394 MANUAL OF PHYSIOLOGY. 



In the kidney, then, we have three sets of capillary vessels, 

 which differ in their position, the form of their meshes, and their 

 relation to their parent artery. Probably the pressure exerted 

 by the blood in them, and the rapidity of its flow through them, 

 differ also : 



1. The capillaries in the glomeruli are loops collected into a 

 tuft by their covering of delicate epithelium. On account of 

 their relation to the afferent artery which ends abruptly in these 

 capillaries, and to the smaller efferent vessel that leads to a sec- 

 ondary plexus of capillaries, the pressure within the glomerulus 

 must be very great compared with that of the general capillaries 

 of the body, and must vary much with changes in local blood- 

 pressure. 



2. The secondary capillary plexus, with its narrow meshwork 

 closely investing the tubules, can only be under comparatively 

 trifling pressure which varies but little, on account of the blood 

 having first to pass through the capillaries of the glomerulus. 

 Their current of blood must also move slowly, since the bed of 

 the stream is here very great. 



3. The straight vessels, with long-meshed capillaries, in the 

 pyramids between the looped and straight tubules are unlike the 

 two preceding. In these straight vessels the blood probably flows 

 with greater velocity than in those around the convoluted tubes ; 

 and their blood pressure is less than that in the glomeruli, but 

 greater than that in the intertubular capillaries. 



THE URINE. 



When freshly voided the urine of man in health is a clear, 

 straw-colored fluid, with a peculiar, aromatic odor. The intensity 

 of the color varies with the amount of solids the color being a 

 rough indication of the degree of concentration. On standing 

 and cooling, a slight cloud of mucus often appears floating in the 

 fluid. This comes from the lining membrane of the bladder, and 

 it usually entangles a few flattened epithelial cells, which are the 

 only organized structural elements found in it in health. 



The fresh urine has a distinctly acid reaction. This does not 

 depend upon the presence of free acid, as is suggested by the fact 



