McKenna's Pharmacology for Nursing, 2e

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

T he gastrointestinal (GI) system is the only system in the body that is open to the external environment. It begins at the mouth and ends at the anus. The GI system is responsible for only a very small part of waste excre- tion. The kidneys and lungs are responsible for excreting most of the waste products of normal metabolism. STRUCTURE AND FUNCTION OF THE GASTROINTESTINAL SYSTEM The GI system is composed of one continuous tube that begins at the mouth, progresses through the oesopha- gus, stomach and small and large intestines and ends at the anus. The pancreas, liver and gallbladder are acces- sory organs that support the functions of the GI system (see Figure 56.1). Structures The tube that comprises the GI tract is continuous with the external environment, opening at the mouth and again at the anus. Because of this, the GI tract contains many foreign agents and bacteria that are not found in the rest of the body. The tube begins in the mouth, which has salivary glands that secrete digestive

enzymes and lubricants to facilitate swallowing. The mouth leads to the oesophagus, which connects to the stomach. The stomach is responsible for mechanical and chemical breakdown of foods into usable nutrients. The stomach empties into the small intestine, where absorption of nutrients occurs. The pancreas deposits digestive enzymes and sodium bicarbonate into the beginning of the small intestine to neutralise the acid from the stomach and to further facilitate digestion. The liver produces bile, which is stored in the gallbladder. The bile is very important in the digestion of fats and is deposited into the small intestine when the gallbladder is stimulated to contract by the presence of fats. All of the nutrients absorbed from the small intestine pass into the liver, which is responsible for processing, storing or clearing them from the system. The small intestine leads to the large intestine, which is responsible for excret- ing any waste products that are in the gastrointestinal system. The excretion occurs through the rectum and is an activity that one learns to control. The peritoneum lines the abdominal wall and also the viscera, with a small “free space” between the two layers. It helps to keep the GI tract in place and prevents a build-up of friction with movement. The greater and lesser omenta hang from the stomach over the lower GI tract and are full of lymph nodes, lymphocytes, monocytes and

Tongue

Parotid gland

Salivary glands

Pharynx Epiglottis

Oesophagus (peristalsis)

Liver

Stomach (churning) Duodenum

Gall bladder

Pancreas

Large intestine (mass movement)

Small intestine (segmentation)

Rectum

FIGURE 56.1  The gastrointestinal tract.