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Specific learning outcomes
•Describe the embryonic development and gross anatomy of the gastrointestinal tract and accessory organs.
•Describe the general histology and physiological functions of the gastrointestinal tract and accessory organs.
•Identify and describe the neuroendocrine systems involved in the control and regulation of mechanical and chemical digestion.
•Discuss the neuroendocrine interface and its role in control and regulation of digestion.
Overview of the digestive system
The human body is made up of trillions of cells, each of which requires access to nutrients to survive. These include carbohydrates, proteins, fats, vitamins and minerals. These nutrients provide energy for metabolic processes and are the building blocks for cell growth and repair, existing within the food we eat (macromolecules). For the cells to be able to gain access to these nutrients, the food material must be broken down into much smaller micromolecules (amino acids, fatty acids, glucose) that are absorbed via the small intestine and transported via the circulatory system to the cells. Conversion of large molecules to small molecules is achieved through the action of two interrelated processes: mechanical digestion and chemical digestion. This is carried out by the gastrointestinal tract, which can be defined as a hollow tube running from the mouth to the anus.
•Mechanical digestion is the use of muscles to mix and break up food to increase the surface area for the action of enzymes. Mechanical digestion starts with chewing where the teeth and the tongue break up the food material. This is assisted by saliva that helps dissolve the nutrients within the food. Mechanical digestion continues in the stomach where a churning action, generated by smooth muscle contractions, pushes the gastric contents in different directions. Mechanical digestion is also assisted by the process of segmentation, which consists of localised rhythmic constrictions of the intestines which helps mix the food material with enzymes.
•Chemical digestion is provided by the action of enzymes that break the chemical bonds from complex food materials, helping to produce the small micromolecules ready for absorption. Chemical digestion begins in the mouth with the action of the enzymes amylase and lipase, continues in the stomach with the action of pepsin and is then completed in the small intestine with the assistance of enzymes from the pancreas.
The digestive system is divided into two parts: the gastrointestinal tract and the accessory organs. The gastrointestinal (GI) tract is made up of a number of organs that are directly involved in the process of digestion: mouth, pharynx, oesophagus, stomach, small intestine, large intestine and rectum (Figure 1.1). The accessory organs are a group of organs that assist in the process of digestion, and they include the teeth, tongue and gallbladder, which facilitate digestion. There are also a number of digestive glands such as the salivary glands, liver and pancreas that produce secretions that assist in the process of digestion but are not considered part of the gastrointestinal tract. This is why they are termed accessory organs (Figure 1.2). These organs are constructed of specialised cells that perform different complex functions. How the cells are specialised is dependent upon the function they perform.
Figure 1.1 General picture of GI tract.
Figure 1.2 Main organs and accessory organs.
There are a number of complex processes that take place within the gastrointestinal tract and associated accessory organs; however, most of the sections of the gastrointestinal tract will share three fundamental processes: secretion, absorption and motility. The level of specialism in each of these three areas will be dependent upon the function to be carried out:
•Secretion – mucous from goblet cells to aid protection and lubrication; enzymes from the glandular cells of the salivary glands and pancreas as well as the epithelial cells of the stomach and small intestine; hormones from the enteroendocrine cells which line the stomach and duodenum; bile from the gallbladder to aid in the digestion of fats.
•Absorption – water, electrolytes, monosaccharides (e.g. glucose), amino acids and fats from the lumen of the small intestine cross the plasma membrane into enterocytes via the processes of diffusion, co-transport and osmosis and then move into the blood circulation by diffusion.
•Motility – the movement of smooth muscle contractions during peristalsis and segmentation in the wall of the gastrointestinal tract mix and propel food material.
Introduction to the digestive system
Disorders of the gastrointestinal tract make up some of the most common illnesses experienced across the lifespan affecting individuals in infancy, childhood and adulthood. Ingestion, transportation, digestion and absorption can be affected by a number of disorders that relate to factors such as structural changes, inflammatory disorders, malabsorption and motility disorders. Any changes to the structure and function of the specialised areas of the gastrointestinal tract can also have an impact on the accessory organs that assist digestion. This book will explore normal anatomy and physiology and look at the changes associated with each stage of the lifespan from the developing embryo to old age. These developmental elements will be considered at the point where the general anatomy, physiology and function are explored for each section of the gastrointestinal tract and related accessory organs. An understanding of anatomy and physiology through the lifespan is essential to the understanding of disorders associated with the digestive system.
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