McKenna's Pharmacology for Nursing, 2e

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P A R T 1 1  Drugs acting on the gastrointestinal system

vast surface area to increase the absorption. The small intestine maintains a BER of 11 contractions per minute. This regular movement is assessed when listening for bowel sounds. The large intestine uses a process of mass movement with an occasional peristaltic wave. When the beginning segment of the large intestine is stimulated, it contracts and sends a massive peristaltic movement throughout the entire large intestine. The end result of the mass movement is usually excretion of waste products. Rectal distension after mass movement stimulates a defecation reflex that causes relaxation of the external and internal sphincters. Control of the external sphinc- ter is a learned behaviour. The receptors in the external sphincter adapt relatively quickly and will stretch and require more and more distension to stimulate the reflex if the reflex is ignored. ■■ The GI system begins at the mouth and ends at the anus; a long tube extends between them and comprises the oesophagus, the stomach, the small intestine and the large intestine. Essential functions are digestion and absorption of nutrients. ■■ The GI system secretes enzymes, acid, bicarbonate and mucus to facilitate the digestion and absorption of nutrients. ■■ The small intestine is the organ where most absorption occurs. The veins of the small intestine carry the absorbed products to the liver for filtering, cleaning and metabolism, or the breaking down of maintaining electrical rhythm and responding to local stimuli (increasing or decreasing activity). The autonomic nervous system influences GI activity, with the sympathetic system slowing and the parasympathetic system increasing activity. GASTROINTESTINAL REFLEXES To function effectively, several local and central reflexes occur. Local reflexes involve stimulation of the nerves in the GI tract and cause movement and secretion. Central reflexes, which include swallowing and vomiting, are controlled by the medulla. Local reflexes Stimulation of local nerves within the GI tract causes increased or decreased movement within the system, maintaining homeostasis. Loss of reflexes or stimulation can result in constipation and the lack of movement of the bolus along the GI tract or diarrhoea with increased KEY POINTS absorbed products into usable substances. ■■ The nerve plexus controls the GI system by

motility and excretion. The longer a faecal bolus remains in the large intestine, the more sodium and water are absorbed from it and the harder and less mobile it can become. There are many local gastrointestinal reflexes . Some knowledge of how these reflexes operate makes it easier to understand what happens when the reflexes are blocked or overstimulated and how therapeutic measures are often used to cause reflex activity. • Gastroenteric reflex : Stimulation of the stomach by stretching, the presence of food or cephalic stimulation (the body’s response to smelling, seeing, tasting or thinking about food) causes an increase in activity in the small intestine. It is thought that this prepares the small intestine for the coming chyme. • Gastrocolic reflex : Stimulation of the stomach also causes increased activity in the colon, again preparing it to empty any contents to provide space for the new chyme. • Duodenal–colic reflex : The presence of food or stretching in the duodenum stimulates colon activity and mass movement, again to empty the colon for the new chyme. It is important to remember the gastroenteric, gastro­ colic and duodenal reflexes when helping people to maintain GI movement. Taking advantage of stomach stimulation (e.g. having the person drink prune juice or hot water or eat bran) and providing the opportunity of time and privacy for a bowel movement encourage normal reflexes to keep things in control. Other local GI reflexes include the following: • Ileogastric reflex : The introduction of chyme or stretch to the large intestine slows stomach activity, as does the introduction of chyme into the small and large intestine, allowing time for absorption. In part, this reflex explains why people who are constipated often have no appetite: The continued stretch on the ileum that comes with constipation continues to slow stomach activity and makes the introduction of new food into the stomach undesirable. • Intestinal–intestinal reflex : Excessive irritation to one section of the small intestine causes a cessation of activity above that section to prevent further irritation, and an increase in activity below that section, which leads to a flushing of the irritant. This reflex is active in “Montezuma’s revenge” (traveller’s diarrhoea): Local irritation of the intestine causes increased secretions and movement below that section, resulting in watery diarrhoea and a cessation of movement above that section. Loss of appetite or even nausea may occur. An extreme reaction to this reflex can be seen after abdominal surgery, when the handling of the intestines causes intense irritation and the reflex can cause the entire intestinal system to cease activity, leading to a paralytic ileus.