McKenna's Pharmacology for Nursing, 2e

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

other components of the mononuclear phagocyte system. This barrier provides rapid protection for the rest of the body if any of the bacteria or other foreign agents in the GI tract should be absorbed into the body. Layers of the GI tract The GI tube is composed of four layers: the mucosa, the muscularis mucosa, the nerve plexus (a network of nerve fibres running through the wall of the GI tract that allows local reflexes and control) and the adventitia. Mucosal layer The mucosal layer provides the inner lining of the GI tract. It can be seen in the mouth and is fairly consistent throughout the tube. It is important to remember when assessing a person that if the mouth is very dry or full of lesions, that is a reflection of the state of the entire GI tract and may indicate that the person has difficulty digesting or absorbing nutrients. This layer has an epi- thelial component and a connective tissue component. Muscularis mucosa layer The muscularis mucosa layer is made up of muscles. Most of the GI tract has two muscle layers. One layer runs circularly around the tube, helping keep the tube open and squeezing the tube to aid digestion and motility. The other layer runs horizontally, which helps propel the gastrointestinal contents down the tract. The stomach has a third layer of muscle, which runs obliquely and gives the stomach the ability to move contents in a The nerve plexus has two layers of nerves—one submu- cosal layer and one myenteric layer. These nerves allow the GI tract local control over movement, secretions and digestion. The nerves respond to local stimuli and act on the contents of the GI tract accordingly. The GI tract is also innervated by the sympathetic and parasympa- thetic nervous systems. These systems can slow down or speed up the activity in the GI tract but cannot initiate local activity. The sympathetic system is stimulated during times of stress (“fight-or-flight” response) when digestion is not a priority. To slow the GI tract, the sym- pathetic system decreases muscle tone, secretions and contractions, and increases sphincter tone. By shutting down the GI activity, the body saves energy for other activities. In contrast, the parasympathetic system (“rest- and-digest” response) stimulates the GI tract, increasing muscle tone, secretions and contractions and decreasing sphincter tone, allowing easy movement. Adventitia layer The adventitia is the outer layer of the GI tract. It serves as a supportive layer and helps the tube maintain its shape and position (see Figure 56.2). churning motion. Nerve plexus layer

Nerve plexus

Lumen of the GI tract

Mucosal layer

Adventitia

Circular muscularis mucosa

Longitudinal muscularis mucosa

FIGURE 56.2  Layers of the gastrointestinal tract.

Gastrointestinal activities The GI system has four major activities:

• Secretion of enzymes, acid, bicarbonate and mucus • Absorption of water and almost all of the essential nutrients needed by the body • Digestion of food into usable and absorbable components • Motility (movement) of food and secretions through the system (what is not used is excreted in the form of faeces) These functions are discussed in detail in the following sections. Secretion The GI tract secretes various compounds to aid the movement of the food bolus through the GI tube, to protect the inner layer of the GI tract from injury and to facilitate the digestion and absorption of nutri- ents (see Figure 56.1). Secretions begin in the mouth. Saliva , which contains water and digestive enzymes, is secreted from the salivary glands to begin the digestive process and to facilitate swallowing by making the bolus slippery. Mucus is also produced in the mouth to protect the epithelial lining and to aid in swallowing. The oesopha- gus produces mucus to protect the inner lining of the GI tract and to further facilitate the movement of the bolus down the tube. The stomach produces acid and digestive enzymes. In addition, it generates a large amount of mucus to protect the stomach lining from the acid and the enzymes. In the stomach, secretion begins with what is called the cephalic phase of digestion. The sight, smell or taste of food stimulates the stomach to begin secreting before any food reaches the stomach. Once the bolus of food arrives at the stomach, gastrin is secreted. Gastrin stimulates the stomach muscles to contract, the parietal