THE UROGENITAL SYSTEM — EXCRETION AND REPRODUCTION 



563 



Materials leave the blood in the glomeruli as they do in other 

 capillary beds, but the arrangement ot the blood vessels is such that a 

 large volume of material is forced out. An afferent arteriole leads from 

 a branch of the renal artery to each glomerulus, and an efferent 

 arteriole from the glomerular capillaries to a second capillary network 

 distributed over the rest of the tubule. These capillaries are drained 

 by branches of the renal vein. A glomerulus thus lies between two 

 arterioles. The efferent arteriole is smaller than the afferent one; this 

 insures a high blood pressiue in the glomerular capillaries, and hence 

 a high filtration pressure to drive fluids from the blood. The filtration 

 pressure in a glomerulus is normally about twice as great as that at the 

 arterial end of an ordinary capillary bed. It has been estimated that 

 some 184 liters of glomerular filtrate are normally produced by a man 

 in one day! 



Tubular Reabsorption and Augmentation. Glomerular filtration 

 is not a selecti\e process. Glucose, amino acids, inorganic ions and 

 many other usefid materials leave the blood along with urea, other 

 wastes, and enough water to dehydrate a terrestrial vertebrate in a few 

 hours. Fortunately the glomerular filtrate tmdergoes further treatment 

 as it passes down the tubule. Virtually all of the glucose and amino 

 acids and most of the water and inorganic ions are reabsorbed, entering 

 the capillaries around the tubule. The various parts of the tubule 

 reabsorb different materials. In mammals, most of the glucose, amino 

 acids and water reenter the blood from the proximal convoluted tu- 



Pronephros 

 (deo e ne-r all ng) 



Oviduct 

 MesonephroS ' 



Wolffian duct 

 Metanephros 



Ostium 



Rcte cords 



Indifferent 

 onad. 



Renal pelvis 

 Ureter 



Urinary 

 bladder 



Ventral 

 pa-rt of 

 cloeLca 



Figure 28.3. A ventral view of the urogenital organs of the sexually indifferent 

 stage of the embryo. 



