'47° 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



100- 

 jrmol/ml 



20 



13 

 32 



umol/ml 

 too - 



Urin 

 osmol ZVKonz. 

 ^mol/ml 

 62 



112 

 356 

 212 



fig. 15. Na and CI concentration in kidney tissue (Mmole/ml of tissue fluid,) in hydropenic (solid 

 lines) and diuretic idashed lines) dogs. To the right are the osmotic pressures, and Na and CI concen- 

 trations of bladder urine taken shortly before the tissue analysis. [(After Ullrich & Jarausch (312).] 



125.5 

 106 



creased concentration here, also raising that in the 

 interstitium now favors further movement of fluid out 

 of the descending limb, further increasing concen- 

 tration, and so on. 



In this fashion, an increasing osmotic gradient is 

 established in the direction of the tip of the papilla, 

 and yet at no level is there much osmotic difference 

 among the luminal fluid, interstitial fluid, and blood. 

 The collecting ducts in the presence of pituitary 

 antidiuretic hormone (ADH) are believed to be water 

 permeable and somewhat Na-impermeable (net 

 transport small, although there may be diffusion 

 into and active transport out). This results in diffusion 

 of water out of the collecting ducts into the hyper- 

 osmotic medullary interstitium, and ultimately into 

 the vasa recta to be carried away until fluid in the 

 collecting ducts becomes correspondingly concen- 

 trated. The role of the vasa recta will be dealt with in 

 greater detail below. 



The view is currently favored that ADH <icts in a 

 permissive fashion to let water diffuse out of the 

 nephron, perhaps altering the size of the "pores" 

 in the base of the cells of the tubular epithelium. 

 Figures 18 and 19 illustrate the operation of the 

 countercurrent system in the concentrating and 

 diluting kidney. In the concentrating kidney (fig. 18) 

 ADH restores the hypotonic urine in the distal con- 

 voluted tubule to isotonicity by permissive (passive) 

 loss of water and then permits its concentration to 

 hypertonicity in the collecting duct. In the diluting 



kidney, the distal convoluted tubule and collecting 

 duct remain impermeable to water, and the urine 

 remains hypotonic. 



Mechanisms relating to salt and water handling in 

 the proximal convoluted tubule, though highly 

 important in a bulk sense, will not be dealt with here 

 (see 279). Sodium is probably actively reabsorbed 

 here, with water following passively. 



RENAL BLOOD VOLUME; 



THE INTRARENAL HEMATOCRIT 



The renal blood volume has been estimated in 

 three ways: /) by reconstructions of the vascular bed 

 and from these estimation of the volume; 2) by the 

 product of mean transit time of erythrocytes and 

 plasma and the mean volume flow; and 3) by an 

 injection of labeled erythrocytes (Cr 51 , P 32 ) and 

 labeled albumin (T-1824, I 131 ) with subsequent 

 analysis of distribution in selected, homogenized 

 tissue samples. Measurement of hemoglobin content 

 has also served to assess erythrocyte volume. 



By reconstruction techniques, Weaver el al. (332) 

 computed the following distribution: 4.5 ml per 100 

 g of kidney in arteries, 4.1 ml per 100 g in cortical 

 veins, and 2.6 ml per 100 g in subcortical veins, with 

 about 2 ml per 100 g undetermined, distributed 

 somehow among glomerular capillaries, efferent 

 arterioles, peritubular capillaries, and medullary 



