The Digestive System

Systems of the Body Series

Margaret E. Smith, Dion G. Morton

• Publisher: Churchill Livingstone
• Year: 2011
• eBook ISBN: 9780702048418
• Edition: 2
• Topic: Medizin
• Subtopic: Medizinische Theorie, Praxis & Referenz

This is an integrated textbook on the digestive system, covering the anatomy, physiology and biochemistry of the system, all presented in a clinically relevant context appropriate for the first two years of the medical student course.- One of the seven volumes in the Systems of the Body series.- Concise text covers the core anatomy, physiology and biochemistry in an integrated manner as required by system- and problem-based medical courses.- The basic science is presented in the clinical context in a way appropriate for the early part of the medical course.

1. OVERVIEW OF THE DIGESTIVE SYSTEM

Chapter objectives

After studying this chapter you should be able to:

1. Understand the key mechanisms of secretion, absorption and motility in the gastrointestinal system.

2. Understand the coordinated and integrated functioning of the digestive system.

3. Understand how function of the digestive system depends on other systems, such as the cardiovascular system.

Introduction: overall function of the digestive system

The cells of the body require adequate amounts of raw materials for their energy requiring and synthetic processes. The raw materials are obtained from the external environment through the ingestion of food. The overall function of the digestive system is to transfer the nutrients in food from the external environment to the internal environment, where they can be distributed to the cells of the body via the circulation. In this chapter, the general principles and basic mechanisms involved in the functioning of the digestive system will be discussed in the context of the system as a whole. The importance of the integration of the digestive system with the other body systems is well illustrated by the problems encountered in non-occlusive ischaemic disease of the gut: a condition in which the defect originates in the vascular system, but serious consequences result from abnormal absorption in the small intestine (see Case 1.1 and Case 1.1).

Case 1.1

Non-occlusive ischaemic disease of the gut: 1

An elderly patient, who was being treated with digitalis for congestive heart failure, suddenly developed severe, constant, abdominal pain. The consultant physician examined him and found that he was in circulatory shock, with a low arterial blood pressure, a thready pulse and a sinus tachycardia (rapid heart rate). His abdomen was exquisitely tender to palpitation, with diffuse peritonism (tenderness). The physician suspected from the clinical findings that the patient was suffering from non-occlusive ischaemic disease of the gut. In this condition, the decreased cardiac output results in decreased intestinal perfusion and this, together with other mechanisms, results in the flow of blood to the gastrointestinal tissues being cut off. This disease is often fatal.

Upon consideration of the details of this case, we can ask the following questions:

• What are the main causes of the sudden development of this condition in patients with cardiac failure?

• What are the physiological consequences of reduced flow of blood for the functioning of the small intestine?

• What is the origin of the patient’s pain?

• How are the normal homeostatic mechanisms which control the flow of blood to the gastrointestinal tract perturbed in this condition?

• How can this patient be treated?

Case 1.1

Non-occlusive ischaemic disease of the gut: 2

Defect, diagnosis and treatment

Decreased cardiac output results in decreased intestinal perfusion with blood. As the velocity of flow decreases, the viscosity of the blood increases and the blood tends to stagnate in the small vessels. Then microthrombi develop and disseminate in the blood vessels of the mesenteric circulation. There is also a generalized vasoconstriction of the blood vessels that diverts the arterial flow to essential organs. This causes small vessels to collapse. The consequent increase in resistance to flow in the splanchnic circulation, together with the decreased cardiac output and reduced arterial blood pressure results in severely reduced blood flow to the intestines, which eventually become ischaemic.

Reduced blood flow to the gastrointestinal tract results in lack of oxygen and reduced energy substrate supply to the tissues (hypoxia). The result is widespread necrosis of the gastrointestinal mucosa which is most sensitive to hypoxia. This quickly leads to disruption of its functions. The necrosis starts at the tips of the villi that become hypoxic first. It seems probable also that disruption of the brush border of the enterocytes exposes the underlying tissue to the effects of the digestive proteolytic enzymes in the lumen. The intestines become permeable to toxic substances from the contents of the gut lumen, such as bacteria and bacterial toxins, and toxic substances from the necrotic cells. These substances enter the portal circulation. In summary, the barrier function of the gut is lost. There is a profound toxaemia and impairment of the normal body defences, resulting in septic shock. Loss of fluid, electrolytes and blood from the gut will also occur. (This effect mirrors loss in the skin in burns). The loss of the external barrier allows penetration of bacteria into the body as well as fluid loss from it.

The abdominal pain is due to the inflammatory response to ischaemia that accompanies the necrosis. The abdominal tenderness (peritonism) is due to transmural ischaemia of the intestinal wall, which in turn results in secondary inflammation of the parietal peritoneum. Differentiation of this condition from occlusive arterial disease is difficult. Selective angiography, a technique involving the introduction of a radio-opaque substance into the blood, followed by X-radiography, may show narrowed and irregular branches of the superior mesenteric artery, and impaired filling of intramural vessels. In contrast, occlusive disease (such as an embolus) would more often be associated with loss of blood flow to major branches of the mesenteric arteries.

Management of this condition requires measures to maintain the cardiac output, blood pressure, and tissue oxygenation, treatment of infection, and replacement of fluid and electrolytes lost from the gastrointestinal tract. Surgery for heart failure is not safe in the presence of gut infarction. If peritonitis is present, abdominal surgery is required to remove the necrotic intestinal tissue.

Components of the digestive system

Figure 1.1 illustrates the component organs of the gastrointestinal tract, and the associated organs that are essential for the functioning of the digestive system. The gastrointestinal tract consists of the mouth, oesophagus, stomach, small intestine and large intestine. The food is taken into the mouth and moved into the pharynx by the activity of skeletal muscle, then along the rest of the tract by the activity of smooth muscle. The food material is brought to an appropriate semi-fluid consistency, and the nutrients in it are dissolved and degraded by secretions that enter the tract at different locations. These processes are aided by the contractions of the muscles that serve to mix the secretions with the food.

Fig. 1.1 The digestive system and associated exocrine glands.

The associated organs situated outside the gastrointestinal tract that are essential for the digestive process are exocrine glands that secrete important digestive juices. These are as follows:

• The three pairs of salivary glands which produce saliva which has a range of functions, but most importantly it provides lubrication of the upper gastrointestinal tract to allow the food to be moved along

• The exocrine pancreas which secretes pancreatic juice which contains most of the important digestive enzymes required to degrade the food into molecules which can be absorbed

• The exocrine liver which produces bile, a secretion which is important for fat digestion and absorption. The bile is also a medium for the excretion of waste metabolites and drugs.

Saliva is released into the mouth. Pancreatic juice and bile enter the duodenum in the upper small intestine (Fig. 1.1). The release of these juices is stimulated when a meal is present in the gastrointestinal tract.

Physiological processes of the digestive system

The physiological processes that are important for the functioning of the digestive system are:

• Digestion

• Absorption

• Motility

• Secretion (and excretion).

Digestion

Digestion is the process whereby large molecules are broken down to smaller ones. Food is ingested as large pieces of matter, containing high molecular weight substances such as protein and starch that are unable to cross the cell membranes of the gut epithelium. Before these complex molecules can be utilized they are degraded to smaller molecules, such as glucose and amino acids.

Absorption

The mixture of ingested material and secretions in the gastrointestinal tract contains water, minerals and vitamins as well as complex nutrients. The products of digestion and other small molecules and ions and water are transported across the epithelial cell membranes, mainly in the small intestine. This is the process of absorption. The transported molecules enter the blood or lymph for circulation to the tissues. This process is central to the digestive system, and the other physiological processes of the gastrointestinal tract subserve it.

Motility

The gastrointestinal tract is a tube of variable diameter, approximately 15 feet long in living human adults. It extends through the body from the mouth to the anus. The food must be moved along it to reach the appropriate sites for mixing, digestion and absorption. Two layers of smooth muscle line the gastrointestinal tract, and contractions of this muscle mix the contents of the lumen and move them through the tract. The process of motility is under the control of nerves and hormones.

Secretion

Exocrine glands secrete enzymes, ions, water, mucins and other substances into the digestive tract. The glands are situated within the gastrointestinal tract...

Table of contents

• Cover image
• Table of Contents
• Front-matter
• Copyright
• PREFACE
• ACKNOWLEDGEMENTS
• 1. OVERVIEW OF THE DIGESTIVE SYSTEM
• 2. THE MOUTH, SALIVARY GLANDS AND OESOPHAGUS
• 3. THE STOMACH
• 4. THE STOMACH
• 5. PANCREAS
• 6. LIVER AND BILIARY SYSTEM
• 7. THE SMALL INTESTINE
• 8. DIGESTION AND ABSORPTION
• 9. THE ABSORPTIVE AND POST-ABSORPTIVE STATES
• 10. THE COLON
• 11. GASTROINTESTINAL PATHOLOGY
• GLOSSARY
• Index