THE PATHWAYS FOR LIPID ABSORPTION 117 



of the fat into the intestine to a rate which will prevent flooding the small 

 intestine with a difficultly absorbable foodstuff. 



c. Small Intestine. The small intestine is the main site of absorption of 

 fats and other lipids. As soon as the fats pass into the intestine, the 

 chyme comes in contact with the pancreatic juice, which contains the 

 lipolytic enzyme, steapsin. It is also almost immediately mixed with the 

 bile; emulsification of the fat promptly occurs, and the action of the steap- 

 sin in bringing about hydrolysis of the fat is greatly accelerated. Whether 

 the fat must be broken down completely to fatty acids before absorption 

 can occur is a moot question which will be discussed later (see page 137). 



Little absorption occurs in the upper part of the small intestine. After 

 the admixture of pancreatic juice, bile, and intestinal juice in the duodenum, 

 the process of digestion continues rapidly. Within three to four hours, 

 the liquid mass has reached the lower part of the ileum, and absorption is 

 proceeding with great rapidity. By the time the foodstuff has reached the 

 end of the ileum, almost all of the ingested fat has been removed from the 

 chyme. After five to eight hours, the intestinal contents have passed into 

 the cecum. The splenic flexure fills as early as the tenth hour, although 

 in some cases this is delayed to the twelfth to fifteenth hour. The remnants 

 of the food reach the rectum by the fourteenth to eighteenth hour. It is 

 therefore evident that within ten hours all ingested foodstuffs have 

 passed out of the small intestine. 



d. Large Intestine. Practically all absorption of fat has occurred before 

 the intestinal contents pass through the ileocecal valve into the large intes- 

 tine. The chyme at this point contains almost no absorbable material, 

 and it has a constant water content. In man, the intestinal residues re- 

 main in the large intestine for from twenty-six to thirty-eight hours. Dur- 

 ing this interval, w T ater especially is absorbed; the water content is de- 

 creased to about one-third of that which obtained when the chyme was 

 poured into the large intestine. The material at this stage has assumed a 

 fecal character. 



{2) Anatomical Features of the Small Intestine 

 of Importance in Absorption 



a. The Surface Structure of the Small Intestine. The small intestine is 

 •especially adapted to absorption because of its large surface area. In 

 man, it has a length of about 27 feet, and the total surface area is estimated 

 at about 8 to 10 square meters. The length and area of the small intestine 

 ■of the lower animals is of a magnitude proportionate to those in man. 



