Physiological Pharmaceutics

120 Physiological PharmaceuticsFigure 6.4 Metabolism and transport of fat into the lymphatic and systemic circulationcholesterol esters in chyme. Fat is emulsified by the bile salts into small droplets whichdisperse in water allowing access of digestive enzymes.Lipase in the pancreatic juice and enteric lipase from the epithelial cells of the smallintestine both hydrolyse emulsified triglycerides to monoglycerides and fatty acids. Theshort and medium chain fatty acids are absorbed passively through the epithelium into theblood. The long chain fatty acids and monoglycerides remain as mixed micelles with the bilesalts and are internalized by the epithelium. They are reassembled into triglycerides withinthe cell and excreted into the lymph as small (0.1 µm) droplets called chylomicra (orchylomicrons) (Figure 6.4).IronHeme iron is absorbed from meat more efficiently than dietary inorganic iron and in adifferent manner 8 . Thus, iron deficiency is less frequent in countries where meat constitutesa significant part of the diet. Proteolytic digestion of myoglobin and hemoglobin results inthe release of heme, which is maintained in a soluble form by globin degradation productsso that it remains available for absorption. Heme enters the small intestinal absorptive cellas an intact metalloporphyrin. This may be facilitated by a vesicular transport system 9 . Inthe absorptive cell the porphyrin ring is split by heme oxygenase. The released inorganic ironbecomes associated with mobilferrin and paraferritin, which acts as a ferrireductase to makeiron available for production of iron-containing end products such as heme proteins.Mucosal transfer of iron into the body occurs competitively with dietary iron that enters theabsorptive cell as inorganic iron, because they both share a common pathway within theintestinal cell. Dietary inorganic iron as the ferric iron is solubilized at the acid pH level ofthe stomach where it chelates mucins and certain dietary constituents to keep them solubleand available for absorption in the more alkaline duodenum.PATTERNS OF MOTILITY IN THE SMALL INTESTINEThe small intestine, like the stomach, displays two distinct patterns of motility. The fedpattern is characterized by random motor activity, in groups of 1 to 3 sequentialcontractions, separated by 5 to 40 seconds of inactivity. The physical and chemical nature