McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 5 6 Introduction to the gastrointestinal system

cells to release hydrochloric acid and the chief cells to release pepsin. Parasympathetic stimulation also leads to acid release. Gastrin and the parasympathetic system stimulate histamine-2 (H 2 ) receptors near the parietal cells, causing the cells to release hydrochloric acid into the lumen of the stomach. Proteins, calcium, alcohol and caffeine in the stomach increase gastrin secretion. High levels of acid decrease the secretion of gastrin. Other digestive enzymes are released appropriately, in response to proteins and carbohydrates, to begin digestion. Peptic ulcers can develop when there is a decrease in the pro- tective mucosal layer or an increase in acid production. Digestive: Digestion of carbohydrates As the now-acidic bolus leaves the stomach and enters the small intestine, secretin is released, which stimulates the pancreas to secrete large amounts of sodium bicarbonate (to neutralise the acid bolus), the pancreatic enzymes chymotrypsin and trypsin (to break down proteins to smaller amino acids), other lipases (to break down fat) and amylases (to break down sugars). These enzymes are delivered to the GI tract through the common bile duct, which is shared with the gallbladder. Digestive: Metabolism of amino acids If fat is present in the bolus, the gallbladder contracts and releases bile into the small intestine. Bile contains a detergent-like substance that breaks apart fat molecules so that they can be processed and absorbed. The bile in the gallbladder is produced by the liver during normal metabolism. Once delivered to the gallbladder for storage, it is concentrated; water is removed by the walls of the gallbladder. Some people are prone to developing gallstones in the gallbladder when the concentrated bile crystallises. These stones can move down the duct and cause severe pain or even blockage of the bile duct. In response to the presence of food, the small and large intestines may secrete various endocrine hormones, including growth hormone, aldosterone and glucagon. They also secrete large amounts of mucus to facilitate the movement of the bolus through the rest of the GI tract. Digestion Digestion is the process of breaking food into usable, absorbable nutrients. Digestion begins in the mouth, with the enzymes in the saliva starting the process of breaking down sugars and proteins. The stomach con- tinues the digestion process with muscular churning, breaking down some foodstuffs while mixing them thor- oughly with hydrochloric acid and enzymes. The acid and enzymes further break down sugars and proteins into building blocks and separate vitamins, electrolytes, minerals and other nutrients from ingested food for

absorption. The beginning of the small intestine intro- duces bile to the food bolus, which is now called chyme . Bile breaks down fat molecules for processing and absorption into the bloodstream. Digestion is finished at this point, and absorption of the nutrients begins. Digestive: General digestion Absorption Absorption is the active process of removing water, nutrients and other elements from the GI tract and deliv- ering them to the bloodstream for use by the body. The portal system drains all of the lower GI tract, where absorption occurs, and delivers what is absorbed into the venous system directly to the liver. The liver filters, clears and further processes most of what is absorbed before it is delivered to the body (see Figure 56.1). Some absorption occurs in the lower end of the stomach, most commonly absorption of water and alcohol. The majority of absorption occurs in the small intestine. It about 8500 mL/day, including nutrients, drugs and anything that is taken into the GI tract, as well as any secretions. The small intestine mucosal layer is specially designed to facilitate this absorption, with long villi on the epithelial layer providing a vast surface area for absorption. The large intestine absorbs approximately 350 mL/day, mostly sodium and water. Motility The GI tract depends on an inherent motility to keep things moving through the system. The nerve plexus maintains a basic electrical rhythm (BER), much like the pacemaker rhythm in the heart. The cells within the plexus are somewhat unstable and leak electrolytes, leading to the regular firing of an action potential. This rhythm maintains the tone of the GI tract muscles and can be affected by local or autonomic stimuli to increase or decrease the rate of firing. The basic movement seen in the oesophagus is peri- stalsis , a constant wave of contraction that moves from the top to the bottom of the oesophagus. The act of swallowing , a response to a food bolus in the back of the throat, stimulates the peristaltic movement that directs the food bolus into the stomach. The stomach uses its three muscle layers to produce a churning action. This action mixes the digestive enzymes and acid with the food to increase digestion. A contraction of the lower end of the stomach sends the chyme into the small intestine. The small intestine uses a process of segmentation with an occasional peristaltic wave to clear the segment. Segmentation involves contraction of one segment of small intestine while the next segment is relaxed. The contracted segment then relaxes, and the relaxed segment contracts. This action exposes the chime to a