578 



Prof. T. G. Brodie. 



I do not wish to lay too great a stress upon the actual pressure-head thus 

 obtained, for the possible errors in the measurements are many. It is, for 

 instance, impossible to obtain anything but an approximation to the lengths 

 of the successive portions of the tubule, and also the measurements of their 

 lumina can only be approximate, for they are undoubtedly altered during 

 fixation. Also I have supposed all the tubules to have equal lumina, and 

 have neglected to take into account those tubules which were at rest. To 

 obtain the total pressure within Bowman's capsule a factor for the velocity 

 head should be added to the pressure-head already calculated, but it is so 

 small that we may omit it. (The mean velocity within the narrowest portion 

 of the tubule amounts to about 1 mm. per second.) 



The important point is that during an active diuresis a pressure-head of 

 the order of 80 mm Hg. may be needed within Bowman's capsule to drive the 

 fluid secreted there down the tubule. 



The mean aortic blood-pressure in the first experiment was 120 mm. Hg, 

 and in the second 115 mm. If we allow 30-35 mm. Hg as the loss of 

 pressure-head between the aorta and the glomerular capillaries when the 

 afferent glomerular vessels are dilated, the blood-pressure within the capillary 

 loops would amount to 90-85 mm. Hg in the first experiment, and 85-80 mm. 

 in the second. Hence, on these figures, practically the whole of the blood 

 pressure-head is required to set up a pressure-head in the fluid within the 

 capsule sufficient to drive the secreted fluid down the tubule. Bearing in 

 mind that the estimates given are only approximate, I conclude that the 

 pressure-head within Bowman's capsule only differs from the pressure -head 

 within the glomerular loops by the pressure required to stretch the walls of 

 the loops. This latter probably does not amount to more than one or two 

 millimetres of mercury. 



If in the light of these arguments we criticise once more the assumptions 

 made by Ludwig's theory, we see that that theory becomes less tenable than 

 ever. In the first place, when the kidney is secreting water at its fastest 

 rate, the pressure difference available for filtration is reduced to a minimum 

 At lower rates of secretion, of course, a pressure difference might be available. 

 In the second place, the assumption must be made that the volume of water 

 discharged from the glomerulus is from 30 to 70 times greater than the 

 volume of water entering the pelvis of the kidney. Hence a very much 

 greater pressure-head would be required to drive that fluid down the tubule, 

 though not 30 to 70 times greater than the pressure required to drive a 

 volume equal to that of the discharged urine, since the fluid has to be driven 

 only as far as the absorbing surface. But as the absorbing surface would 

 have to be taken as extending at least to the end of the ascending limb of 



