'77'' 



II VNDBOOK Ol I'HNsH il i H,Y 



Ml KcH'HYSIOLOGY III 



then losses of chloride from the cerebrospinal fluid to 

 l he blood in the meninges would be rapidly compen- 

 sated hv diffusion from the nervous tissue into the 

 cerebrospinal fluid ' 



Rates (if Flow oj Cerebrospinal Fluid 

 ami Aqueous Humm 



The pathological conditions of hydrocephalus and 

 glaucoma indicate unequivocally that the fluids are 

 formed continuously, .1 serious interference with the 

 drainage routes leading inevitably to a rise in the fluid 

 pressure. The question now arises as to the assessment 

 of the rale of renewal, or turnover, of the fluids. This 

 is remarkably difficult in the ocular system since with- 

 drawal of fluid from the eye results, as we have seen, 

 in a breakdown of the blood-aqueous humor barrier. 

 I he fluid reformed under these conditions is abnor- 

 mal, having stronger affinities with an exudate of 

 plasma than with true aqueous humor. Consequently, 

 a mere measurement of the rate of reformation of fluid 

 after withdrawal would provide a most unsound 

 measure of the normal rate of flow. Probably the most 

 accurate measurements are those of Barany & Kinsey 

 (ig) in the rabbit and of Goldmann (110) in man. 

 Space will not permit a detailed description of the 

 theoretical basis of their computations. Essentially 

 they measured the rate at which certain substances 

 passed out of the aqueous humor; the substances 

 chosen were such thai a direct loss by diffusion into 

 the iris and ciliary body was unlikely, the substances 

 leaving almost exclusively by flow into the canal of 

 Schlemm. Results in the rabbit and man nave turn- 

 over rales between 1.4 and 1.2 per cent per min., 

 respectively. Kinsey & Bar.im (133) showed that the 



" I lie extent i" which the central nervous parenchyma ma) 

 determine the concentration of a given ion or nonelectrol) te in 

 lli. 1 .■! ■ In 11sp1n.1l thud has not been seriously considered so far. 

 There is some indirect evidence suggesting that this factor 

 Cannot be ignored I Ims the concentration of chloride in the 

 ventricular lluid is not ure.itlv, if at all, dillerent from that in 

 the lumbar fluid (43; 59, |> 210). If the extracellular fluid "t the 

 parenchyma were simply a filtrate ol plasma, the concentration 



of Mil le in it would be less than that in the cerebrospinal 



fluid, and diffusion from the latter would be expected as the 

 fluid flowed through the subarachnoid space; in other winds, 

 the siiliai.ii hnoid fluid should have a lowei concentration than 

 the wiiti m nl. 11 I In fact that no dilierence has so far been 

 demonstrated would indicate that the chloride concentrations 

 in the two fluids cerebrospinal and extracellular were 

 determined independently, in th< cerebrospinal thud hv thi 



1 11 activity of tip 'l I plexuses and in the extracellulai 



1 In h I l.\ il" activity "t the ne ns or neuroglial cells 



rale at which the isotope, Xa'-' 1 , entered the aqueous 

 humor from the blood was apparently equal to the 

 rate of turnover of the fluid as a whole, i.e. that one 

 may assume that most of the sodium entering the eye 

 from the blood comes in the primary secretion from 

 the ciliarv body, direct exchanges between the iris 

 and the anterior chamber being small by comparison. 

 If the same finding applies to other species, the rates 

 of renewal of fluid in various mammals are as given in 

 table 5. In the rat, the rate of renewal is remarkably 

 high. 17 



In the cerebrospinal system the loss of fluid docs not 

 result in a serious breakdown of the barrier so that 

 more direct measurements of rate of flow are probably 

 feasible. Such studies (83, 86, 91, 116, 152, 184) indi- 

 cate, in general, a rate of flow of the order of 0.2 to 

 0.5 per cent of the total volume per min., i.e. consid- 

 erably less rapid than that of the aqueous humor. 

 From measurements of the rate of clearance of sub- 

 stances injected into the cerebrospinal fluid, for ex- 

 ample those of Dandy & Blackfan (56) with phcnol- 

 sulphonephthalein, one may calculate a turnover rate 

 for the dog of the same order, namely 0.3 per cent per 

 min. (59) 



Mi, hanism oj Dunnage 



In the ocular system, the factors determining the 



speed of drainage will clearly be the difference in 

 pressure between the fluid in the anterior chamber and 

 the blood in the anterior ciliary veins into which the 

 collectors from the canal empty the so-called outflow 

 pressure and the factional resistance to flow across 

 the wall of the canal and along the channels leading 

 to the large veins. Increasing the intraocular pressure, 

 for example liv pressure on the globe, increases the 

 rale of flow, as observed in aqueous veins (107, 108) 

 or by less direct methods (112). Under normal condi- 

 tions the intraocular pressure is about 20 mm Hg, 

 while the pressure in the episcleral veins is of the order 

 of 10 to 14 mm Hg (207) and in .1 laminated aqueous 



17 This high rate of turnover may be related to the relatively 

 low concentration ol bicarbonate in rat aqueous humor. In 

 such a small eye the problem of neutralizing the lactic a< id 

 formed 1 >% the bulky lens must be acute, and it would seem to be 



achieved liv a very rapid renewal of aqueous hiiiniii (62, 63 



Othei studies on rates "I renewal of aqueous humor are those 

 nl Moses \ Bruno 1 1 h 1 1, Grant (l 12), Beck, ji. Barany 



(16-181 and Ross (189, 190) Recent studies on the kinetics ol 



penetration of \,e' into the aqueous humor suggest that values 

 ol the rate of Mow derived from these measurements are low 

 (97. '34) 



