Digestive System PDF

**DIGESTIVE SYSTEM** **INTRODUCTION** The function of the **digestive system** is to change these complex organic nutrient molecules into simple organic and inorganic molecules that can then be absorbed into the blood or lymph to be transported to cells. **DIVISIONS OF THE DIGESTIVE SYSTEM** The...

**DIGESTIVE SYSTEM** **INTRODUCTION** The function of the **digestive system** is to change these complex organic nutrient molecules into simple organic and inorganic molecules that can then be absorbed into the blood or lymph to be transported to cells. **DIVISIONS OF THE DIGESTIVE SYSTEM** The two divisions of the digestive system are the alimentary tube and the accessory organs. The **alimentary tube also known as the gastrointestinal (GI) tract, this is essentially a long tube through which food passes and** extends from the mouth to the anus. It consists of the mouth or oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. Digestion takes place within the oral cavity, stomach, and small intestine; most absorption of nutrients takes place in the small intestine. Undigestible material, primarily cellulose, is eliminated by the large intestine (also called the colon). The **accessory organs** of digestion are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Digestion does not take place *within* these organs, but each contributes something *to* the digestive process. Diagram of the Digestive system **MOUTH/ORAL CAVITY** Food enters the **oral cavity** (or **buccal cavity**) by way of the mouth. The boundaries of the oral cavity are the hard and soft palates superiorly; the cheeks laterally; and the floor of the mouth inferiorly. Within the oral cavity are the teeth and tongue and the openings of the ducts of the salivary glands. The oral cavity is lined throughout with *mucous membrane*, consisting of stratified squamous epithelium containing small mucus-secreting glands. The part of the mouth between the gums and the cheeks is the *vestibule* and the remainder of its interior is the *oral cavity*. The mucous membrane lining of the cheeks and the lips is reflected onto the gums or *alveolar ridges* and is continuous with the skin of the face. The *uvula* is a curved fold of muscle covered with mucous membrane, hanging down from the middle of the free border of the soft palate. Originating from the upper end of the uvula are four folds of mucous membrane, two passing downwards at each side to form membranous arches. The posterior folds, one on each side, are the *palatopharyngeal arches* and the two anterior folds are the *palatoglossal arches*. On each side, between the arches, is a collection of lymphoid tissue called the *palatine* *tonsil*.  **Structure of the mouth** **TEETH** The teeth are embedded in the alveoli or sockets of the alveolar ridges of the mandible and the maxilla. Babies are born with two sets, or *dentitions*, the *temporary* or *deciduous teeth* and the *permanent teeth*. At birth the teeth of both dentitions are present, in immature form, in the mandible and maxilla. There are 20 temporary teeth, 10 in each jaw. They begin to erupt at about 6 months of age, and should all be present by 24 months. The permanent teeth begin to replace the deciduous teeth in the 6th year of age and this dentition, consisting of 32 teeth, is usually complete by the 21st year. **Deciduous/**temporary **teeth: 2-Incisors 1-Canine 2-Molars;** **Permanent teeth: 2-Incisors 1-Canine 2-Premolars 3-Molars** **Structure of a tooth** Although the shapes of the different teeth vary, the structure is the same and consists of: *the crown* -- the part that protrudes from the gum *the root* -- the part embedded in the bone *the neck* -- the slightly narrowed region where the crown merges with the root. In the centre of the tooth is the *pulp cavity* containing blood vessels, lymph vessels and nerves, and surrounding this is a hard ivory-like substance called *dentine*. The dentine of the crown is covered by a thin layer of very hard substance, *enamel*. The root of the tooth, on the other hand, is covered with a substance resembling bone, called *cementum*, which secures the tooth in its socket. Blood vessels and nerves pass to the tooth through a small foramen (hole) at the apex of each root. **Blood supply** Most of the arterial blood supply to the teeth is by branches of the *maxillary arteries*. The venous drainage is by a number of veins which empty into the *internal* *jugular veins*. **Nerve supply** The nerve supply to the upper teeth is by branches of the *maxillary nerves* and to the lower teeth by branches of the *mandibular nerves*. These are both branches of the *trigeminal* *nerves* (5th cranial nerves) **Functions of the teeth** Teeth have different shapes depending on their functions. *Incisors* and *canine* teeth are the cutting teeth and are used for biting off pieces of food, whereas the *premolar* and *molar* teeth, with broad, flat surfaces, are used for grinding or chewing food. Tooth structure. Longitudinal section of a tooth showing internal structure. **TONGUE** The **tongue** is made of skeletal muscle that is innervated by the hypoglossal nerves (12th cranial). It is attached by its base to the *hyoid bone* and by a fold of its mucous membrane covering, called the *frenulum*, to the floor of the mouth. The superior surface consists of stratified squamous epithelium, with numerous *papillae* (little projections). Many of these contain sensory receptors (specialised nerve endings) for the sense of taste in the *taste buds* **Blood supply** The main arterial blood supply to the tongue is by the *lingual branch* of the *external carotid artery*. Venous drainage is by the *lingual vein*, which joins the *internal* *jugular vein*. **Nerve supply** The nerves involved are: the *hypoglossal nerves* (12th cranial nerves), which supply the voluntary muscle the *lingual branch of the mandibular nerves*, which arise from the 5th cranial nerves, are the nerves of somatic (ordinary) sensation, i.e. pain, temperature and touch the *facial* and *glossopharyngeal nerves* (7th and 9^th^ cranial nerves), the nerves of taste. **Functions of the tongue** The tongue plays an important part in: chewing (mastication) swallowing (deglutition) speech taste. Nerve endings of the sense of taste are present in the papillae and widely distributed in the epithelium of the tongue. **SALIVARY GLANDS** The digestive secretion in the oral cavity is **saliva**, produced by three pairs of **salivary glands**. The **parotid glands** are just below and in front of the ears. The **submandibular** (also called submaxillary) glands are at the posterior corners of the mandible, and the **sublingual** glands are below the floor of the mouth. Each gland has at least one duct that takes saliva to the oral cavity. Secretion of saliva is continuous, but the amount varies in different situations. The presence of food (or anything else) in the mouth increases saliva secretion. **Parotid glands** These are situated one on each side of the face just below the external acoustic meatus. Each gland has a *parotid duct* opening into the mouth at the level of the second upper molar tooth. **Submandibular glands** These lie one on each side of the face under the angle of the jaw. The two *submandibular ducts* open on the floor of the mouth, one on each side of the frenulum of the tongue. **Sublingual glands** These glands lie under the mucous membrane of the floor of the mouth in front of the submandibular glands. They have numerous small ducts that open into the floor of the mouth. **Structure of the salivary glands** The glands are all surrounded by a *fibrous capsule*. They consist of a number of *lobules* made up of small acini lined with secretory cells. The secretions are poured into ductules that join up to form larger ducts leading into the mouth. **Blood supply** Arterial supply is by various branches from the external carotid arteries and venous drainage is into the external jugular veins. **PHARYNX** The pharynx is divided for descriptive purpose into three parts, the nasopharynx, oropharynx and laryngopharynx. The nasopharynx is important in respiration. The oropharynx and laryngopharynx are passages common to both the respiratory and the digestive systems. Food passes from the oral cavity into the pharynx then to the oesophagus below, with which it is continuous. The walls of the pharynx consist of three layers of tissue. The *lining membrane* (mucosa) is stratified squamous epithelium, continuous with the lining of the mouth at one end and the oesophagus at the other. Stratified epithelial tissue provides a lining well suited to the wear and tear of swallowing ingested food. The *middle layer* consists of connective tissue, which becomes thinner towards the lower end and contains blood and lymph vessels and nerves. The *outer layer* consists of a number of involuntary muscles that are involved in swallowing. When food reaches the pharynx, swallowing is no longer under voluntary control. **Blood supply** The blood supply to the pharynx is by several branches of the facial arteries. Venous drainage is into the facial veins and the internal jugular veins. **Nerve supply** This is from the pharyngeal plexus and consists of parasympathetic and sympathetic nerves. Parasympathetic supply is mainly by the glossopharyngeal and vagus nerves and sympathetic from the cervical ganglia. **ESOPHAGUS** The oesophagus is about 25 cm long and about 2 cm in diameter and lies in the median plane in the thorax in front of the vertebral column behind the trachea and the heart. It is continuous with the pharynx above and just below the diaphragm it joins the stomach. It passes between muscle fibres of the diaphragm behind the central tendon at the level of the 10th thoracic vertebra. Immediately the oesophagus has passed through the diaphragm it curves upwards before opening into the stomach. This sharp angle is believed to be one of the factors that prevents the regurgitation (backflow) of gastric contents into the oesophagus. The upper and lower ends of the oesophagus are closed by sphincters. The upper *cricopharyngeal* or *upper oesphageal sphincter* prevents air passing into the oesophagus during inspiration and the aspiration of oesophageal contents. The *cardiac* or *lower oesophageal sphincter* prevents the reflux of acid gastric contents into the oesophagus. There is no thickening of the circular muscle in this area and this sphincter is therefore 'physiological', i.e. this region can act as a sphincter without the presence of the anatomical features. When intra-abdominal pressure is raised, e.g. during inspiration and defaecation, the tone of the lower oesophageal sphincter increases. There is an added pinching effect by the contracting muscle fibres of the diaphragm. **Structure of the oesophagus** There are four layers of tissue. As the oesophagus is almost entirely in the thorax the outer covering, the adventitia, consists of *elastic fibrous tissue* that attaches the oesophagus to the surrounding structures. The proximal third is lined by stratified squamous epithelium and the distal third by columnar epithelium. The middle third is lined by a mixture of the two. **Blood supply** **Arterial.** The thoracic region is supplied mainly by the paired oesophageal arteries, branches from the thoracic aorta. The abdominal region is supplied by branches from the inferior phrenic arteries and the left gastric branch of the coeliac artery. **Venous drainage.** From the thoracic region venous drainage is into the azygos and hemiazygos veins. The abdominal part drains into the left gastric vein. There is a venous plexus at the distal end that links the upward and downward venous drainage, i.e. the general and portal circulations. **STRUCTURAL LAYERS OF THE DIGESTIVE TRACT** The layers of the walls of the digestive tract follow a consistent pattern from the oesophagus onwards. This basic structure does not apply so obviously to the mouth and the pharynx. In the organs from the oesophagus onwards, modifications of structure are found which are associated with specific functions. The walls of the alimentary tract are formed by four layers of tissue: adventitia or serosa -- outer covering muscle layer submucosa mucosa -- lining. **Adventitia or serosa** This is the outermost layer. In the thorax it consists of *loose fibrous tissue* and in the abdomen the organs are covered by a serous membrane (serosa) called *peritoneum*. **Muscle layer** With some exceptions this consists of two layers of *smooth (involuntary) muscle*. The muscle fibres of the outer layer are arranged longitudinally, and those of the inner layer encircle the wall of the tube. Between these two muscle layers are blood vessels, lymph vessels and a plexus (network) of sympathetic and parasympathetic nerves, called the *myenteric plexus.* These nerves supply the adjacent smooth muscle and blood vessels. Contraction and relaxation of these muscle layers occurs in waves, which push the contents of the tract onwards. This type of contraction of smooth muscle is called *peristalsis* and is under the influence of sympathetic and parasympathetic nerves. Muscle contraction also mixes food with the digestive juices. Onward movement of the contents of the tract is controlled at various points by *sphincters*, which are thickened rings of circular muscle. Contraction of sphincters regulates forward movement. They also act as valves, preventing backflow in the tract. This control allows time for digestion and absorption to take place. **Submucosa** This layer consists of loose areolar connective tissue containing collagen and some elastic fibres, which binds the muscle layer to the mucosa. Within it are blood vessels and nerves, lymph vessels and varying amounts of lymphoid tissue. The blood vessels are arterioles, venules and capillaries. The nerve plexus is the *submucosal plexus*, which contains sympathetic and parasympathetic nerves that supply the mucosal lining. **Mucosa** This consists of three layers of tissue: *mucous membrane* formed by columnar epithelium is the innermost layer, and has three main functions: *protection*, *secretion* and *absorption* *lamina propria* consisting of loose connective tissue, which supports the blood vessels that nourish the inner epithelial layer, and varying amounts of lymphoid tissue that protects against microbial invaders *muscularis mucosa*, a thin outer layer of smooth muscle that provides involutions of the mucosal layer, e.g. gastric glands, villi. **Mucous membrane** In parts of the tract that are subject to great wear and tear or mechanical injury, this layer consists of *stratified squamous* *epithelium* with mucus-secreting glands just below the surface. In areas where the food is already soft and moist and where secretion of digestive juices and absorption occur, the mucous membrane consists of *columnar* *epithelial cells* interspersed with mucus-secreting goblet cells. Mucus lubricates the walls of the tract and provides a physical barrier that protects them from the damaging effects of digestive enzymes. Below the surface in the regions lined with columnar epithelium are collections of specialised cells, or glands, which release their secretions into the lumen of the tract. The secretions include: *saliva* from the salivary glands *gastric juice* from the gastric glands *intestinal juice* from the intestinal glands *pancreatic juice* from the pancreas *bile* from the liver. These are *digestive juices* and most contain enzymes that chemically break down food. Under the epithelial lining are varying amounts of lymphoid tissue that provide protection against ingested microbes. **Nerve supply** The digestive tract and its related accessory organs are supplied by nerves from both divisions of the autonomic nervous system, i.e. both parasympathetic and sympathetic parts. Their actions are generally antagonistic to each other and at any particular time one has a greater influence than the other, according to body needs, at that time. When digestion is required, this is normally through increased activity of the parasympathetic nervous system.