Anatomy and Physiology of the Gastrointestinal System
The gastrointestinal tract (GIT) consists of a hollow muscular tube starting from the oral cavity, where food enters the mouth, continuing through the pharynx, oesophagus, stomach and intestines to the rectum and anus, where food is expelled. There are various accessory organs that assist the tract by secreting enzymes to help break down food into its component nutrients. Thus the salivary glands, liver, pancreas and gall bladder have important functions in the digestive system. Food is propelled along the length of the GIT by peristaltic movements of the muscular walls.
The primary purpose of the gastrointestinal tract is to break food down into nutrients, which can be absorbed into the body to provide energy. First food must be ingested into the mouth to be mechanically processed and moistened. Secondly, digestion occurs mainly in the stomach and small intestine where proteins, fats and carbohydrates are chemically broken down into their basic building blocks. Smaller molecules are then absorbed across the epithelium of the small intestine and subsequently enter the circulation. The large intestine plays a key role in reabsorbing excess water. Finally, undigested material and secreted waste products are excreted from the body via defecation (passing of faeces). In the case of gastrointestinal disease or disorders, these functions of the gastrointestinal tract are not achieved successfully. Patients may develop symptoms of nausea, vomiting, diarrhoea, malabsorption, constipation or obstruction. Gastrointestinal problems are very common and most people will have experienced some of the above symptoms several times throughout their lives.
Individual Components of the Gastrointestinal System
The oral cavity or mouth is responsible for the intake of food. It is lined by a stratified squamous oral mucosa with keratin covering those areas subject to significant abrasion, such as the tongue, hard palate and roof of the mouth. Mastication refers to the mechanical breakdown of food by chewing and chopping actions of the teeth. The tongue, a strong muscular organ, manipulates the food bolus to come in contact with the teeth. It is also the sensing organ of the mouth for touch, temperature and taste using its specialised sensors known as papillae. Insalivation refers to the mixing of the oral cavity contents with salivary gland secretions. The mucin (a glycoprotein) in saliva acts as a lubricant. The oral cavity also plays a limited role in the digestion of carbohydrates. The enzyme serum amylase, a component of saliva, starts the process of digestion of complex carbohydrates. The final function of the oral cavity is absorption of small molecules such as glucose and water, across the mucosa. From the mouth, food passes through the pharynx and oesophagus via the action of swallowing.
Three pairs of salivary glands communicate with the oral cavity. Each is a complex gland with numerous acini lined by secretory epithelium. The acini secrete their contents into specialised ducts. Each gland is divided into smaller segments called lobes. Salivation occurs in response to the taste, smell or even appearance of food. This occurs due to nerve signals that tell the salivary glands to secrete saliva to prepare and moisten the mouth. Each pair of salivary glands secretes saliva with slightly different compositions.
The parotid glands are large, irregular shaped glands located under the skin on the side of the face. They secrete 25% of saliva. They are situated below the zygomatic arch (cheekbone) and cover part of the mandible (lower jaw bone). An enlarged parotid gland can be easier felt when one clenches their teeth. The parotids produce a watery secretion which is also rich in proteins. Immunoglobins are secreted help to fight microorganisms and a-amylase proteins start to break down complex carbohydrates....
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