Chap. 14 THE BY-PRODUCTS OF METABOLISM EXCRETION 251 



(Fig. 14.6). About one cubic centimeter per minute accounts for the three 

 pints or thereabout of urine usually secreted from the kidneys per day. After 

 reabsorption of the water, the concentration of urea and nitrogenous sub- 

 stances in the urine may be over a hundred times greater than in the blood 

 plasma. Up to a certain amount, the cells in the walls of the tubules are evi- 

 dently capable of taking up glucose and other useful constituents from the 

 excreted fluid in the tubule and passing them back to the blood. The blood 

 plasma and the filtrate in the tubule are always coming to a balance with one 

 another in their content of water, sugar, salts, and urea. 



Water-saving insects, reptiles, and birds dispose of their nitrogenous waste 

 as semisolid uric acid. The kidneys of all land vertebrates take back by reab- 

 sorption much of the contents of the filtrate, the watery urine that is first made 

 according to the ancient ancestral pattern and then brought up to the modern 

 pattern. This is a roundabout way; it is also a physiological reminiscence. 



Ureters and Bladder 



Function. Urine is propelled through the ureters by peristaltic contractions 

 and enters the bladder in jets at the rate of one to five per minute. As the blad- 

 der becomes distended it presses against the oblique openings of the ureters, 

 preventing backflow into them. It also sets up afferent nerve impulses to the 

 spinal cord. These in turn set up impulses from the cord which stimulate 

 rhythmic contraction of muscles in the bladder, and eventually cause relaxa- 

 tion of the sphincter valve at its opening into the urethra. In very young ani- 

 mals this action is involuntary, but later it becomes a habit formed by volun- 

 tary behavior. 



Conditions and Diseases Affecting the Work of the Kidneys 



Nephritis. Various kinds of inflammation of the kidney tubules are called 

 nephritis. Although the term is used commonly it gains real meaning with the 

 knowledge that the nephric tubule is the essential working unit of all kidneys. 

 The type of nephritis commonly known as Bright's disease was described by 

 Richard Bright (1789-1858), a British physician, one of the great modern 

 pathologists. He did not theorize or experiment but did the observing upon 

 which theory and experiment are based. He was the first to connect with the 

 kidney the symptoms of a disease known since the time of Hippocrates. Bright 

 enjoyed life, his work, his travels, and the sketches that he made to illustrate 

 the accounts of them. 



Floating Kidney. A floating kidney is due to a shift in the position of the 

 kidney either posteriorly, or tilted away from the dorsal wall. The kidneys of 

 fishes, reptiles, and birds fit snugly along each side of the backbone; those of 

 amphibians and mammals are attached loosely beneath the peritoneum. 



