Porth's Essentials of Pathophysiology, 4e

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Gastrointestinal and Hepatobiliary Function

U N I T 8

compartment, excessive secretion or impaired absorp- tion can lead to extracellular fluid deficit. Salivary Secretions Saliva is secreted by the salivary glands. The salivary glands consist of the parotid, submaxillary, sublingual, and buccal glands. Saliva has three functions. The first is protection and lubrication. Saliva is rich in mucus, which protects the oral mucosa and coats the food as it passes through the mouth, pharynx, and esophagus. The sublingual and buc- cal glands produce only mucus-type secretions. The second function of saliva is its protective antimicrobial action. The saliva cleans the mouth and contains the enzyme lysozyme, which has an antibacterial action. Third, saliva contains ptyalin and amylase, which initiate the digestion of dietary starches. Secretions from the salivary glands are primarily regulated by the ANS. Parasympathetic stimulation increases flow and sympathetic stimulation decreases flow of saliva. The dry mouth that accompanies anxiety attests to the effects of sympathetic activity on salivary secretions. Gastric Secretions The otherwise smooth lining of the stomach is dot- ted with gastric pits that form the openings for two types of glands: pyloric and oxyntic or gastric glands (Fig. 28-7). The pyloric glands secrete gastrin and mucus for protection of the pyloric mucosa, and are located in the antral portion, or distal 20%, of the stomach. The oxyntic glands are located on the inside surfaces of the body and fundus of the stomach, which occupies about 80% of the stomach. They contain three types of cells: mucous neck cells, which secrete mainly mucus; peptic (chief cells), which secrete large quantities of pepsinogen; and parietal (oxyntic cells), which secrete HCl and intrin- sic factor, which is needed for vitamin B 12 absorption (Fig. 28-7). The pepsinogen that is secreted by the peptic cells is rapidly converted to pepsin (a protein-digesting enzyme) when exposed to the low pH of the gastric juices. A few stem cells are also found in the gastric pits. These cells are the parent cells for all new cells of the gastric mucosa. The cellular mechanism for HCl secretion by the pari- etal cells in the stomach involves the hydrogen ion (H + )/ potassium ion (K + )-adenosine triphosphatase (ATPase) transporter and chloride ion (Cl – ) channels located on their luminal membrane (Fig. 28-8). During the process of HCl secretion, carbon dioxide (CO 2 ) produced by aer- obic metabolism combines with water (H 2 O), catalyzed by the enzyme carbonic anhydrase, to form carbonic acid (H 2 CO 3 ), which readily dissociates into H + and bicarbon- ate (HCO 3 – ). The HCO 3 – moves out of the cell and into blood from the basolateral membrane. At the luminal side of the membrane, H + is secreted into the stomach by the H + /K + -ATPase transporter (also known as the proton pump ). Chloride follows H + into the stomach by diffus- ing through Cl – channels in the luminal membrane. Three substances stimulate HCl secretion by the pari- etal cells: acetylcholine, gastrin, and histamine. Although each substance binds to different receptors on the pari- etal cell and has a different mechanism of action, they all serve to stimulate an increase in H + secretion through the H + /K + -ATPase transporter. Acetylcholine is released

SUMMARY CONCEPTS (continued)

secretions, and absorptive functions of the different segments of the GI tract.The activity of the neurons in the enteric nervous system is regulated by both local influences, such as stretch and distention of the GI tract wall, and input from the autonomic nervous system. ■■ Autonomic signals to the gut are carried from the brain and spinal cord by the parasympathetic and sympathetic nervous systems, with parasympathetic stimulation increasing the activity of the enteric nervous system and sympathetic stimulation inhibiting its activity.

Secretory Functions of the Gastrointestinal Tract

The GI tract produces a number of secretions, including mucus, hydrochloric acid, digestive enzymes, and hor- mones. Secretory activity, like motility, is influenced by local, humoral, and neural influences. Neural control of GI secretory activity is mediated through the ANS, with parasympathetic stimulation increasing secretory activ- ity and sympathetic stimulation inhibiting secretory activity. Many local influences, including the pH and osmolarity of the GI contents, act as stimuli for neural and hormonal mechanisms. Gastrointestinal Secretions Throughout the GI tract, secretory glands serve two basic functions: production of mucus to lubricate and protect the mucosal layer of the GI tract wall and secretion of fluids and enzymes to aid in the digestion and absorption of nutrients. Each day, approximately 7000 mL of fluid is secreted into the GI tract (Table 28-1). Approximately 50 to 200 mL of this fluid leaves the body in the stool; the remainder is reabsorbed in the small and large intestines. These secretions are mainly water and have sodium and potassium concentrations similar to those of extracel- lular fluid. Because water and electrolytes for digestive tract secretions are derived from the extracellular fluid

TABLE 28-1 Secretions of the

Gastrointestinal Tract

Secretions

Amount Daily (mL)

Salivary Gastric

1200 2000 1200

Pancreatic

Biliary

700

Intestinal

2000 7100

Total