The digestive system

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The digestive system consists of the alimentary canal, which is a tube connecting the mouth and anus, and associated structures that facilitate digestion of ingested food and drink (Fig. 12.1). The alimentary canal comprises the mouth, oesophagus and gastrointestinal tract. The gastrointestinal tract comprises the stomach, duodenum, jejunum, ileum, colon, appendix, rectum and the upper part of the anal canal. The associated structures are the teeth, tongue, salivary glands, liver and pancreas. At the mouth, lips intervene between skin and the mouth and they are covered by a stratified squamous epithelium which is not keratinised. At the anus the stratified, keratinised squamous epithelium of skin extends a short distance into the anal canal. The upper part of the anal canal is lined by a stratified, squamous epithelium which is not keratinised.

The digestive system has two primary functions. It breaks down food and drink into small molecules such as glucose, amino acids, fatty acids and triglycerides, a process which involves enzymic activity and is known as digestion. The second primary function of the digestive system ensures the small molecules produced by digestion in the lumen of the alimentary canal enter the body ‘proper’ by being absorbed across the epithelium lining the alimentary canal and into the blood or lymphatic vessels. In addition, the digestive system absorbs water, minerals, vitamins and ions from the material in the lumen of the gastrointestinal tract. The terminal part of the tract functions as a store for components of food that have not been digested. The stored, undigested food and waste products of the body, e.g. from the breakdown of red blood cells, are emptied from the alimentary canal at defecation.
The structure of the alimentary canal is related to its functions and different aspects of function occur at different locations along the canal. The gastrointestinal tract itself is several metres in length in humans, which is a reflection of the space required to accomplish all aspects of its function and is related to the transit times required for these functions.

General structure of the alimentary canal

All the alimentary canal is lined by a mucosa (Chapter 3), which consists of an epithelium, connective tissue lamina propria and, in many regions, a layer of smooth muscle forming the muscularis mucosae (Fig. 12.2). The epithelium of the mucosa reflects the primary function(s) occurring in that region. However, in all regions the mucosa is important in forming a barrier between substances ingested, including microorganisms, and the internal environment of the body. In regions where the muscularis mucosae is present, contraction of this muscle moves and folds the mucosa, aiding contact between the contents of the lumen and the surface epithelial cells.
A submucosal layer of connective tissue, which supports nerves and blood and lymph vessels, attaches the mucosa in most parts of the alimentary canal to outer layers of muscle (the muscularis externa) (Fig. 12.2). In most regions of the alimentary canal two layers of muscle are present in the muscularis externa. In the outer, longitudinal layer the long axis of each muscle cell lies roughly parallel to the length of the lumen. In the inner, circular layer the long axes of the muscle cells lie around the lumen. Contractions of the muscularis externa cause peristalsis, which provides the motive force transporting the luminal contents of the canal towards the anus. In the submucosal layer and the muscularis externa, parasympathetic neuronal cell bodies are present and some are involved in co-ordinating muscle activity.
In some regions of the alimentary canal the muscularis externa is attached to adjacent structures by connective tissue known as the adventitia. However, in most regions of the canal in the abdominal cavity, the muscularis externa is covered by a serosal membrane (Chapter 3) which is the inner (visceral) layer of the peritoneum. The visceral peritoneal membrane is continuous with the parietal peritoneal membrane, which is attached to the abdominal walls (and part of the liver). Squamous epithelial cells on the surface of the peritoneal membranes secrete small amounts of fluid into the peritoneal cavity. The surface of peritoneal membranes is kept moist by these secretions, which provide lubrication to ensure that, as gut tubes move against each other, potentially damaging friction does not occur. In addition, the peritoneal membranes enclose the vessels and nerves supplying the gut tube.
Specialised gut-associated lymphoid tissue (GALT) is present in the walls of the alimentary canal. In some regions, GALT forms large structures, e.g. tonsils; in other regions, only small clusters of lymphoid cells are present (Fig. 12.2). These cells are part of the primary defence mechanisms against pathogens entering the body across the mucosa lining the gut tube.

Mouth

The mouth (oral cavity) contains the tongue and teeth. The epithelium of the mucosa lining the cheeks and tongue is able to resist the wear and tear in-volved in chewing food, and in most regions it is a stratified, squamous (non-keratinised) epithelium. This epithelial surface is moistened by secretions from small serous and mucous glands in the submucosal layer and from salivary glands (Fig. 12.3) which drain their saliva into the oral cavity. Physical breakdown of food occurs in the mouth and it is mixed with salivary gland secretions which begin to break down large carbohydrate molecules.

Salivary glands

There are three, paired salivary glands, the parotid, submandibular and sublingual glands. Each gland is invested by a connective tissue capsule, and connective tissue forms trabeculae which penetrate the glands and provide support for the blood vessels and nerves supplying the glands and for the ducts draining them. Each gland has secretory cells arranged as compound, tubuloacinar, exocrine glands (Chapter 3). The secretory acini drain into small ducts (lined by a simple cuboidal epithelium) which join together in a branching system; the larger ducts are lined by columnar epithelial cells, which may be stratified. The ducts drain the saliva into the oral cavity.
Two types of secretory cell, serous and mucous cells, are present in salivary gland acini (Fig. 12.3) and, respectively, they produce a watery secretion containing proteins which function as enzymes and a viscous mucus in which large carbohydrate complexes are major components. In addition, myoepithelial cells wrap around the acini and their contractions help to expel the secretions. The secretions assist the process of digestion by moistening and lubricating the food, and by providing enzymes. The main enzyme is amylase, which breaks down large carbohydrate molecules. Saliva is also responsible for lubricating and cleansing the oral cavity. It contains bactericidal substances such as lysozyme secreted by serous cells and may contain immunoglobulin A secreted by plasma cells.

Parotid glands

The parotid glands are the largest of the salivary glands. Each parotid gland is located close to an ear, and each duct drains into the oral cavity near the ipsilateral second upper molar tooth. Serous cells line the acini and form the majority of the parenchyma of the parotid glands (Fig. 12.4). Adipose cells appear in the parotid glands with age. Many small ducts are distributed throughout the parenchyma of parotid glands and they are lined by epithelial cells which stain readily with eosin (Fig. 12.4).

Submandibular glands

A submandibular gland lies beneath the mandible on each side, wrapped around the muscle which supports the tongue. Each gland drains via a duct that opens on the ipsilateral side of the ridge (the frenulum) on the undersurface of the tongue. Mucous glands are a prominent feature of submandibular glands, but serous cells are also present. The arrangement of the serous and mucous gland cells together in some acini is such that these are described as mucous glands with serous demilunes (Fig. 12.3).

Sublingual glands

The sublingual glands are the smallest of the salivary glands. Mucous gland cells are predominant in sublingual glands and the ducts open directly into the floor of the oral cavity, rather than via a single duct system.

Tongue

The tongue is covered by a stratified, non-keratinised, squamous epithelium which on the undersurface is similar to the epithelium lining the mouth. The upper surface of the tongue is divided by a ‘V’-shaped groove into an anterior (two-thirds) and a posterior region developed from different parts of the embryo (see Mitchell B, Sharma R. Embryology: An Illustrated Colour Text. Elsevier: 2004). The stratified squamous epithelium on the upper surface is studded by prominent projections. In the posterior region, many of the projections are due to aggregations of lymphocytes deep to the epithelium. In addition, three distinct types of projection known as papillae are also described, and all assist in macerating food and resisting abrasion:

Filiform papillae. These are numerous on the upper surface and are in parallel rows which converge towards the midline. The surface of these papillae is covered by a keratinised, stratified squamous epithelium.
Fungiform papillae. These are covered by a stratified (non-keratinised) squamous epithelium. Specialised clusters of sensory cells (taste buds) are present in the epithelium covering this type of papilla.
Circumvallate papillae. There are 8 to 12 large, circumvallate papillae visible to the unaided eye just anterior to the ‘V’-shaped groove of the tongue. They are also covered by a stratified (non-keratinised) squamous epithelium which contains taste buds.
Deep to the mucosa of the tongue there are numerous, small serous and mucous glands which secrete onto the surface of the epithelium. The major component of the inner part of the tongue is skeletal muscle (see Fig. 5.7). The bundles of muscle cells are arranged in a complex three-dimensional meshwork and coordinated contraction of the muscle cells is important in chewing, swallowing and in sound production.

Pharynx and oesophagus

Swallowed food and drink pass via the pharynx to the oesophagus (gullet). The structure and function of the epithelium lining the component parts of the pharynx are described in Chapter 11. Aggregations of lymphoid cells are a prominent feature of the walls of the pharynx and form tonsils (Fig. 12.5). The lymphoid cells are important components of the immune mechanisms defending the alimentary canal (and the respiratory tract).
The major part of the oesophagus is in the thorax, but a small segment lies in the abdominal cavity and is continuous with the stomach. The structures in the walls of the oesophagus conform to the general plan for the gastrointestinal tract (Fig. 12.2). In the thorax, the connective tissue of the adventitia binds the muscularis externa of the oesophagus to adjacent structures. The muscularis externa consists of an outer, approximately longitudinal layer and an inner circular layer. In the upper part of the muscularis externa, striated voluntary muscle (Fig. 12.6) is present, whereas in the lower region it is replaced by smooth involuntary muscle. In the middle region there is a transition between the two types of muscle. Coordinated contraction of muscle cells in the different regions of the oesophagus ensures that swallowed food and drink normally pass to the stomach. In some regions, the submucosal tissue of the oesophagus contains serous and mucous glands in addition to blood vessels, nerves and aggregations of lymphoid cells. The epithelium of the mucosa lining the oesophagus is stratified and squamous (Fig. 12.6) and is able to resist friction from swallowed food and chemical attack by swallowed drinks, e.g. alcohol.

The gastrointestinal tract

Stomach

The stomach is a dilated portion of the gastrointestinal tract. It is described as having four regions: the cardiac region, the fundus, the body and the pylorus (Fig. 12.1). The cardiac region is continuous with the lower end of the oesophagus. The fundus is the part of the stomach lying closest to the diaphragm and is a site, in humans, where gas collects. The body of the stomach is the largest part of the stomach and the pylorus connects the body of the stomach to the duodenum. The stomach produces 2–3 litres/day of gastric secretions (juices) and the main constituents are mucus, water, hydrochloric acid and enzymes able to digest carbohydrates, fats and proteins. The mixture of gastric juices and ingested food and drink is known as chyme. The structure of the stomach conforms to the general plan (Fig. 12.2) with some modifications which relate to its functions.
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