Physiological Pharmaceutics

The stomach 89Figure 5.13 MRI cross section through the body. The large organ at the top is thestomach, which clearly shows the liquid layer (white) floating on the solid (grey) matterduodenal bulb rises above than that in the antrum, and the pylorus prevents reflux byclosing. The vagus nerve has an important role in the control of emptying, but studies onparaplegia caused by injury to the spinal cord indicate that the sympathetic division of theautonomic nervous system is also involved.Effect of meal size and composition on gastric emptyingThe empty stomach has a volume of approximately 50 ml which increases to over 1 litrewhen full. The stomach empties the three different components of the meal, liquid,digestible solid and indigestible solid, at different rates. For example, in a study of theemtying of a mixed meal consisting of soft drink, scrambled egg and radioopaque markers,the separate components had T 50’s (the time or half the component to be emptied) of 30±7minutes, 154±11 minutes and 3 to 4 hours respectively 19 .In general, the larger the amount of food ingested, the longer the period of fed activity;large meals tend to empty more slowly in the first hour and then more quickly comparedto a small meal. Gastric emptying rates correlate with the nutritive density of the meal;foodstuffs slow gastric emptying equally when their concentration is expressed askilocalories per milliliter. It is now believed that two types of receptors exist which controlthe rate at which the energy density is delivered to the duodenum. Two sets of duodenalreceptors are involved, one stimulated by the osmotic properties of the digestion productsof carbohydrate and protein, and one by the digestion products of fat. Energy is not actuallysensed, but the two sets of receptors behave in tandem to control the delivery of the chymeto the duodenum by energy density.The emptying of amino acids appears to be solely dependent upon their osmolarity,except for L-tryptophan which delays gastric emptying in concentrations which can beobtained from normal protein digestion 20 . Fatty acids, monoglycerides and diglycerides all