

 not for the air reservoir the sound would issue in spasmodic screams ; 



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