18. 1 Introduction
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The gut and nutrition   18.1 Introduction Food is required by the body for the production of energy and for the growth and repair of tissues. Each day an average adult consumes around 1 kg of solid food and 1-2 liters of fluid. The majority of this material is in a form that cannot be used immediately by the body for cellular metabolism, so it must be broken down into simple molecules which may be absorbed into the bloodstream. The digestive or gastrointestinal (GI) system performs this task. Specifically, the major functions of the digestive system are:
18.2 General organization of the gastrointestinal system The major anatomic components of the gastrointestinal (GI) tract and its accessory organs are illustrated in Fig. 18.1. Although the tract is located within the body, it is, in reality, a hollow tube which is open at both ends (mouth and anus). The lumen of the tube is therefore an extension of the external environment. The GI tract includes the oral cavity, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, and ileum), large intestine (ascending, transverse, and sigmoid colon), rectum, and anal canal. In life, the gastrointestinal tract is about 5.5 m (18 ft) long. The accessory organs are the teeth, tongue, salivary glands, pancreas, liver, and gallbladder. Each organ or part of the tract is adapted to carry out specific functions. Histologic features of the wall of the gastrointestinal tract Although the detailed structure of the GI tract varies throughout its length according to the particular functions of each Fig. 18.1 A diagrammatic representation of the gastrointestinal tract and its principal accessory organs. region, there are common features in the overall organization of the tissues of the gut wall. Figure 18.2 illustrates the basic plan of the layers of the gut wall. From the outside inwards these layers are: the serosa, a layer of longitudinal smooth muscle, a layer of circular smooth muscle, the submucosa, and the mucosa. 394 18 The gut and nutrition   Fig. 18.2 Transverse section through a portion of small intestine to illustrate the general organization of the gut wall. Although this figure shows the basic organization, there are individual variations in structure in different regions of the gut. A layer of smooth muscle fibers, the muscularis mucosa, lies deep within the mucosa.The serosa (also called the adventitia) is the covering of the GI tract. It forms the visceral peritoneum and is continuous with the parietal peritoneum which lines the body cavity. It is a binding and protective layer which consists mainly of loose connective tissue covered by a layer of simple squamous epithelium. The two layers of smooth muscle arranged longitudinally and circularly are together known as the muscularis externa. Contraction of these muscle layers mixes the food with digestive enzymes and propels it along the GI tract. The circular layer is 3—5 times thicker than the longitudinal layer. The submucosa is made up of loose connective tissue with collagen and elastin fibrils, blood vessels, lymphatics and, in some regions, submucosal glands. The innermost layer of the gut wall, the mucosa, is further subdivided into three regions: a layer of epithelial cells, a basement membrane (lamina propria), and the muscularis mucosa. The characteristics of the epithelium vary greatly from one region of the GI tract to another. For example, it is smooth in the esophagus but is thrown into finger-like projections called villi in the small intestine. The nature of the smooth muscle of the gastrointestinal tract The motor functions of the GI tract are performed almost entirely by the action of smooth muscles. They are responsible for the mixing of food with digestive juices and the propulsion of food along the GI tract at a rate which allows the optimal digestion of food and absorption of the digestion products. The general anatomic and electrophysiologic characteristics of smooth muscle are discussed fully in Chaptet 7. Only the specific characteristics of the smooth muscle of the GI tract will be considered here. The smooth muscle in the gut is of the single unit or visceral type. It operates as a functional syncytium whereby electrical signals originating in one fiber are propagated to neighboring fibers so that sections of smooth muscle contract synchronously. The smooth muscle fibers maintain a level of tone that determines the length and diameter of the GI tract. Various types of contractile responses are superimposed upon this basal tone. The most important of these are segmentation and peristaltic contraction. Segmentation, which occurs principally in the small intestine, facilitates the mixing of food with digestive enzymes and exposes the digestion products to the absorptive surfaces of the GI tract. It is characterized by closely spaced contractions of the circular smooth muscle layer separated by short regions of relaxation, as shown in Fig. 18.3a. Peristaltic contractions are concerned mainly with the propulsion of food along the tract and consist of successive waves of contraction and relaxation of the smooth muscle, as shown in Fig. 18.3b. The longitudinal smooth muscle contracts first and, halfway through its contraction, the circular muscle begins to contract. The longitudinal muscle relaxes during the latter half of the circular muscle contraction. This pattern of contraction is repeated, resulting in the slow but progressive movement of material along the GI tract. The normal trigget for a wave of peristalsis is distension. The contractile properties of smooth muscle are determined by the underlying electrical activity of its cells. The resting membrane potential of the cells shows spontaneous rhythmical 18.2 General organization of the gastrointestinal system 395   Fig. 18.3 Principal patterns of contractile activity in the smooth muscle of the GI tract:
fluctuations (the basic electrical rhythm or slow wave rhythm). The frequency of these fluctuations varies along the length of the GI tract, generally decreasing with distance from the mouth. For example, in the duodenum there are 11 or 12 slow waves per minute and in the colon only 3 or 4. These differences in rate create a gradient of pressure along the GI tract which contributes to the steady movement of its contents towards the ileo-cecal sphincter.Although the smooth muscle layers of the gut can contract in the absence of action potentials, the depolarizing phases of the slow waves are sometimes accompanied by bursts of action potentials. Such bursts are associated with vigorous propulsive movements such as those seen in the antral region of the stomach. Innervation of the gastrointestinal tract The complex afferent and efferent innervation of the GI tract allows for fine control of secretory and motor activity via intrinsic (enteric) and extrinsic (sympathetic and parasympathetic) pathways. ^ Two well-defined networks of nerve fibers and ganglion cell bodies are found within the wall of the GI tract from the esophagus to the anus. The networks are called intramural plexuses and constitute the enteric nervous system (ENS). The myenteric plexus (also known as Auerbach's plexus) lies between the circular and longitudinal smooth muscle layers of the muscularis externa, while the less extensive submucosal plexus (or Meissner's plexus) lies within the submucosa (see Fig. 18.2) The former is largely motor in function and the latter mainly sensory, receiving signals from the intestinal epithelium and from stretch receptors in the gut wall. The ENS has a more complex organization than the auto-nomic ganglia that supply, other visceral organs. For this reason the ENS is sometimes referred to as the 'gut brain'. It utilizes many neurotransmitters and neuromodulators, including chole-cystokinin (CCK), substance P, VIP (vasoactive intestinal polypeptide), somatostatin, and the enkephalins. The enteric nervous system is responsible for coordinating much of the secretory activity and motility of the GI tract through intrinsic pathways often called 'short-loop' reflexes. ^ Although much of the activity of the GI tract is controlled through the intrinsic nerves of the ENS, the nerve plexuses are themselves linked to the central nervous system via afferent fibers and receive efferent input from the autonomic nervous system. Afferent innervation of the 61 tract Chemoreceptor and mechanoreceptor endings are present in the mucosa and the muscularis externa. Some of these send afferent axons back to the central nervous system, to mediate reflexes via the CNS ('long-loop' reflexes) while the axons of others synapse with cells within the plexuses to mediate local reflexes. Many of the extrinsic sensory afferents travel in the vagus nerve (vagal afferents) and have their cell bodies in the nodose ganglion of the brainstem. Some, particularly those forming part of the reflex arcs which control motility, travel to the spinal cord via the sympathetic nerves. The cell bodies of these fibers lie within the dorsal root ganglia. It is important to realize that at least 80 per cent of fibers in the vagus and up to 70 per cent of splanchnic nerve fibers are afferent. Indeed, in vago-vagal reflexes, both the afferent and efferent fibers travel in the vagus nerve. Such reflexes play a significant role in the control of motility in the GI tract. Sympathetic innervation Most sympathetic fibers supplying the GI tract are post-ganglionic and their cell bodies lie within the celiac, superior 396 18 The gut and nutrition  and inferior mesenreric, and hypogastric plexuses. Some of the sympathetic fibers innervate the smooth muscle of blood vessels within the GI tract, causing vasoconstriction, while others enter glandular tissue and innervate secretory cells. However, the majority of the sympathetic fibers terminate within the sub-mucosal and myenteric plexuses where they appear to inhibit synaptic transmission, possibly by presynaptic inhibition.The sphincters of the GI tract are supplied with adrenergic fibers whose actions are usually excitatory. There is also some innervation of the circular smooth muscle layers of the small and large intestine by sympathetic fibers. Here their effect is largely inhibitory. Parasympathetic innervation Parasympathetic input to the gut stimulates both its motility and secretory activity. The vagus nerve relays the parasympathetic innervation to the stomach, small intestine, cecum, appendix, ascending colon, and transverse colon. The remainder of the colon receives parasympathetic innervation from pelvic nerves via the hypogastric plexus. All the parasympathetic fibers terminate within the myenteric plexus and are predominantly cholinergic. Further details of the autonomic innervation of the gut are given in Chapter 10. Hormonal regulation of the gastrointestinal tract In addition to its extensive innervation, the GI tract is regulated by a number of peptide hormones which act through endocrine and/or paracrine pathways (Chapter 12). The GI tract utilizes at least 20 different regulatory peptides. Eight poly-peptides are known to act as circulating (endocrine) hormones. These are:
CCK and neurotensin also act as paracrine agents, exerting their effects close to their site of secretion. Other paracrine hormones include somatostatin and histamine. In general the effects of these agents on motility and secretory activity supplement those of the gastrointestinal innervation, although the relative importance of nervous and hormonal influences differs throughout the tract. In the salivary glands, for example, nervous control is the dominant influence on secretion. In other areas such as the stomach, nervous and endocrine influences are of equal importance, while hormones are the principal regulators of secretion from the exocrine pancreas. General characteristics of the blood flow to the GI tract The various digestive functions of the gut require a rich and highly organized blood supply. The combined circulation to the stomach, liver, pancreas, intestine, and spleen (although this organ has no digestive function) is called the splanchnic circulation. At rest the splanchnic vessels receive 20—25 per cent of the cardiac output. Two large capillary beds are partially in series with one another in the splanchnic circulation. Branches of the splanchnic artery supply the capillary beds of the GI tract, spleen, and pancreas. From here, venous blood flows ultimately into the portal vein which supplies about 70 per cent of the blood supply to the liver. The portal blood leaves the liver via the hepatic vein to enter the inferior vena cava. The remainder of the hepatic blood supply is provided by the hepatic artery which supplies most of the oxygen required by the liver. The chief purpose of the portal circulation is to allow rapid delivery of the products of digestion from the intestine to the liver where they will undergo further processing. The general organization of the splanchnic and portal circulations is illustrated in Fig. 18.4 The intestine is supplied by the superior and inferior mesenteric arteries, small arterial branches of which form a vascular network in the submucosal layer that penetrates the longitudinal and circular smooth muscle. The ingestion of food increases intestinal blood flow. This increase is partly mediated by the secretion of gastrin and cholecystokinin (CCK). Summary
18.3 Intake of food, chewing, and salivary secretion 397  Fig. 18.4 A diagrammatic representation of the blood supply to the liver and gastrointestinal 18.3 Intake of food, chewing, and salivary secretionFood is ingested via the mouth (also called the oral or buccal cavity), the only part of the GI tract which has a bony skeleton. In the mouth the food is broken into smaller pieces by the process of chewing (mastication) and mixed with saliva which softens and lubricates the food mass. As food moves around the mouth, the taste buds are stimulated and odors are released from the food. The mouth is divided into two regions, the vestibule and the oral cavity. The vestibule is the region between the teeth, lips, and cheeks. The oral cavity is the inner area bounded by the teeth. The epithelium here is typically 15-20 layers of cells thick and is structurally adapted to withstand the frictional forces generated during mastication. The teeth A child has 20 deciduous (milk) teeth, an adult 32 permanent teeth, embedded in the alveoli or sockets of the alveolar ridges of the mandible and maxilla. In adults the upper and lower jaws each possess four incisors, two canines, four premolars, and six molars. The incisors and canines are the cutting teeth while the premolars and molars have broad, flat surfaces for grinding or chewing food. Although the shapes of the different teeth vary, they share a similar basic structure (Fig. 18.5). The crown is the part of the tooth that protrudes from the gum, while the root is embedded in the jaw bone. In the center of the tooth is the pulp cavity which contains blood vessels, lymphatics, and nerves, and surrounding this is the hard dentin. Outside the dentin of the crown is a layer of even harder enamel. The root of the tooth is surrounded by softer cement which fixes the tooth into its socket. Blood vessels and nerves pass to the tooth through a small opening at the tip of each root. The nerve supply to the upper teeth is by branches of the maxillary nerves and to the lower teeth by branches of the mandibular nerves. These are both branches of the trigeminal nerve (cranial nerve V). The tongue The tongue, which is formed from skeletal (voluntary) muscle, is inserted into the hyoid bone and attached to the anterior floor of the mouth, behind the lower incisor teeth, by a fold of its mucous membrane covering, called the frenulum. The role of the tongue in the perception of taste has been described in Chapter 8. It also plays an important role in speech and is necessary for swallowing. 398 18 The gut and nutrition   Fig. 18.5 A longitudinal section through a canine tooth, illustrating the major structural features. Mastication The teeth perform several functions during mastication—tearing (canines), cutting (incisors), and grinding (premolars and molars). A crushing force of 50—80 kg can be generated on the molars during chewing, a value which far exceeds the forces generally required for a normal diet. Both the tongue and the cheeks also have important roles to play in the process of chewing. Their movements help to keep the food in the correct position for effective chewing, while the sensory receptors on the tongue provide information regarding the readiness of the food for swallowing. The secretion of saliva Approximately 1500 ml of saliva is produced each day by the salivary glands. It performs several important functions: it lubricates the food to facilitate swallowing, it aids in speech, and it contains an enzyme, salivary a-amylase (ptyalin), which begins the process of starch digestion. The saliva dissolves certain substances in foods making them available to the taste cells. Saliva also exerts an antibacterial action which contributes significantly to oral comfort. In individuals who lack functional salivary glands, a condition called xerostomia (dry mouth) is seen in which there is a predisposition to dental caries and infections of the buccal mucosa. There are three main pairs of salivary glands: the parotid, submandibular, and sublingual glands. Other, smaller glands exist over the surface of the palate and tongue and inside the lips, though these do not appear to be under nervous control. Each gland is surrounded by a fibrous capsule and consists of a number of lobules made up of small acini (alveoli) lined with secretory (acinar) cells. The acini are drained by ductules which join to form larger ducts leading into the mouth. All the major salivary glands receive both sympathetic and parasympathetic innervation. Noradrenergic sympathetic fibers from the superior cervical ganglion are distributed to both blood vessels and acinar cells. Preganglionic parasympathetic fibers arrive by way of the facial and glossopharyngeal nerves and synapse with postganglionic neurons close to the salivary glands themselves. Both the secretory cells and the duct cells receive parasympathetic postganglionic fibers. The parotid glands are situated at the angle of the jaw, lying posterior to the mandible and inferior to the ear. They are the largest of the salivary glands and produce an entirely serous secretion, a watery fluid lacking mucus. Saliva from the parotid glands accounts for around 25 per cent of the total output of the salivary glands at rest. It contains a-amylase and antibody (immunoglobulin A). The submandibular glands (also known as the submaxillary glands) lie below the lower jaw (mandible). They produce a more viscid saliva which forms 70 per cent of the daily output. These glands contain acinar cells which secrete mucoproteins, as well as cells producing serous fluid. The sublingual glands lie in the floor of the mouth below the tongue. They contribute the remaining 5 per cent or so of the total salivary output, producing a secretion which is rich in mucoprotein and gives the saliva its somewhat sticky character. ^ The formation of saliva is a two-stage process. An isotonic fluid (primary secretion) is formed by the acinar cells as a result of the active transport of electrolytes followed by the passive transfer of water. Secondary modification of this fluid then occurs by means of ion transport processes occurring in the epithelial cells lining the ducts. The concentrations of sodium, chloride, and bicarbonate ions in the primary secretion resemble those of plasma. Modification of the primary secretion involves the active reabsorption of sodium and bicarbonate by the epithelial cells lining the ducts as the saliva flows past them. Figure 18.6 illustrates the ion movements believed to occur during the formation of saliva. The final electrolyte content of saliva depends upon the rate at which it is secreted and flows along the salivary ducts. At low rates of secretion there is ample time for ductal modification of 18.3 Intake of food, chewing, and salivary secretion 399   Fig. 18.6 Water and electrolyte transport leading to the formation of saliva by acinar and ductal cells of a salivary gland. the primary secretion, and the saliva is relatively dilute. As the flow rate increases, the salivary content of sodium, bicarbonate, and chloride increases, while there is a small drop in the potassium concentration (Fig. 18.7). The pH of saliva rises from 6.2 to 7.4 with increasing rates of secretion as a result of the increase in bicarbonate concentration.  Fig. 18.7 The variation of the ionic composition and osmolality of the saliva with flow rate. The composition of plasma is shown for comparison in the bar diagram on the right. The regulation of salivary secretion The rate of salivary secretion is controlled primarily by reflexes mediated by the autonomic nervous system. The resting rate of salivary secretion is about 0.5mlmin l. During maximal stimulation by sapid substances, the smell of food, and chewing, the rate of secretion may increase to 7 ml min"1. Sensory receptors in the mouth, pharynx, and olfactory area relay information about the presence of food in the mouth, its taste and smell, to the salivatory nuclei which are located in the medulla. The medulla also receives both facilitatory and inhibitory impulses from the hypothalamic appetite area and regions of the cerebral cortex concerned with the perception of taste and smell. Most salivatory responses are mediated by parasympathetic efferent fibers originating in the salivatory nuclei. Parasympathetic stimulation promotes an abundant secretion of watery saliva which is rich in amylase and mucins. The transporting characteristics of the ductal epithelium are also altered. Bicarbonate secretion is stimulated while the reabsorption of sodium and the secretion of potassium is inhibited. These changes are mediated by muscarinic receptors and are inhibited by atropine (see Chapter 10). The response to parasympathetic stimulation also includes a significant increase in blood flow to the salivary glands. This effect is not atropine-sensitive. Several mechanisms are thought to contribute to the increase in blood flow. These include the release of VIP (vasoactive intestinal polypeptide) from parasympathetic nerve terminals and the release of the proteolytic enzyme kallikrein into the interstitial fluid from the acinar cells themselves. Kallikrein, in turn, promotes the production of the powerful vasodilator, bradykinin, from plasma a-2 globulin. The response of the salivary glands to sympathetic stimulation is variable. Sympathetic fibers stimulate the secretory cells and enhance the output of amylase. At the same time, however, blood flow to the glands is usually reduced through vasoconstriction and the net result is a fall in the rate of salivary secretion. Indeed, a dry mouth is an important characteristic of the sympathetic response to fear and stress. A summary of the mechanisms involved in the control of salivary secretion is shown in Fig. 18.8. Digestive actions of saliva The digestive enzyme, salivary amylase, is stored within zymogen granules in the serous acinar cells. It is able to degrade complex polysaccharides such as starch and glycogen to maltose, mal-totriose, and dextrins, working optimally at pH 6.9- Although food remains in the mouth for only a short time and the contents of the stomach are highly acidic, salivary amylase is believed to continue working within the food bolus for some time after the entry of food into the stomach. It is probably inactivated only after complete mixing of the bolus contents with the gastric juice. Swallowing Once mastication is complete and the lubricated food bolus has been formed, it is swallowed. From the mouth, food passes 400 |
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