528 



ORGAN SYSTEMS OF MAN 



much water the blood pressure rises and 

 more fluids are ehminated as a conse- 

 quence. The amount of urine that is ex- 

 creted depends not only on the amount of 

 fluids taken into the body but also on the 

 amount of salt that is to be eliminated from 

 the blood. On a salty diet the urine output 

 is greater because more salts must be re- 

 moved from the blood if a normal balance 

 of these ions is to be maintained. Also it 

 requires more water to pass the high pro- 

 portion of solids that appears in the urine 

 after a heavy intake of salts. In the case of 

 diabetes there is too much sugar in the 

 capsular filtrate and this produces a high 

 osmotic pressure. For this reason, not 

 much water can be reabsorbed into the 

 blood and so a large urine output results. 



Urine volume is also controlled by a hor- 

 mone secreted by the posterior lobe of the 

 pituitary (see p. 449). This substance con- 

 trols the rate of water reabsorption in the 

 tubule. If it is insufficient or completely 

 lacking, a disease known as diabetes insipi- 

 dus results. People suffering from this dis- 

 ease may have a urine output of 30-40 liters 

 per day instead of 1.3-1.5 liters, which is 

 average. As might be expected, they also 

 suffer from an insatiable thirst. 



A normal kidney restores all of the utiliz- 

 able substances in the capsular filtrate to 

 the blood except in cases where the reten- 

 tion of such substances might be harmful. 

 However, urea and other substances are 

 highly concentrated as the capsular filtrate 

 flows down the tubule toward the pelvis of 

 the kidney. Once in the bladder it is known 

 as urine. Its concentration will vary, of 

 course, with the amount of water included 

 with it. This can be measured by deter- 

 mining its specific gravity, a routine proce- 

 dure in diagnosis. 



Like most organs of the body, the kid- 

 ney has a tremendous latitude within which 

 to operate, and can withstand considerable 

 abuse and still do its job satisfactorily. 

 Actually, only one-half of one kidney, or 

 one-fourth of the total kidney tissue, is 



necessary to handle the normal business of 

 living. 



HISTORY OF FOOD 

 IN THE BODY 



It might be well at this point to review 

 the entire story of the food pathway 

 through the body. This can best be done by 

 some such scheme as that shown in Fig. 

 20-4. 



The food taken into the digestive tract in 

 the form of large molecules is broken down 

 by enzymatic action into its absorbable end 

 products: amino acids, simple sugars such as 

 glucose, and fatty acids and glycerol. The 

 first two are taken directly into the blood 

 stream through the hepatic portal system, 

 whereas the last enter the blood via the 

 lymphatics. In the liver, under the influence 

 of hormones, glucose is either stored in the 

 form of glycogen (by the loss of water) or 

 is passed out to the tissues in the general 

 circulation. Amino acids either pass into 

 tlie general circulation where they function 

 in growth and repair or they become deam- 

 inized in the liver. Here the nitrogen- 

 containing fragment forms urea which is 

 eliminated through the kidneys, while the 

 carbon fragment follows the glucose path- 

 way. 



In the tissues, glucose oxidizes to carbon 

 dioxide and water, thus releasing the en- 

 ergy necessary for life. Tlie oxidation prod- 

 ucts are eliminated through the lungs. Some 

 of the amino acids are resynthesized into 

 the specific proteins of the body, whereas 

 others are broken down into their nitrogen 

 end products, namely, ammonia and even- 

 tually urea, which is eliminated through the 

 kidneys. 



The fatty acids and glycerol resynthesize 

 into fats the moment they pass from the gut 

 into the lymph channels. They then flow in 

 the blood to the tissues where they are 

 stored or where their stored-up energy is 

 released by oxidation, the resulting carbon 

 dioxide leaving the body through the lungs. 



