Physiological Pharmaceutics

86 Physiological PharmaceuticsFigure 5.10 The migrating myoelectric complexStarting from a state in which the stomach and small intestine show no motor activity(Phase I), the activity front begins simultaneously in the antrum and duodenum producinga series of mixing contractions which build up over a period of 60–90 minutes (Phase II).These end in a series of powerful circular peristaltic waves which sweep from the site oforigin down the entire small bowel to the caecum emptying all the large solid particles fromthe stomach and small intestine (Phase III). The waves then subside to the resting phase. Thewhole cycle repeats every two hours until a meal is eaten, when they are immediatelyinterrupted to initiate the digestive phase of motility. The peristaltic waves can also be haltedby an intravenous infusion of a hormone, motilin.Gastric emptying of the basal gastric secretion occurs even during fasting to preventaccumulation of fluid, as there is very little net absorption by the gastric mucosa.The fed stateThe adult capacity of the stomach is about 1500 ml. The average (western) daily intake offood and drink is 3 to 4 kg, and it is estimated that another 5 litres of fluids such as saliva,gastric juice, pancreatic juice and other body liquids are added to this. The greatest secretoryactivity occurs in the stomach within the first hour of eating and the volume of gastric juicesproduced may be up to twice that of the meal.The uppermost third of the stomach, or fundus, adapts to the varying volume ofingested food by relaxing its muscular wall, holding the food while it undergoes the firststages of digestion. The adaptive process allows it to accommodate the ingested foodwithout increasing intragastric pressure.The stomach not only elicits a fed pattern of motility in response to the presence offood with calorific value, but it will also respond to the presence of a large quantity of small