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18. 1 Introduction




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^ 18 The gut and nutrition




Fig. 18.8 A summary of the major factors influencing the secretion of saliva.

posteriorly into the oropharynx and then into the laryngophar-ynx, both of which are common passageways for food, fluids, and air.

The first (oral) phase of swallowing is voluntary but the subse­quent pharyngeal and esophageal stages of the process are under reflex autonomic (involuntary) control. During the oral phase of swallowing, the tip of the tongue is placed against the hard palate and the tongue is then contracted to force the food bolus into the oropharynx (the part of the pharynx lying immediately behind the mouth). As food enters the pharynx and stimulates mechanorecep-tors there, the involuntary phase of swallowing begins.

Contraction of the superior constrictor muscle raises the soft palate towards the posterior pharyngeal wall to prevent food entering the nasopharynx. It also initiates a wave of peristaltic contraction which propels the bolus through the relaxed upper esophageal sphincter into the esophagus. The larynx rises so that the epiglottis covers the opening of the nasopharynx into the trachea while the opening of the esophagus is stretched. At the same time, respiration is inhibited {deglutition apned). In this way food is prevented from entering the trachea.

In the final esophageal phase of contraction, the wave of peristaltic contraction which was initiated in the pharynx continues along the length of the esophagus. The wave lasts for 7—10 seconds and is usually sufficient to propel the bolus to the stomach. If it fails to do so, the resulting distension of the esophagus initiates a vago-vagal reflex which triggers a secondary peristaltic wave. The esophageal submucosa contains

glands that secrete mucus in response to pressure from a food bolus. This mucus helps to lubricate the esophagus and facilitates the transport of food.

At the junction between the esophagus and the stomach there is a slight thickening of the circular smooth muscle of the GI tract called the lower esophageal sphincter (also called the cardiac or gastro-esophageal sphincter). This acts as a valve and remains closed when food is not being swallowed, preventing regurgitation of food, gastric juices, and air.

Just before the peristaltic wave (and the food) reaches the end of the esophagus, the lower esophageal sphincter relaxes to permit the entry of the bolus into the stomach. This normally takes around 1—2 seconds after swallowing. The time taken for food to travel down rhe esophagus depends on the consistency of the food and the position of the body. Liquids take only 1—2 seconds to reach the cardiac sphincter but solid foods can take much longer. The process is normally aided by gravity.

The pharyngeal and esophageal phases of deglutition are controlled by neurons located in the medulla and lower pons. Afferent information from mechanoreceptors located around the pharynx is carried to the brain by afferents in the glosso-pharyngeal nerve. When food stimulates these receptors a complex sequence of events is initiated. Motor impulses travel from the brain to the muscles of the pharynx and upper eso­phagus via cranial nerves, including the vagus. Lesions of the swallowing area of the brain or of the glossopharyngeal and vagal nerves carrying the efferent impulses result in difficulty in swallowing (dysphagia). The major events of the swallowing reflex are illustrated in Fig. 18.9.

Summary

  1. In the mouth, food is mixed with saliva as it is chewed. Three pairs of
    salivary glands (parotid, submaxillary, and sublingual) secrete about
    1500 ml of saliva each day. This contains mucus, which helps to
    lubricate the food, and an a-amylase, which initiates the breakdown
    of carbohydrate.

  2. Isotonic fluid is formed by the acinar cells of the salivary glands by
    the secretion of electrolytes and water. This undergoes modification
    as it flows along the salivary ducts so that the final composition of
    saliva depends upon its flow rate.

  3. Salivary secretion is controlled by reflexes mediated by the autonomic

nervous system. Parasympathetic stimulation promotes an abundant secretion of watery saliva rich in amylase and mucus. Blood flow to the salivary glands is also enhanced. Sympathetic stimulation pro­motes the output of amylase but reduces blood flow to the glands. The overall effect of sympathetic stimulation is generally a reduction in the rate of salivary secretion.

4. Swallowing occurs in three phases. The first, oral phase is voluntary
but subsequent phases are under reflex autonomic control. As swal­
lowing occurs, a wave of peristalsis is initiated which propels the food
bolus through the upper esophageal sphincter into the esophagus.
This wave of contraction continues for several seconds and moves rhe
bolus down to the lower esophageal sphincter which relaxes to permit
the entry of food into the stomach.

18.4 The stomach 401



Fig. 18.9 The events occurring during swallowing as a food bolus moves from the mouth to the esophagus.

^ 18.4 The stomach

Below the esophagus the GI tract expands to form the stomach, which lies in the left side of the abdominal cavity. The functions of the stomach include:

  1. the temporary storage of food;

  2. chemical digestion of proteins to polypeptides by pepsins;

  3. mechanical digestion by movements of the stomach;

  4. regulation of the passage of chyme into the small
    intestine; and

  5. the secretion of intrinsic factor which is essential for the
    absorption of vitamin B12.

The stomach is continuous with the esophagus at the lower esophageal (cardiac) sphincter and with the duodenum at the pyloric sphincter. Its position in relation to other abdominal organs is shown in Fig. 18.1. It is atound 25 cm long, and roughly J-shaped, although its size and shape varies between individuals and with its degree of fullness. When the stomach is empty it has a volume of about 50 ml and its mucosa and sub-

mucosa are thrown into large longitudinal folds called rugae. When fully distended the stomach can hold up to 4 liters.

The major regions of the stomach are illustrated in Fig. 18.10. The cardiac region (cardia) surrounds the cardiac orifice through which food enters the stomach. The part of the stomach which extends above the cardiac orifice is called the fundus while the mid portion is called the body. This is continu­ous with the funnel-shaped pyloric region. The upper, widest portion of the pyloric region is the antrum which narrows to form the pyloric canal, terminating in the pylorus itself. The convex portion of the stomach is called the greater curvature while the concave region is the lesser curvature.

The fundus and body of the stomach act as a temporary reservoir for food. They can accommodate large increases in volume without appreciable changes in intragastric pressure because the smooth muscle in these areas relaxes in response to the presence of food. This reflex is mediated at least in part by afferent and efferent fibers running in the vagus nerve causing an inhibition of muscle tone. Contractile activity in the fundus and body is relatively weak so that food remains here largely undisturbed for fairly long periods. By contrast, vigorous con­tractions take place in the antral region where food is broken down into smaller particles and mixed with gastric juice to form chyme, the semiliquid form in which it is passed to the duodenum.

Blood supply and innervation of the stomach

Arterial blood is supplied to the stomach by the gastric arteries, branches of the celiac artery which form a plexus of vessels within the submucosa. From here, the vessels branch extensively to provide the mucosal layer with a rich vascular network. Venous drainage is via the gastric veins which empty into the portal vein which conveys blood to the livet. Blood flow to the gastric mucosa increases significantly when the stomach is stim­ulated to secrete gastric juice in response to a meal.

Esophagus

Lesser curvature.

Fundus

Internal folds—rugae

; Body

Pyloric sphincter

Greater curvature

Duodenum'

Pyloric antrum

Fig. 18.10 A longitudinal view to show the major anatomic regions and rugae of the stomach.

402

18 The gut and nutrition


The stomach is richly innervated with both intrinsic and extrinsic nerves. Cell bodies of the intrinsic neurons lie within the plexuses, particularly the myenteric plexus. Their axons synapse with other intrinsic nerves and with extrinsic fibers and supply the smooth muscle and gastric secretory cells. The extrin­sic innervation includes sympathetic fibers from the celiac plexus and parasympathetic vagal neurons. A number of afferent fibers also innervate the stomach. These are sensitive to a variety of stimuli, including distension and pain. Many afferent fibers leave the GI tract via vagal and sympathetic nerves. Others form the afferent arms of intrinsic reflex arcs.

Structure of the gastric mucosa

The basic organization of the stomach wall resembles that shown in Fig. 18.2. In addition to the longitudinal and circular muscle layers, there is, on the anterior and posterior sides of the stomach, an additional layer of muscle between the mucous membrane and the circular layer. The smooth muscle cells in this layer are obliquely orientated and may play a role in the grinding and churning movements displayed by the stomach.

The gastric mucosa contains a variety of secretory cells. The surface epithelium of the gastric mucosa is a simple columnar epithelium composed almost entirely of secretory cells which produce a protective alkaline fluid containing mucus. This epithelial layer is dotted with millions of deep gastric pits. These are depressions into which the secretions of gastric glands empty. There are around 100 pits per square millimeter of the mucosa, occupying about 50 per cent of its total surface.

The gastric glands contain several types of cells, the exact nature of which differs according to the region of the stomach. A diagrammatic representation of a gastric gland, showing the locations of the various glandular cell types is shown in Fig. 18.11.

The gastric glands contain four types of cells:

  1. Mucous neck cells, which are situated at the opening of the
    gastric glands. These cells secrete a mucus distinct from
    that secreted by the surface epithelial cells. Its special
    significance is unclear.

  2. Chief cells, which are located in the basal regions of the
    gastric glands. These cells secrete pepsinogen, the inactive
    form of the proteolytic enzyme pepsin.

  3. Parietal or oxyntic cells ate scattered amongst the chief
    cells. They secrete hydrochloric acid and intrinsic factor.

  4. Entero-endocrine cells secrete a variety of regulatory peptides
    which enter the bloodstream and exert effects on motility
    and secretory processes within the GI tract.

In the fundus and body of the stomach both chief and oxyntic cells are numerous. In the antral and pyloric regions, oxyntic cells are much less numerous, with mucus, and gastrin the pre­dominant secretions. In the cardiac region of the stomach, the gastric glands consist almost entirely of mucus-secreting cells.



Fig. 18.11 A diagrammatic representation of a gastric gland to show the various secretory cell types present.

Summary

  1. The functions of the stomach include storage of food; mixing, churn­
    ing, and kneading the food to produce chyme; and the secretion of
    acid, enzymes, mucus, and intrinsic factor.

  2. In addition to the circular and longitudinal smooth muscle layers,
    the stomach wall possesses a third, obliquely arranged, muscle layer
    which promotes churning movements.

  3. The surface epithelium of the gastric mucosa is composed almost
    entirely of goblet cells secreting an alkaline fluid containing mucus.
    Gastric glands empty into gastric pits in the epithelium. The glands
    contain mucous cells, chief cells which secrete pepsinogens, and pari­
    etal cells which produce gastric acid and intrinsic factor. A variety of
    endocrine cells are also present, for example the G-cells which secrete
    gastrin.

^ 18.5 The composilion of gastric juice

The fluid secreted into the stomach by the gastric glands is called gastric juice. It contains salts, water, hydrochloric acid, pepsinogens, intrinsic factor, and mucus. The exact composition and flow rate of gastric juice is determined by the relative activities of the different types of cell within the gastric glands and will vary according to the time that has elapsed since the ingestion of food. Some 2—3 liters of gastric juice are secreted each day in adults.

] 8.5 The composition of gastric juice

403


Clinical tests of gastric secretory function, involving the col­lection of gastric juice by means of a swallowed tube, have shown that during fasting there is little or no secretion of gastric juice and that the stomach contains only about 30 ml of fluid. Following the ingestion of a test meal (such as thin gruel), the pH of the stomach contents first rises, as the acid present is neu­tralized, and then falls progressively over the ensuing 90 minutes or so, as hydrochloric acid is secreted by the oxyntic cells of the gastric glands. The output of all the other con­stituents of gastric juice also increases following a meal.

The electrolyte composition of gastric juice depends upon its rate of secretion. As the rate of secretion rises, the sodium con­centration falls while that of hydrogen ions is increased. The level of potassium ions in gastric juice is always higher than that of the plasma, which is why prolonged vomiting may lead to hypokalemia (low plasma potassium).

The formation of stomach acid

The pH of gastric juice is very low (pH 1—3). Although HC1 is not essential for the overall digestive process, the highly acidic environment it creates is important for several reasons:

  1. It helps in the breakdown of connective tissue and muscle
    fibers of ingested meat.

  2. It activates pepsinogens.




  1. It provides optimal conditions for the activity of pepsins.

  1. By combining with calcium and iron to form soluble salts,
    HC1 aids in the absorption of these minerals.

  2. It acts as an important defense mechanism for the stomach,
    killing many of the bacteria which may cause infection
    (e.g. typhoid, salmonella, cholera, and dysentery).

Gastric acid is secreted by the parietal cells of the gastric glands, predominantly in the fundus and body of the stomach. The majority of these cells only secrete HC1 after they have been stimulated. The cytoplasm of unstimulated cells is filled with an elaborate branching system of tubular structures derived from the endoplasmic reticulum. These are lined by microvilli and possess the hydrogen ion secretory apparatus. When the parietal cells are stimulated to secrete (i.e. following a meal), the tubular structures fuse to form deep invaginations of the apical mem­brane which are known as secretory canaliculi. The formation of canaliculi results in a large (up to tenfold) increase in the surface area of the parietal cell membrane and brings large numbers of hydrogen ion pumping sites into close proximity with the luminal fluid.

The metabolic steps involved in the production of acid by a parietal cell are shown in Fig. 18.12. Chloride ions are moved out of the cell against both a chemical and an electrical gradient. Hydrogen ions move down an electrical gradient but against a



Fig. 18.12 The steps involved in the secretion of gastric acid by a parietal (oxyntic) cell.

404

18 The gut and nutrition


massive concentration gradient (as much as a million to one). The process of acid production is therefore highly energy-dependent and parietal cells contain numerous mitochondria.

A unique membrane transport system is now known to be located on the canalicular membrane of parietal cells. The system is driven by a H + ,K+-ATPase and uses energy derived from the hydrolysis of ATP to pump hydrogen ions out of the cell in exchange for potassium ions. There are two routes by which chloride ions may leave the cell. First, there is a chloride channel in the secretory canaliculus and, secondly, a potassium-chloride cotransport system which facilitates the exit of chloride. The potassium ions, therefore, shuttle in and out of the cells, leaving with chloride ions and re-entering in exchange for hydrogen ions.

The hydrogen ions themselves are derived from oxidative processes within the cell (Fig. 18.12). Hydroxyl ions are also produced as a result of this oxidation and, with carbonic acid, generate bicarbonate ions which leave the cell in exchange for chloride ions at the basolateral membrane. These chloride ions provide the chloride which will leave the cell at the canalicular membrane via the chloride channel and the potassium—chloride cotransporter. As bicarbonate is added to the plasma during the secretion of acid by the stomach, the venous blood draining the stomach is more alkaline than the arterial blood. This is known as the 'alkaline tide'.

The secretion of enzymes by the gastric glands

Gastric acid secretion is accompanied by the release of a number of proteolytic enzymes from the chief cells of the gastric glands. These are collectively known as pepsin. They are secreted in the form of inactive precursors called pepsinogens which are con­tained in membrane-bound zymogen granules released by exo-cytosis when the gastric glands are stimulated to secrete. In the acid environment of the stomach, pepsinogens are converted to active pepsins. Pepsins show their greatest proteolytic activity at pH values below 3. The gastric pepsins are endopeptidases, i.e. they hydrolyze peptide bonds within the protein molecule to liberate polypeptides and a few free amino acids.

Although fat digestion by the stomach is probably negligible, the gastric glands do secrete a lipase that works over the broad pH range 4-7 and is stable at the very low pH levels found in the gastric juice. It is most active against the short-chain triglyc-erides found in milk and is therefore probably more important in children than in adults.

The secretion of intrinsic factor by the gastric glands

The secretion of the glycoprotein intrinsic factor by the parietal cells of the stomach is essential for life. Intrinsic factor is secreted in response to the same stimuli which promote acid secretion. It binds to vitamin B12 in food in the upper small

intestine and protects it from the enzymatic actions of the gut. The complex of B!2 and intrinsic factor is absorbed by the mucosal epithelial cells of the lower ileum. Vitamin B12 is needed for the production of mature red blood cells and its absence gives rise to pernicious anemia (see Section 13.7). Lifelong treatment by intramuscular injections of vitamin B12 reverse this anemia and can enable patients to survive even following total gastrectomy.

Why doesn't the stomach digest itself?

The gastric mucosa is exposed to extremely harsh chemical con­ditions. Gastric juice is corrosively acidic and contains protein-digesting enzymes. To protect itself the stomach creates a so-called mucosal barrier.

Three factors contribute to this barrier. First, the tight junc­tions between the cells of the mucosal epithelium help to prevent the gastric juice from leaking into the underlying layers of tissue. Secondly, mucus secreted by the surface epithelial cells and the neck cells of the gastric glands adheres to the gastric mucosa and forms a protective layer 5—200 /Am in thickness. This mucus is alkaline as a result of the secretion by the surface epithelial cells of a watery fluid which is rich in bicarbonate and potassium ions. When food is eaten, the rates of secretion of both the mucus and the alkaline fluid from the surface epithelial cells are enhanced. As a result, the surfaces of the gastric epithe­lial cells themselves are shielded from direct contact with the potentially damaging gastric contents and remain bathed in their own protective fluid.

Finally, prostaglandins, particularly those of the E series, appear to play an important role in the protection of the gastric mucosa. They increase the thickness of the mucus gel layer, stimulate the production of bicarbonate, and cause vasodilata-tion of the microvasculature of the mucosa. This improves the supply of nutrients to any damaged areas of mucosa while the increased bicarbonate content of the fluid neutralizes the gastric acid, thus optimizing conditions for tissue repair.

The epithelial cells of the gastric mucosa are in a dynamic state of growth, migration, and desquamation (shedding). Indeed, the gastric epithelium is continuously renewed, pro­viding further protection against damage from the harsh envi­ronment. Damaged epithelial cells are shed and replaced by new cells derived from relatively undifferentiated stem cells which migrate up from the necks of the gastric glands.

Anything that breaches the mucosal barrier produces inflam­mation of the underlying tissue, a condition known as gastritis. Persistent erosion of the stomach wall can lead to the formation of gastric ulcers. Common predisposing factors for the formation of gastric ulcers include hypersecretion of acid and/or reduced mucus secretion. A variety of drugs appear to promote ulcer for­mation by altering the rates of acid and mucus production. They include nonsteroidal anti-inflammatory drugs, such as ibuprofen, caffeine, nicotine, and aspirin, which act by interfering with the production of prostaglandins. Occasionally, bile acids are regur-

^ 18.6 The regulation of gastric secretion
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