The Digestive System PDF

The digestive system the The digestive system is also known as the gastrointestinal system or the alimentary canal. This vast system is approximately 10 m long. It travels the length of the body from mouth through the thoracic, abdominal and pelvic cavities, where it ends at the anus. The...

The digestive system the The digestive system is also known as the gastrointestinal system or the alimentary canal. This vast system is approximately 10 m long. It travels the length of the body from mouth through the thoracic, abdominal and pelvic cavities, where it ends at the anus. The digestive system consists of the main digestive system structures and the accessory organs. The main digestive system structures include the mouth, pharynx, oesophagus, stomach, small intestine and large intestine. Accessory organs also contribute to the function of the digestive system. The accessory organs are the salivary glands, the liver, the gallbladder and the pancreas. The activity of the digestive system The activity of the digestive system can be categorised into five processes: Ingestion: taking food into the digestive system. Propulsion: moving the food along the length of the digestive system. Digestion: breaking down food. This can be achieved mechanically as food is chewed or moved through the digestive system, or chemically by the action of enzymes mixed with the food as it moves through the digestive system. Absorption: the products of digestion exit the digestive system and enter the blood or lymph capillaries for distribution to where they are required. Elimination: the waste products of digestion are excreted from the body as faeces. The mouth (oral cavity) The teeth is grinding and tearing food. The lips and cheeks move food and mix with saliva. This process of chewing and mixing food with saliva is called mastication. The lips and cheeks are also involved in speech and facial expression. The tongue is a large, voluntary muscular structure. The superior surface contains many little projections called papillae(taste buds). The papillae contain the nerve endings responsible for the sense of taste. Other functions of the tongue include swallowing (deglutition), moving food around the oral cavity and speech. Salivary glands The salivary glands are exocrine glands that produce saliva through a system of ducts. Humans have 3 paired major salivary glands: Parotid Submandibular Sublingual as well hundreds of minor salivary glands. In health, approximately 1–1.5 L of saliva are secreted daily. Saliva consists of: Water, salivary amylase, mucus, mineral salts, lysozyme, immunoglobulins and blood clotting factors. Saliva has 3 digestive enzymes : Salivary Amylase – it is a carbohydrate – digestive enzyme. It acts on cooked or boiled starch and converts into dextrin and maltose. Maltase – It is present only in traces in human saliva and it converts maltose into glucose Lingual Lipase – lingual lipase is a lipid digesting (lipolytic) enzyme It is secreted from serous glands situated on the posterior aspect of the tongue It digests milk fats. It hydrolyses triglycerides into fatty acids. The other functions of saliva are moisten and lubricate food, maintain moisture of the oral cavity, Lysozyme has an antibacterial action. Immunoglobulin and clotting factors also contribute to the prevention of infection. Pharynx The pharynx consists of three parts: the oropharynx, the nasopharynx and the laryngopharynx. It is a passage for air and food. Oesophagus esophagus is a muscular tube about 25 cm connecting the throat (pharynx) with the stomach. It end with oesophageal sphincter (cardiac sphincter) that regulates the movement of substances from the oesophagus to the stomach. Swallowing (deglutition) Once ingested food has been adequately chewed and formed into a bolus it is ready to be swallowed. Swallowing (deglutition) occurs in three phases : 1. The voluntary phase: During this phase the action of the voluntary muscles of the oral cavity manipulates the food bolus into the oropharynx. 2. The pharyngeal phase: During this phase a reflex action is initiated in response to the sensation of the food bolus in the oropharynx. 3. The oesophageal phase: The food bolus moves from the pharynx into the esophagus. This wave of muscle contraction is known as peristalsis. The structure of the digestive system There are four layers of tissue or tunicas that exist throughout the length of the digestive tract from oesophagus to anus. 1.The mucosa is the innermost layer. The mucosa consists of three layers: the mucous epithelium (mucous membrane), which is involved in the secretion of mucus and other digestive system secretions, protection and absorption of the products of digestion. the lamina propria, which consists of loose connective tissue that has a role in supporting the blood vessels and lymphatic tissue of the mucosa. the muscularis mucosa and consists of a thin smooth muscle layer that helps to form the gastric pits or the microvilli of the digestive system. 2. The submucosa is a thick layer of connective tissue. It contains blood and lymphatic vessels and some small glands. It also contains Meissner’s plexus – nerves that stimulate the intestinal glands to secrete their products. 3. The muscularis consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle. Blood and lymph vessels and the myenteric plexus (a network of sympathetic and parasympathetic nerves) are located between the two layers of smooth muscle. The wave-like contraction and relaxation of this muscle layer are responsible for moving food along the digestive tract – a process known as peristalsis 4. The serosa (adventitia) is The outer layer of the digestive tract. Stomach Is a J- like muscular structure lie between oesophagus and duodenum and has two sphincter:  Cardiac sphincter which lie between stomach and oesophugus  Pyloric sphincter lie between stomach and duodenum. Stomach consists of 4 regions : 1. Cardiac region 2. fundus (dom shape) 3. body (greater and lesser curvature) 4. pyloric region The vagus nerve innervates the stomach with parasympathetic fibres that stimulate gastric motility and the secretion of gastric juice. Sympathetic fibres from the celiac plexus reduce gastric activity. The muscularis contains three layers of smooth muscle instead of two. It has longitudinal, circular and oblique muscle fibres. A very full stomach can contain approximately 4 L, while an empty stomach contains only about 50 mL.  The mucosa of stomach contains many gastric glands that secrete many different substances : Surface mucous cells produce thick bicarbonate‐coated mucus. This thick layer of mucus protects the stomach mucosal epithelia from corrosion by acidic gastric juice. Mucous neck cells also secrete mucus Parietal cells produce hydrochloric acid and intrinsic factor. Intrinsic factor is necessary for the absorption of vitamin B12. Chief cells produce pepsinogen, which is converted to pepsin in the presence of hydrochloric acid. Pepsin is necessary for the breakdown of protein into smaller peptide chains. Enteroendocrine cells, such as G cells, produce a variety of hormones, including gastrin. These hormones help regulate gastric motility. Regulation of gastric juice secretion is divided into three phases: 1. The cephalic phase: The sight, taste or smell of food stimulates the secretion of gastric juice. 2. The gastric phase: When food enters the stomach, the hormone gastrin is secreted into the bloodstream, and this stimulates the secretion of gastric juice. 3. The intestinal phase: As the acidic contents of the stomach enter the duodenum of the small intestine the hormones secretin and cholecystokinin (CKK) are secreted. These hormones also act to reduce the secretion of gastric juice and gastric motility. The functions of the stomach are: to act as a store for food the production of mucus to protect the stomach mechanical digestion, by the churning action facilitated by an additional layer of smooth muscle the mixing food with hydrochloric acid to help eradicate pathogens and denature proteins in preparation for the action of pepsin the production of chyme the production of intrinsic factor. Small intestine The small intestine is approximately 6 m long. In the small intestine food is further broken down by mechanical and chemical digestion, and absorption of the products of digestion takes place. The small intestine is divided into three parts : 1. The duodenum is approximately 25 cm long. It is the entrance to the small intestine. 2. The jejunum measures 2.5 m and is the middle part of the small intestine. 3. The ileum measures 3.5 m. It meets the large intestine at the ileocaecal valve. There are four types of cell present in the mucosa of the small intestine : The absorptive cell produces digestive enzymes and absorbs digested foods. Goblet cells secrete mucus to protect the intestine from abrasion and from the acidic chyme entering the small intestine Enteroendocrine cells produce regulatory hormones such as secretin and CKK. These hormones are secreted into the bloodstream and act on their target organs to release pancreatic juice and bile. Paneth cells produce lysozyme, which protects the small intestine from pathogens that have survived the acid conditions of the stomach. The smooth muscle activity within the small intestine continues the process of mechanical digestion. There are two types of mechanical digestion in the small intestine: Segmental contractions, which help to mix the various enzymes in the small intestine with the contents of the chyme, and peristalsis, which propels the food down the length of the small intestine as well as facilitating mixing. Chemical digestion  Within the small intestine, any carbohydrates that have not been broken down by the action of salivary amylase will be broken down by pancreatic amylase.  Bile will emulsify fat and fatty acids, making it easier for lipase (also from the pancreatic juice) to break the fats into fatty acids and glycerol.  Proteins are denatured by hydrochloric acid in the stomach. In the small intestine they are further acted upon by the enzymes trypsin, chymotrypsin and carboxypeptidase. The end product of protein digestion is tripeptidases, dipeptidases and amino acids. Function of the small intestine Production of mucus to protect the duodenum from the effects of the acidic chyme. Secretion of intestinal juice and pancreatic juice from the pancreas increase the pH of the chyme to facilitate the action of the enzymes. Bile enters the small intestine to emulsify fat so that it can be further broken down by the action of lipase. Many enzymes are secreted to complete the chemical digestion of carbohydrates, proteins and fats. Mechanical digestion is by peristalsis and segmentation, and slows down to allow adequate mixing and maximum absorption. The small intestine is structurally designed with a large surface area for maximum absorption of the products of digestion. The small intestine is where the majority of nutrients, electrolytes and water are absorbed. The pancreas The pancreas is composed of exocrine and endocrine tissue. It consists of a head, body and tail. The cells of the pancreas are responsible for making the endocrine and exocrine products: The islet cells of the islets of Langerhans produce the endocrine hormones insulin and glucagon. The acini glands of the exocrine pancreas produce 1.2–1.5 L of pancreatic juice daily. Pancreatic juice travels from the pancreas via the pancreatic duct into the duodenum at the hepatopancreatic ampulla( ampulla of vater). The cells of the pancreatic ducts secrete bicarbonate ions, which gives pancreatic juice its high pH (pH 8). This helps to neutralise acidic chyme from the stomach, thus protecting the small intestine from damage by the acidity. Additionally, the actions of amylase and lipase are most effective at the higher pH (pH 6–8). Pancreatic juice consists of:  water; mineral salts;  pancreatic amylase, which completes the digestion of carbohydrates;  lipase, used in the digestion of fat;  trypsinogen, chymotrypsinogen and procarboxypeptidase, which are released in an inactive form to protect the digestive system structures from the protein‐digesting enzymes that they become – once they enter the duodenum they are activated by enterokinase from intestinal juice and become trypsin, chymotrypsin and carboxypeptidase respectively and are then used in the digestion of protein. Two hormones regulate the secretion of pancreatic juice.  Secretin, produced in response to the presence of hydrochloric acid in the duodenum, promotes the secretion of bicarbonate ions.  CKK, secreted in response to the intake of protein and fat, promotes the secretion of the enzymes present in pancreatic juice. Parasympathetic vagus nerve stimulation also promotes the release of pancreatic juice. The liver and production of bile The liver is the body’s largest gland. It weighs between 1 and 2 kg. It lies under the diaphragm protected by the ribs. The liver occupies most of the right hypochondriac region and extends through part of the epigastric region into the left hypochondriac region. The liver produces and secretes up to 1 L of yellow/green alkaline bile per day.The function of bile is to emulsify fats. Bile is composed of: bile salts such as bilirubin from the breakdown of haemoglobin cholesterol fat‐soluble vitamins fat mineral salts mucus. The functions of the liver Apart from the production of bile and the metabolism of carbohydrate, fat and protein, the liver has many additional functions: detoxification of drugs – the liver deals with medication, alcohol, ingested toxins and the toxins produced by microbes; recycling of erythrocytes; deactivation of many hormones, including the sex hormones, thyroxine, insulin, glucagon, cortisol and aldosterone; production of clotting proteins; storage of vitamins, minerals and glycogen; synthesis of vitamin A; heat production. The gallbladder The gallbladder is a small, green, muscular sac that lies posterior to the liver. It functions as a reservoir for bile. It also concentrates bile by absorbing water. The stimulus for gallbladder contraction is the hormone CKK which secreted from the small intestine into the blood and is produced in response to the presence of fatty chyme in the duodenum. CKK stimulates the secretion of pancreatic juice and the relaxation of the hepatopancreatic sphincter. When the sphincter is relaxed, both bile and pancreatic juice can enter the duodenum. The large intestine The large intestine measures 1.5 m in length and 7 cm in diameter. It is continuous with the small intestine from the ileocaecal valve and ends at the anus. Food residue enters the caecum and has to pass up the ascending colon along the transverse colon, down the descending colon and out of the body via the rectum, anal canal and anus. The large intestine mucosa contains large numbers of goblet cells that secrete mucus to ease the passage of faeces and protect the walls of the large intestine. The food residue from the ileum is fluid when it enters the caecum and contains few nutrients. The small intestine is responsible for some of the absorption of water, but the primary function of the large intestine is to absorb water and turn the food residue into semi‐solid faeces. The large intestine also absorbs some vitamins, minerals, electrolytes and drugs. Food residue usually takes 24–48 h to pass through the large intestine; 500 mL of food residue enters the large intestine daily and approximately 150 mL leaves as faeces.