320 



DIGESTION 



lymph vessels. These breakdown products of foods enter the general 

 circulation from the smaller vessels and are taken up by the various 

 organs of the body. Amino acids and monosaccharides pass into the 

 capillaries and from there into the portal blood. Some authorities claim 

 that small quantities are transported into the lymph. Over 90 per cent 

 of the fatty acids of fat absorbed in the intestines of the rat have been 

 demonstrated to be transferred to intestinal lymph. It is doubtful if the 

 fatty acids of fats can be transported directly into the portal blood. 



Calcium and iron are absorbed mainly from the upper part of the 

 small intestines. The intestinal content has a profound effect on the 

 extent of calcium absorption. Soluble calcium salts such as the gluconate, 

 lactate, and chloride are readily available, but the phosphate is not. 

 Cereals reduce calcium absorption since they contain phytic acid (inositol 

 hexaphosphoric acid), which binds calcium; spinach has a similar effect 

 because of its oxalic acid content. As has been mentioned previously, 

 vitamin D enhances calcium absorption (p. 211). Iron absorption is 

 notoriously inefficient. For example, if a normal child is fed 5 mg. of iron, 

 only about 12 per cent of the iron is absorbed. However, the greater the 

 need of the body for iron, the greater is tlie increase in the uptake of 

 this element. It has been shown that in cases of iron-deficiency anemia, 

 absorption increases many times over normal. Most of the other inor- 

 ganic salts and the bulk of the water are absorbed in the colon. 



As a result of water removal in Ihe colon, the materials there acquire 

 a semisolid consistency. A large number of bacteria act in the colon 

 on materials which have been passed there from the small intestines or 

 which are secreted by the walls of the colon. It has been estimated that 

 10-90 per cent of the weight of the feces is derived from bacteria. The 

 characteristic odor of feces is due mainly to indole and skatole. These 

 are products of degradation, presumably of tryptophan. Part of the 



NH2 

 I 

 CH.— CH— COOH 



Tryptophan 



indole and skatole are reabsorbed into the blood and are mainly con- 

 verted into the sulfate esters (indoxyl sulfate and skatoxyl sulfate) 



