McKenna's Pharmacology for Nursing, 2e

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

stomach, this movement is known as retching, and it can be quite tiring and uncomfortable. This action causes a backward peristalsis and movement of stomach contents up the oesophagus and out the mouth. The body thus rids itself of offending irritants. The vomiting reflex is complex and protective, but it can be undesirable in certain clinical situations, when the stimulant is not something that can be vomited or when the various components of the vomiting reflex could be detrimental to a person’s health status. ■■ Swallowing, a centrally mediated reflex important in delivering food to the GI tract for processing, is controlled by the medulla. It involves a complex series of timed reflexes. ■■ Vomiting is controlled by the chemoreceptor trigger zone (CTZ) in the medulla or by the emetic zone in immature or injured brains. The CTZ is stimulated by several different processes and initiates a complex series of responses that first prepare the system for vomiting and then cause a strong backward peristalsis to rid the stomach of its contents. CHAPTER SUMMARY ■■ The gastrointestinal (GI) system is composed of one long tube that starts at the mouth, includes the oesophagus, the stomach, the small intestine and the large intestine, and ends at the anus. The GI system is responsible for digestion and absorption of nutrients. ■■ Secretion of digestive enzymes, acid, bicarbonate and mucus facilitates the digestion and absorption of nutrients. ■■ The GI system is controlled by a nerve plexus, which maintains a basic electrical rhythm and responds to local stimuli to increase or decrease activity. The sympathetic nervous system, if stimulated, slows GI activity; stimulation of the parasympathetic nervous system increases activity. Initiation of activity depends on local reflexes. ■■ A series of local reflexes within the GI tract helps to maintain homeostasis within the system. Overstimulation of any of these reflexes can result in constipation (underactivity) or diarrhoea (overactivity). KEY POINTS

■■ Swallowing, a centrally mediated reflex important in delivering food to the GI tract for processing, is controlled by the medulla. It involves a complex series of timed reflexes. ■■ Vomiting is controlled by the chemoreceptor trigger zone (CTZ) in the medulla or by the emetic zone in immature or injured brains. The CTZ is stimulated by several different processes and initiates a complex series of responses that first prepare the system for vomiting and then cause a strong backward peristalsis to rid the stomach of its contents.

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ONLINE RESOURCES

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BIBLIOGRAPHY

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