Chap. 14 



THE BY-PRODUCTS OF METABOLISM EXCRETION 



249 



Ca'pillaries joining 



artery and vein 



Renal artery 

 Renal vein \ 



Ureter 



Single 

 t'glomerulua 



Artei-y 



-Vein 



^ Collecting- 

 tubules 



■yi Papillary 

 ' ' duct 



Fig. 14.5. Finer structure of a mammalian kidney. Left, Cut surface showing 

 the veins and the arteries (black) with their many branches in the cortex and the 

 collecting tubes that converge and open through the calyx. Right, One functional 

 unit of a kidney showing a glomerulus such as those represented by dots in the 

 cortex in Figure 14.4. The afferent artery to the glomerulus is actually larger 

 than the efferent one. (Courtesy, Clendening: The Human Body. New York, 

 Alfred A. Knopf, Inc., 1930.) 



tubules, then come together to make minute veins that empty into the arcuate 

 vein. This and its branches finally connect with the renal veins that open into 

 the vena cava which carries blood directly to the heart. 



Units of the Kidney and Their Work. Each tubule with its accompanying 

 capillaries is one of about one million working units in each human kidney 

 (Fig. 14.6). In outlining the function of the kidney the parts named in the 

 preceding paragraphs will be mentioned again. 



Each unit begins near the outer surface of the kidney where it holds the 

 glomerulus in a double-walled cup, the renal capsule. Both walls are very 

 thin and slightly separated by a space that is continued into the kidney 

 tubule. The plasma of the blood in the glomerulus is continually under pres- 

 sure by the drive from the heart, plus additional pressure due to the fact 

 that the afferent artery through which blood flows into the glomerulus is larger 

 than the efferent one through which it flows out. Except for proteins and other 

 large molecules the contents of the renal capsule are continuously filtered 

 through its thin inner wall into the cavity that leads into the tubule. Reabsorp- 

 tion occurs farther on where capillaries surround the many loops of the tubule. 



