Physiological Pharmaceutics

The stomach 87particle size indigestible material, suggesting that gastric distention is also a contributoryfactor 13 . Indigestible material of small diameter (1–3 mm) will elicit the fed pattern of motoractivity in dogs, and the duration of postprandial antroduodenal motility patterns can beinfluenced solely by the size of a meal which comprises only of indigestible material.The mixing, grinding and emptying of food all occur together. The dispersion of foodis more a mechanical process than an enzymatic one. Mixing and grinding are carried outby a series of peristaltic waves which originate in mid-body as a shallow indentation andgradually deepen as they progress toward the duodenum. The velocity of the wave increasesuntil the final 3 to 4 cm of the antrum are reached, at which stage the antrum and pylorusappear to contract simultaneously. This is often called antral systole. Liquids and solidswithin the distal antrum are compressed as the antral wave deepens. The wave does notocclude the lumen and hence liquid and suspended particles are retropelled through thewave, but larger and denser solids are trapped ahead of the constriction. Once the pylorushas closed, the antral systole grinds and then retropels the solids into the proximal antrum(Figure 5.11). The grinding action, combined with the shear forces produced duringretropulsion, reduce the particle size and mixes the particles with gastric juice. The motionand acid-pepsin digestion accounts for the physical breakdown of solid food in this region.The rate of emptying from the human stomach decreases if the ingested solid food iscomposed of larger masses. Solids are only emptied after they are ground to particles smallerthan approximately 1 mm, and the larger, harder particles will take longer to reach this size.Tonic or sustained motor activity decreases the size of the lumen of the stomach, asall parts of the gastric wall seem to contract simultaneously. It is this type of activity thataccounts for the stomach’s ability to accommodate itself to varying volumes of gastriccontent. Mixing contractions and peristaltic contractions are superimposed upon the toniccontraction, which is independent of the other contractions. Both the mixing and theperistaltic contractions occur at a constant rate of three per minute when recorded from thegastric antrum. This rate is now recognised as the basic rhythm, although some drugs arecapable of abolishing both types of contractions or of stimulating the strength ofcontractions. The distension of the body of the stomach by food activates a neural reflexthat initiates the activity of the muscle of the antrum.About twice per minute between 1 and 5 ml of antral contents escape into theduodenum, and decrease the duodenal pH. Emptying from the pylorus occurs in discreteepisodes of 2 to 5 seconds only, and the majority of these occur as the terminal antrum,Figure 5.11 Grinding and mixing action of the antral mill Note retropulsion of largeparticles i.e. sieving