The Gastrointestinal System at a Glance

Preface

Organization of the Book

The Gastrointestinal System at a Glance is organized in four parts, each starting with a structural and functional overview of the main components of the system and followed by chapters dealing with integrated gastrointestinal function. The clinical relevance of aspects of anatomy, physiology and function is discussed in each chapter in order to highlight the practical importance of each subject. The third and fourth sections are more clinical, covering the most important gastrointestinal and hepatobiliary diseases and the major aspects of diagnosis and treatment. Endoscopy and radiology are described in dedicated chapters. Self-assessment questions on the accompanying website are all based on the text, and can be used to check understanding and recall.

How to Use This Book

This book offers a visual and graphic scaffold for further detailed study. The aim is to provide pictures that will illustrate concepts and make them more memorable. Thus, the book can be read before starting on coursework, annotated with additional details from lectures, tutorials and self-directed study, and then used for revision before examinations. It will therefore be useful for students approaching a subject for the first time, particularly as part of an integrated systems-based medical curriculum. The diagrams, many of which will also be available as online flashcards, should trigger recall of facts that might otherwise be lost in plain text.

Anatomical and Clinical Detail

The anatomical diagrams are representations, and not exact reproductions, to illustrate how structure supports function, rather than to provide exact detail. For more thorough anatomy, students may use Anatomy at a Glance, also available in this series. Similarly, specific diseases are discussed to demonstrate pathogenic mechanisms and general principles, rather than to provide exhaustive detail. This book should be used to understand the normal physiology, how it goes wrong in disease, and the principles underlying modern clinical practice in gastroenterology and hepatology.

Satish Keshav

Adam Bailey

Acknowledgements

We thank all the staff at Wiley-Blackwell Publishing, particularly Martin Sugden, Fiona Pattision, Ben Townsend, Martin Davies, and Karen Moore, who encouraged us through the gestation of this edition. Professor Darrell Evans of Brighton and Sussex Medicine School co-authored the chapter on Embryology, for which we are grateful.

List of Abbreviations

Introduction and Overview

Structure and Function

The gastrointestinal system comprises the hollow organs from mouth to anus that form the gastrointestinal tract, the pancreas, which mainly secretes digestive juices into the small intestine, and the liver and biliary system, which perform vital metabolic functions in addition to their contribution to digestion and the absorption of nutrients.

The Intestinal Tract

A hollow tubular structure into which nutrient-rich food is coerced, and from which wastes are expelled, this is found in the most primitive multicellular organisms, from the hydra onwards. In humans, the tract is highly specialized throughout, both structurally and functionally. The mouth and teeth are the first structures in this tract and are connected by a powerful muscular tube, the oesophagus, to the stomach. The stomach stores food after meals and is the site where major digestive processes commence. The small intestine is the main digestive and absorptive surface. The large intestine acts mainly as a reservoir for food waste and allows reabsorption of water from the mainly liquid material leaving the small intestine; it can be affected by a number of common, serious diseases, such as inflammatory bowel disease and colorectal cancer.

The Pancreas

Digestive enzymes are produced in many parts of the gastrointestinal tract, including the mouth (salivary glands) and small intestine (enterocytes), although the exocrine pancreas is the most prodigious producer of digestive enzymes. Pancreatic failure causes malabsorption, which can be reversed by artificial enzyme supplements.

The Liver and Biliary System

Without the liver, survival is measured in hours, and no artificial system has yet been devised to substitute for hepatic function. The liver is the largest solid organ in the body, and its essential functions include regulation of protein, fat and carbohydrate metabolism, synthesis of plasma proteins, ketones and lipoproteins, and detoxification and excretion. Via the hepatic portal circulation, it receives and filters the entire venous drainage of the spleen, gastrointestinal tract and pancreas. Through the production of bile, it is also essential for digestion and absorption, particularly of dietary fats and fat-soluble vitamins.

Integrated Function

The gastrointestinal system is controlled by both intrinsic and extrinsic neuronal and endocrine mechanisms. Enteric nerves and endocrine cells are particularly important in coordinating motility, digestion and absorption, and in regulating feeding and overall nutrition, including the control of body weight.

The gastrointestinal system presents a huge surface area that has to be protected against injury, particularly from microbial pathogens that are ingested with food and from the large, diverse population of commensal bacteria that populate the intestine. Estimates of the total number of species of bacteria vary from 500 to 1000, and may be greater. In faeces, the number of bacteria is huge, 10⁸ to 10¹⁰ per gram, so that the total number of bacterial cells in the body may be approximately 10¹³. The mucosal immune system is critically important in regulating how the intestine responds to these challenges, providing protection and not reacting inappropriately to normal components of the diet.

Diseases and Disorders

Nausea, vomiting, diarrhoea and constipation are common symptoms, and their basic pathophysiology illustrates important aspects of gastrointestinal function.

Gastrointestinal symptoms are frequently not associated with any discernible pathological abnormality. These medically unexplained symptoms are often labelled functional disorders and, as our understanding of gastrointestinal physiology becomes more sophisticated, we may discover new explanations and treatments that are more effective.

Gastrointestinal system infections are common and are associated with significant morbidity and mortality worldwide. They range from self-limiting food poisoning to life-threatening local and systemic infections. Even peptic ulceration is most frequently caused by infection, with the Helicobacter pylori bacterium.

For some major diseases, such as inflammatory bowel disease, the aetiological agent has not been identified, despite rapidly advancing genetic and molecular research. Conversely, coeliac disease, another serious and common gastrointestinal inflammatory disease, is caused by a well-characterized immune response to wheat-derived proteins.

Colon cancer is a major cause of cancer-related death, and our molecular and cellular understanding of its pathogenesis, and the pathophysiology of other gastrointestinal, pancreatic and liver tumours, is rapidly increasing.

Liver damage is often caused by infections or drugs and may be acute or chronic. Acute liver disease can rapidly progress to liver failure, or can resolve, either spontaneously or with appropriate treatment. Chronic liver disease may cause cirrhosis, which is characterized by a variety of signs and symptoms and changes throughout the body, including the effects of hepatic portal venous hypertension.

The gastrointestinal system is essential to nutrition, and disordered nutrition is a major issue worldwide – both through undernutrition and starvation and through overnutrition, which causes obesity, possibly the single most important modern health problem in the affluent world.

Diagnosis and Treatment

Clinical assessment, including a focused history and examination, is the foundation of diagnosis. In addition, the gastrointestinal system can be investigated by endoscopy, radiology and specific functional tests. Endoscopy and radiology may also be used therapeutically, and pharmacotherapy and surgery for gastrointestinal disorders exploit many unique features of the structure and function of the system.

1

Mouth and Teeth

The mouth and teeth admit food into the gastrointestinal tract. They cut and break large pieces, chop, grind and moisten what can be chewed, and prepare a smooth, round bolus that can be swallowed and passed on to the rest of the system. Of course, the lips and mouth also serve other functions.

Structure

The sensitive, flexible, muscular lips that form the anterior border of the mouth can assess food by palpation, and their flexibility enables them to seal off the oral cavity and form variously a funnel, suction tube or shallow ladle to ingest fluids and food of varying consistency. The main muscles of the lips are orbicularis ori.

The maxilla and mandible support the roof and floor of the mouth, respectively. The arch of the mandible supports a sling of muscles that forms the floor, including the tongue. The maxilla is continuous with the rest of the skull and forms the roof of the mouth anteriorly and, simultaneously, the floor of the nasal cavity and paranasal maxillary sinus. Posteriorly, the roof is formed by the soft palate, composed of connective tissue.

The sides of the mouth comprise the cheek muscles, chiefly the buccinator, and supporting connective tissue. Posteriorly, the oral cavity opens into the oropharynx, and the tonsils are situated between the fauces laterally, marking the posterior limit of the oral cavity.

The entire mouth, including the gingivae or gums, is lined with a tough, mainly non-cornified stratified squamous epithelium, which changes to skin (cornified stratified squamous epithelium) at the vermillion border of the lips.

Teeth arise in the alveolar bone of the mandible and maxilla. Infants are born without external teeth and with precursors within the jaw. A transient set of 20 ‘milk’ teeth erupts through the surface of the bone between 6 months and 3 years of age. They are shed between 6 and 13 years of age, and permanent teeth take their place. There are 32 permanent teeth and the most posterior molars, also known as wisdom teeth, may only erupt in young adulthood.

Teeth are living structures with a vascular and nerve supply (derived from the trigeminal, or Vth cranial, nerve) in the centre of each tooth, which is termed the pulp. Surrounding the pulp is a bony layer called dentine, and surrounding this is an extremely hard, calcified layer called cementum within the tooth socket, the enamel crown protruding into mouth. Teeth lie in sockets within the alveolar bone, and the joint is filled with a layer of tough fibrous tissue (the periodontal membrane) allowing a small amount of flexibility. The margins of the tooth joint are surrounded by gingivae, which are a continuation of the mucosal lining of the mouth.

Function

The lips, cheeks and tongue help to keep food moving and place it in the optimal position for effective chewing. The main muscles of chewing or mastication are the masseter and temporalis, which powerfully bring the lower jaw up against the upper jaw, and the pterygoids, which open the jaws, keep them aligned, and move them sideways, and backwards, and forwards for grinding. The trigeminal (Vth cranial) nerve controls the muscles of mastication.

Teeth are specialized for different tasks as follows:

Incisors have flat, sharp edges for cutting tough foods, such as meat and hard fruits.

Canines have pointed, sharp ends for gripping food, particularly meat, and tearing pieces away.

Premolars and molars have flattened, complex surfaces that capture tiny bits of food, such as grains, and allow them to be crushed between the surfaces of two opposed teeth. As people get older, the grinding surfaces of the molars are gradually worn down.

Certain drugs can be absorbed across the oral mucosa and may be prescribed sublingually (under the tongue). In this way, the need to swallow is avoided and the absorbed drug bypasses the liver and avoids hepatic first-pass metabolism. Glyceryl trinitrate is one of the most common drugs administered in this way.

Common Disorders

Herpes simplex infection of the mouth is very common, causing cold sores, which often erupt on the lips when people have other illnesses. Serious oral infections, usually caused by a mixture of anaerobic bacteria, are less common.

The corners of the mouth may be ulcerated or fissured in patients who cannot take care of their mouths, for example after a stroke, so careful oral hygiene is important in these cases. Nutritional deficiency, particularly of B complex vitamins and iron, is also associated with fissures at the edge of the mouth, known as angular stomatitis.

Shallow ‘aphthous’ ulcers in the mouth are common and are usually not associated with a more serious condition. Rarely, squamous cell carcinoma can develop in the mouth. Risk factors for this include smoking and chewing tobacco or betel nut, which is particularly common on the Indian subcontinent.

Dental caries is the commonest disorder of teeth, resulting in tooth loss with advancing age. It is caused by the action of bacteria, producing acids that demineralize the teeth. There is also infection of the gums and periodontal membrane, encouraged by carbohydrate and sugar-rich food residues left in the mouth. Bacteria grow in the gap between the tooth enamel and gums, forming a layer called plaque, within which they multiply. Their metabolic products, including organic acids, damage tooth enamel. Gradual erosion of enamel and retraction of