THE RENAL CIRCULATION 



I507 



which are consonant with the findings in the experi- 

 mental animal. Filtration rate rapidly declined at a 

 mean arterial pressure of 60 mm, and often ceased en- 

 tirely between 40 and 50 mm Hg. Estimated blood 

 flow was drastically reduced, and calculated renal re- 

 sistance was high. The renal fraction of the cardiac 

 output was much below the normal range, but the 

 criticism of Balint & Fekete (8) must be kept in mind. 

 Despite this, and its autonomy under or during 

 such circumstances, it is tempting to conclude that the 

 renal circulation in man is subordinate to the welfare 

 of the body as a whole. 



CONCLUDING REMARKS 



The kidney is an organ characterized by a high 

 volume of blood flow resulting in a narrow A-V oxy- 

 gen difference despite a high rate of oxygen utiliza- 

 tion. The A-V oxygen difference tends to remain 

 constant in the face of minor fluctuations in flow, but 

 at very low rates of flow, an increase in the A-V oxy- 



gen difference has been observed by several investi- 

 gators. The remarkable autonomy of the renal 

 circulation may be an adaptation to insure steady de- 

 livery of oxygen to the renal tissue. 



The constancy of flow appears to be desirable for 

 another reason. The countercurrent system for the 

 concentration and dilution of the urine operates 

 optimally with constant blood flow. When this has 

 been experimentally altered (e.g., increased flow 

 through the medullary circuits) the osmotic stratifica- 

 tion in the vasa recta and loop of Henle system has 

 been dissipated, and concentrating power impaired. 



Indications are that a countercurrent multiplier 

 system for concentration of serum albumin exists in 

 the vasa recta, a mechanism which would aid inter- 

 stitial fluid uptake and removal into the systemic 

 circulation. The interesting interrelationship of the 

 vasa recta system to the loop of Henle system in the 

 role of water and salt absorption merits much further 

 study, particularly in the direction of quantitative 

 measurement of regional flow (cortical versus medul- 

 lary), and the factors which alter it. 



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