The digestive system
This chapter focuses on the anatomy and histology of digestive system. The digestive system, or the gut, is a muscular tube lined with endoderm that traverses the body from mouth to anus, sealed at various points by muscular sphincters controlled by the autonomic nervous system. The digestive tube is subdivided into a number of organs on a basis of the anatomical and functional breaks as a result of the sphincters. The major part of the tube from esophagus to anus is made up of layers of muscle separated from each other by connective tissue in which lie blood vessels and nerves. The characteristic movement of the digestive tract is peristalsis. A peristaltic wave is a wave of contraction originating at the cranial end of a piece of gut and passing caudally. In man the peristaltic wave always travels in the same direction. The function of the peristaltic wave is to move the contents of the gut along the tube in the direction of the wave. In the oesophagus peristaltic waves are initiated by swallowing. Gases in the stomach may result from the action of the digestive secretions on foodstuffs or may even reach the stomach from subsequent parts of the digestive tract. Most commonly, they are simply air that has been swallowed with the food. Such gases may be eliminated if they become uncomfortable by eructation.
The digestive system, or the gut, is a muscular tube lined with endoderm which traverses the body from mouth to anus, sealed at various points by muscular sphincters controlled by the autonomic nervous system. The cephalic end of the tube, the mouth cavity, is closed by the lips; entry to the tube is therefore under voluntary control. The caudal end, the anus, is sealed by the anal sphincters, the outermost of which is under voluntary control in the adult. Since the tube may be open to the external environment at both ends, its inside and contents must be regarded as external to the body. Food and drink are taken into this tube, where they are physically and chemically processed. Their components are then absorbed actively or passively across its wall into the body cavity proper, or else remain unabsorbed and are carried down the tube to be excreted as faeces. The walls of the gut are made up of muscle whose contractions knead and mix the food after the initial chopping and grinding in the mouth, and also propel the food mixture onwards. The mural epithelium is specialised to form glands of varying complexity: these produce mucoid substances to lubricate the passage of the food mixture and to protect the walls, and enzymes which break down complex foodstuffs into simple molecules. The glands may be situated in the thickness of the walls or may be so deeply invaginated as to form separate structures – as, for example, the pancreas. Other parts of the tube are specialised for absorption, with infoldings of the surface mucous membrane (the mucosa) which enormously increase the area available for diffusion. The mucosa, like other epithelial surfaces, is constantly losing its outer layer of cells and replacing them. This helps to protect it against the abrasion due to the passage of food. The cells lost in this way in the lower part of the gut are excreted in the faeces, together with some of the gut bacteria and any secretions which have not been reabsorbed.
In life the muscular walls of the gut always maintain some degree of contraction, or tone. Measurements in adults by X-ray and other methods, therefore, suggest that the gut is only about 4.5 m long even though it appears twice this length in cadavers.
The digestive tube is subdivided into a number of organs on a basis of the anatomical and functional breaks due to the sphincters (Fig. 11.1). The mouth, or oral cavity, is separated from the oro-pharynx only by a ring of lymphoid tissue and is continuous through the naso-pharynx with the nasal cavity. The pharynx is not anatomically separated from the oesophagus, although a functional sphincter, the cricopharyngeal sphincter, has been described. The oesophagus extends from there to the cardiac sphincter at the entrance to the stomach. It is situated within the thoracic cavity and at rest its lumen is in equilibrium with thoracic pressures because of the thinness of its walls. A balloon in the oesophagus may therefore be used as a pressure transducer to measure intrathoracic pressures. The next portion is the stomach, separated above from the oesophagus by the cardiac, or gastroesophageal sphincter, and below from the small intestine by the gastroduodenal, or pyloric, sphincter. Functionally the stomach has a cardiac portion, a fundus or body, and the tubular pyloric antrum leading to the pylorus. The small intestine is made up of the short C-shaped duodenum holding the pancreas within its loop, and the longer jejunum and ileum. Since there is no obvious division between jejunum and ileum their lengths are arbitrarily estimated as in the ratio of 2:3. The small intestine as a whole is about 270 cm long in the living subject. Its termination is marked by the ileocaecal valve or sphincter. The ileum projects into the colon (the next part of the gut) and since it enters laterally, the opening is closed by colonic contractions. The colon extends a short distance downwards as the caecum, which ends in a small blind-ended extension, the appendix. The colon proper passes up towards the liver, crosses the abdominal cavity from right to left and then continues downwards to join the rectum at the colonic sphincter. The rectum is the final part of the gut and ends in the anus with the internal anal sphincter.
Figure 11.1 The main organs of the digestive system as seen when the abdomen is opened. The outlines are continued as dotted lines when they pass behind another organ: the complex folds of the small intestine are indicated by curved lines of shading, and only the beginning of the jejunum and the end of the ileum, where it joins the caecum, are shown for localisation.
The major part of the tube from oesophagus to anus is made up of layers of muscle separated from each other by connective tissue in which lie blood vessels and nerves. Characteristically the muscle is arranged in three layers: the submucosal, the circular and the longitudinal layers (Fig. 11.2). In fact, both circular and longitudinal layers are spiral muscles, the circular a tightly coiled spiral, and the longitudinal an extended spiral. The lumen of the gut is lined with a mucosa whose surface varies from a stratified squamous epithelium in the mouth and oesophagus to a columnar epithelium in the intestine. The lamina propria is invaginated by glands producing mucus and digestive secretions. The blood vessels and lymphatics are specially arranged in the parts of the intestine whose function is primarily absorptive. Although the gut is composed of smooth muscle with its own intrinsic rhythmicity, it has an extensive innervation to modify and co-ordinate its activity as well as to control its secretions. Two nerve plexuses are described: a submucous plexus (of Meissner) between the muscularis mucosae and the circular muscle layer; and the myenteric plexus (of Auerbach) between the circular and longitudinal muscle layers. These plexuses are both sensory and motor. The extrinsic nerves connect mainly with the myenteric plexus. Parasympathetic fibres travel in the vagus to the upper part of the gut, and in the pelvic and splanchnic nerves to the colon and rectum (Fig. 9.18). Sympathetic fibres pass from the coeliac plexus to the stomach and small intestine, from the superior mesenteric plexus to the small intestine, caecum, ascending and transverse colon, and from the inferior mesenteric plexus to the descending colon and the rectum (Fig. 9.19).
In different parts of the gut there are variations from the general plan. A marked variation in the muscle layers occurs in the colon where the longitudinal muscle is not uniform over the surface but is arranged in ribbon-like bands, the taenia coli. Their length and state of contraction causes the colon to be pulled together into a series of sacs. The muscle layers of the oesophagus are not so well characterised as those elsewhere and in the stomach there is an intermediate layer of muscle between the circular and longitudinal layers.
The control and co-ordination of the processes of ingestion, digestion and excretion are discussed later (Chaps. 19 and 21) but a general description of the muscular activities seen in the gut is given here.
The characteristic movement of the digestive tract is peristalsis. A peristaltic wave is a wave of contraction originating at the cranial end of a piece of gut and passing caudally. It is usually initiated by stretching of the gut, which probably causes depolarisation and the development of an action potential. The wave of electrical activity passes caudally followed by the contraction. This myogenic effect is supplemented and integrated by nerves in the submucosal plexus and may be modified by the myenteric plexus as a result of sympathetic or parasympathetic stimulation. Generation of the peristaltic wave begins in the longitudinal muscle and, when that is approximately half completed, contraction of the circular muscle follows. The intrinsic plexuses co-ordinate the response of the circular muscle. Distension of the gut not only causes excitation of the longitudinal muscle but also activates the myenteric plexus so that contraction of the circular muscle is briefly inhibited before it is in turn excited. In this way the delay between the two muscles is maintained. In man the peristaltic wave always travels in the same direction. In isolated gut preparations the wave of contraction is preceded by a wave of relaxation. The function of the peristaltic wave is to move the contents of the gut along the tube in the direction of the wave. In the oesophagus peristaltic waves are initiated by swallowing. Some 10 to 20 cm of the oesophagus is seen to be contracting at once and the wave takes about a second to reach its peak, holds it for about half a second, and then dies away over the next second. The wave moves at 2-4cm/s. Although it generates an internal pressure of up to 1.5 kPa, the actual propulsive force is small – of the order of 0.05-0.10N. The stomach is traversed by rhythmic peristaltic waves at a frequency of around 3/min generated by a pacemaker area near the cardia. In the duodenum a basic rhythm of 7-8 waves/min is seen. These regular rhythms of muscle contraction all originate in rhythmic action potentials from identifiable pacemaker areas and are known as the basic electrical rhythms (BERs) of the particular sections of the gut.
Peristaltic waves in the small intestine generally are short weak movements travelling at about 1 cm/min which do not pass the length of the small intestine but die away after a short distance. They are only weakly propulsive: as a result a pendular movement of the intestinal contents takes place despite the absence of any actual reversal of peristaltic direction. This slow passage of the intestinal contents provides ample time for absorption to occur across the wall of the gut. A pacemaker area has been described near the entrance of the bile duct but the portion of the duodenum cranial to this has its own independent rhythm of contraction.
The peristaltic waves of the colon have a much slower frequency, between 2 and 10/min, and travel slightly more slowly, at between 1 and 2 cm/min. A massive peristaltic wave is seen infrequently in the colon and extending down into the rectum. It is apparently initiated by the entry of food into the stomach. Such a wave, mass peristalsis, or mass movement, takes about 15 min to traverse the length of the colon. It is not to be confused with defaecation: this also involves a massive peristaltic wave, but one triggered by distension of the rectum which is under over-riding control from conscious centres in the adult.
A number of other movements are described in the large and small intestines. In the small intestine a type of contraction termed segmentation is probably the major form of movement. In this, ring-like contractions appear at intervals, fade away, and then re-appear in new sites between those of the previous contractions. This process is only slowly propulsive and serves to mix the contents of the small intestine. After feeding these contractions have been observed to occur some 10 or 11 times/ min. In the colon, similar annular contractions produce what are known as haustral movements. The section between two haustral contractions is called a haustral sac. These haustrations occur between 2 and 10times/min; like the segmental contractions of the small intestine, they result in kneading of the intestinal contents.
The movements of the muscularis mucosae are independent of those of the longitudinal and circular muscles: they originate as responses to the contact of the mucosal surface with any material within the gut lumen. Extrinsic nerves, however, are able to modify the excitability of the muscularis mucosae via the nerve plexuses in the same way as they modify the excitability of the longitudinal and circular muscles: the parasympathetic generally increasing, and the sympathetic inhibiting, activity.
As a result of the propulsive movements the food ingested is passed through the gastro-intestinal tract. The time taken for this process ranges from a minimum of between 4 and 10 h up to maxima in the range of 68-165 h. A meal rarely disappears from the body in less than 12 h, and it may remain as long as a week. Some mixing of consecutive meals does, in fact, occur in the intestine. If a patient inadvertently swallows a gold inlay it may not appear in the faeces for several days. So-called regular bowel habits do not shorten the period. The time taken to traverse individual segments of the digestive tract has also been measured. Movement is rapid down the oesophagus – partly under the influence of gravity-water taking about 1 s and chewed food about 5 s. If the subject is horizontal the time is extended slightly, but the force generated by swallowing together with the peristaltic force (some 1.5 kPa above resting levels) drives the food on into the stomach. Even a subject in the vertical position with the head downwards can still swallow: the cricopharyngeal sphincter constricts to give a pressure of about 1.2 kpa and this is sufficient to support a column of water as the peristaltic wave carries it upwards. The gastro-oesophageal sphincter can constrict to give a pressure 2.5-4.0 kPa above resting, and this will prevent reflux of material from the stomach.
The stomach in the resting state is of small volume – perhaps 50ml – but as it fills it stretches, showing the plastic and elastic properties of smooth muscle (see Chap. 5). As food passes into the stomach, layering occurs in the fundus, but by the time the food reaches the pylorus mixing has taken place. The appearance of the stomach in X-ray pictures depends upon the density of its contents – the stomach as a whole is equivalent to a bag of fluid in a fluid-filled cavity and if its density is low it floats. Within the stomach the same principle holds good: solid materials sink within the fundus, liquids ‘float’, and gases rise to the top – to give the appearance of a cap in some X-ray pictures. The passage of solid material, however, towards the pyloric antrum is relatively slow. Although the rate of emptying varies, when it begins each peristaltic wave transfers about 1ml to the duodenum, resulting in a transfer rate of about 20 ml/min. About 35% of a 500 ml fluid meal leaves the stomach in one hour and it usually takes around 4h for a normal meal to leave the stomach. Patients about to undergo general anaesthesia are therefore advised to refrain from meals for some 4h before the operation, so that the stomach may be empty and the risk of vomiting (with the consequent danger of vomitus passing into the respiratory tract) reduced.
The pressure in the stomach is normally around 2 kPa and hence less than the closing pressure of the gastro-oesophageal sphincter. Increased abdominal pressure, as in pregnancy, may lead to reflux of the stomach contents, giving rise to the sensation of heartburn.
Gases in the stomach may result from the action of the digestive secretions on foodstuffs, or may even reach the stomach from subsequent parts of the digestive tract; most commonly they are simply air which has been swallowed with the food. Such gases may be eliminated if they become uncomfortable by eructation (in U.K. ‘belch’, in U.S. ‘burp’). This swallowing of air (aerophagy) and its expulsion is utilised in patients who have had the larynx removed to eliminate carcinoma: by adjusting the strength and frequency of the eructation they can learn to produce oesophageal speech. The dental surgeon may sometimes assist these patients by fitting an upper denture with a vibrating plate which can be activated by oesophageal airflow.
The minimum transit time through the small intestine is again something of the order of 4h, with the intestinal contents often delayed at the ileocolic junction for several hours. Movement in the colon is even slower.
Gases in the intestine, often generated by the bacteria normally present in the gut, pass more rapidly than do the solid contents. They contain appreciable quantities of methane. The expulsion of these gases is termed a flatus – strictly, a rectoflatus. Movement of gases around the intestinal coils can produce the sounds known as borborygmi.
The secretions of the oral cavity are collectively known as whole or mixed saliva, although some writers prefer the term ‘oral fluid’. All three terms refer to the fluid which is collected by expectoration; secretions of individual glands are named appropriately as parotid saliva, accessory gland saliva, etc. In the resting mouth the lips and cheeks are almost in contact with the teeth anteriorly, and the tongue is in close proximity with the teeth and the palate posteriorly. Only a thin film of whole saliva separates these structures and this constitutes their immediate environment. How far saliva can be considered the environment of the teeth themselves is debatable, since most of the dental enamel surfaces are covered either by a glycoprotein pellicle or the bacterial film known as dental plaque. However, water and small molecules appear to diffuse easily through thin layers of dental plaque.
Whole saliva is made up of the secretions of the three pairs of major salivary glands – the parotid, the submandibular and the sublingual – together with the secretions of the minor, or accessory, glands which are distributed in the mucosa of the cheek, lips, hard and soft palates, and tongue. In addition to these secretions, the total oral fluid includes the gingival crevicular fluid, which passes between the teeth and the gingival cuff tissue to reach the oral cavity (p. 23), and, if it is formed by human teeth, enamel fluid (p. 23).
Centrifugation of whole saliva separates a plug of solid material known as salivary sediment. This makes up 5-10% of the volume of whole saliva collected after mechanical stimulation such as chewing wax. Sediment only rarely contains food debris: it is made up of cellular material in varying states of viability, ranging from completely normal cells to unidentifiable fragments. The cells are of three types: epithelial cells which have been desquamated from the oral mucosa, white blood cells derived from the tissues at the gingival margins of the teeth, and the commensal bacteria of the oral and dental surfaces. Whole saliva contains between 6 and 600 × 103 buccal squames/ml, usually identifiable as intact cells, although many of the unrecognisable fragments may also be derived from this type of cell. In saliva from subjects with natural teeth there are 25-650 × 103 leucocytes/ml, mainly polymorphonuclear neutrophils, together with a few lymphocytes and basophils. These cells gain access to saliva by diapedesis through the walls of the gingival vessels (which in most subjects show the dilatation and enhanced permeability typical of mildly inflamed tissues) and then through the gingival cuff tissue along the sides of the teeth. The bacterial count is very variable but a representative figure would be between 60 and 70 × 106/ml. The majority (about 70%) of the bacteria are streptococcal. These include Streptococcus mutans, currently identified as the main causative agent of dental caries. Subjects with active dental caries have a higher proportion of lactobacilli in their saliva, but the numbers are very small (1% or less of the total bacterial count).
Estimates based on typical flow rates and durations of flow in response to differing stimuli suggest that the total volume of saliva produced in 24 h is between 0.5 and 0.6 litres. Of this, about half is due to a resting, unstimulated, flow, and the other half is produced in response to various stimuli associated with food intake. Sleeping subjects produce only some 10 ml of saliva from the major glands over 8h, probably because of the lack of stimulation.
The relative proportions of the secretions in whole saliva vary with the degree of stimulation and with the nature of the stimulus. In sleeping subjects there is no observable flow of saliva from the parotid glands; the submandibular glands contribute 80% and the sublingual 20% of the saliva produced. The contribution of accessory gland saliva during sleep is unknown, but could be substantial at such low total flow rates. Unstimulated, or resting, flow in subjects who are awake amounts to 200-300 ml over 12-14 h: about 20ml/h. Of this the submandibular glands contribute about 70%, the parotid glands about 20%, the sublingual glands 1-2%, and the accessory glands almost 7%. Gustatory stimulation gives a whole saliva with almost equal contributions from submandibular and parotid glands at fast flow rates, whilst vigorous mechanical stimulation by chewing rubber bands or paraffin wax results in a whole saliva containing twice as much parotid saliva as submandibular saliva. Accessory glands contribute around 7.5% of the volume of stimulated whole saliva, regardless of the nature of the stimulus.
Gingival crevicular fluid may account for 10-100 μl/h of resting flow – less than 0.1%. Despite the small volume, however, it may be very important in the immediate environment near the gingival margin.
Each parotid gland is pyramidal in shape with the base of the pyramid lying directly below the skin (Fig. 11.3). Each is enclosed in a fibrous capsule and weighs about 25 g. The main duct passes forward over the masseter muscle, turns inward to pierce the buccinator muscle and then terminates in a small papilla on the mucosa of the cheek close to the upper first molar tooth. The situation of the duct renders it easy to attach a cup (a Lashley or Carlsen-Crittenden cup) in position over it by suction.