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• 1.
  Digestive System The processby which food is broken down into simple chemical compounds that can be absorbed and used as nutrients or eliminated by the body is called digestion. Carbohydrate Protein Fats Monosaccharides Amino Acids Glycerol and Fatty Acids
• 2.
  Digestive System requires: 1.Movement of food through the alimentary tract. 2. Secretion of digestive juices and digestion of food. 3. Absorption of digestive products ،water, electrolytes, and vitamins. 4. Circulation of blood to carry away absorbed Substances. 5. Nervous and hormonal control of all these functions Major Organs 1. Mouth 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine 6. Large intestine Accessory Organs 1. Teeth 2. Tongue 3. Liver 4. Pancreas 5. Gall Bladder 6. Salivary Gland *Together this makes a tube called the Digestive tract.
• 4.
  Stages of Digestion 1.Ingestion 2. Movement/Propulsion (Swallowing and Peristalsis) 3. Digestion- Mechanical (Chewing, Churning, Segmentation) and Chemical (Mouth, Stomach, intestine) Mechanical (physical) - Chew - Tear - Grind - Mash - Mix Chemical - Enzymatic reactions to improve digestion of – Carbohydrates – Proteins – Lipids 4. Absorption 5. Elimination
• 5.
  1. Ingestion •The entryof food into the alimentary canal through the mouth is called ingestion. Simply put, the act of eating and drinking is called ingestion. 2. Propulsion •Propulsion refers to the movement of food through the digestive tract. •It includes both the voluntary process of swallowing and the involuntary process of peristalsis. •Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along. •These waves also play a role in mixing food with digestive juices. This both mixes and moves the contents along the alimentary tract. 3. Mechanical Digestion •Digestion is a purely physical process that does not change the chemical nature of the food. •Instead, it makes the food smaller to increase both surface area and mobility. •It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. •The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme. •Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. •By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates
• 6.
  4. Chemical digestion •Chemicaldigestion of food by enzymes present in secretions produced by glands and accessory organs of the digestive system. •In chemical digestion, starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). •These secretions vary in composition but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine. 5. Absorption •This is the process by which digested food substances pass through the walls of some organs of the alimentary canal into the blood and lymph capillaries for circulation around the body. •It takes place primarily within the small intestine. •There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa. 6. Elimination •Food substances that have been eaten but cannot be digested and absorbed are excreted by the bowel as faeces.
• 7.
  A. Mouth •The mouthor oral cavity is bounded by muscles and bones: anteriorly —by the lips, posteriorly — it is continuous with the oropharynx, laterally —by the muscles of the cheeks, superiorly —by the bony hard palate and muscular soft palate, inferiorly —by the muscular tongue and the soft tissues of the floor of the mouth. •lined throughout with mucous membrane, consisting of stratified squamous •The palate forms the roof of the mouth and is divided into the anterior hard palate and the posterior soft palate. The soft palate is muscular, curves downwards from the posterior end of the hard palate, and blends with the walls of the pharynx at the sides. •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. •It consists of the following important parts: The Tongue •The tongue is a voluntary muscular structure that occupies the floor of the mouth. •The superior surface consists of stratified squamous epithelium, with numerous papillae (little projections), containing nerve endings of the sense of taste, sometimes called the taste buds. •The tongue plays an important part in: mastication (chewing), deglutition (swallowing), speech, taste The Teeth •The teeth are embedded in the ridges of the mandible and the maxilla. •Each individual has two sets, the temporary or deciduous teeth, and the permanent teeth.
• 8.
  The Pharynx Food passesfrom the oral cavity into the pharynx then to the esophagus below, with which it is continuous. The pharynx is divided for descriptive purposes 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. Function of Pharynx The pharynx has roles in both the respiratory and digestive systems and can be thought of as the point where these systems diverge. For the digestive system, its muscular walls function in the process of swallowing, and it serves as a pathway for the movement of food from the mouth to the oesophagus. The Oesophagus •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. •The upper and lower ends of the oesophagus are closed by sphincter muscles namely upper cricopharyngeal sphincter and lower oesophageal sphincter to prevent the reflux of acid gastric contents into the oesophagus. Functions of Oesophagus •The oesophagus serves to pass food and liquids from the mouth down to the stomach. This is accomplished by periodic contractions (peristalsis). •The oesophagus is an important connection to the digestive system through the thoracic cavity, which protects the heart and lungs. •Two sphincters on either side of the oesophagus separate food into small units known as a bolus.
• 9.
  Physiology of Digestion Inthe Oral Cavity •After ingestion, the food is chewed and mixed with saliva, which contains enzymes amylase that begin breaking down the carbohydrates - breakdown of complex sugars, reducing them to the disaccharide maltose. And some lipid digestion via lingual lipase. •Chewing by the teeth increases the surface area of the food and allows an appropriately sized bolus to be produced. •Food leaves the mouth when the tongue and pharyngeal muscles propel it into the oesophagus. Pharynx and Oesophagus •The pharynx is divided for descriptive purposes into three parts, the nasopharynx, oropharynx, and laryngopharynx. •The presence of the bolus in the pharynx, lubricated by mucus, stimulates a wave of peristalsis which propels the bolus through the oesophagus to the stomach.
• 10.
  The Stomach •The stomachis a J-shaped , situated in the abdominal cavity. •Continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter. •It has two curvatures. -lesser curvature and the greater curvature •divided into three regions: the fundus, the body, and the antrum. •About 2 liters of gastric juice are secreted daily by special secretory glands in the mucosa consists of Water, mineral salts, mucus and on the stomach surface, hydrochloric acid, Intrinsic factor, inactive enzyme precursors, etc. •In the stomach, Gastric muscle contraction consists of a churning movement to break down the bolus and mixes it with gastric juice and peristaltic waves that propel the stomach contents towards the pylorus.
• 11.
  In the Stomach •Mealeaten accumulates in the stomach in layers, the last part of the meal remaining in the fundus for some time. •Gastric juice has an acidic pH and consists of water, mineral salts, mucus secreted by goblet cells, hydrochloric acid secreted by parietal cells, intrinsic factor, and inactive enzyme precursors: pepsinogens secreted by chief cells in the glands. •The hydrochloric acid present in the juice acidifies the food and stops the action of salivary amylase, kills ingested microbes, and provides the acid environment needed for effective digestion by pepsins. Pepsinogens are activated to pepsins by hydrochloric acid and by pepsins already present in the stomach. •They begin the digestion of proteins, breaking them into smaller molecules. •Preparation of iron for absorption further along the track — the acid environment of the stomach solubilizes iron salts, which is required before iron can be absorbed •Production of intrinsic factor needed for absorption of vitamin B12 in the terminal ileum. •When the stomach is active the pyloric sphincter closes. Later, Strong peristaltic contraction of the pyloric antrum forces gastric contents, after they are sufficiently liquefied, through the pylorus into the duodenum in small spurts. •Regulation of the passage of gastric contents into the duodenum. When the chyme is sufficiently acidified and liquefied, the pyloric antrum forces small jets of gastric contents through the pyloric sphincter into the duodenum.
• 12.
  The Small Intestine •Thesmall intestine is over 5 meters long, continuous with the stomach at the pyloric sphincter and leads into the large intestine at the ileocaecal valve. •The small intestine comprises three main sections continuous with each other: 1.The duodenum: 25 cm long and curves around the head of the pancreas. Secretions from the gall bladder and pancreas are released into the duodenum through a common structure, the hepatopancreatic ampulla, and the opening into the duodenum is guarded by the hepatopancreatic sphincter (of Oddi). 2.The jejunum: It is the middle section of the small intestine and is about 2 meters long. 3.The ileum, or terminal section, is about 3 meters long and ends at the ileocaecal valve, which controls the flow of material from the ileum to the caecum, the first part of the large intestine. The surface area of the small intestine mucosa is greatly increased by permanent circular folds, villi, and microvilli consist of columnar epithelial cells with tiny
• 13.
  In the SmallIntestines •The small intestine is the part of the intestines where 90% of the digestion and absorption of food occurs, the other 10% taking place in the stomach and large intestine. •The main function of the small intestine is the absorption of nutrients and minerals from food. •When acid chyme passes into the small intestine it is mixed with pancreatic juice, bile, and intestinal juice, and is in contact with the enterocytes of the villi. •When a meal has been eaten the hormone CCK (Cholecystokinin) is secreted by the duodenum during the intestinal phase of secretion of gastric juice. This stimulates the contraction of the gall bladder and relaxation of the hepatopancreatic sphincter, enabling the bile and pancreatic juice to pass into the duodenum together. Digestion by Pancreatic Juice •Pancreatic juice enters the duodenum at the hepatopancreatic ampulla. Pancreatic juice is alkaline (pH 8) because it contains significant quantities of bicarbonate ions, which are alkaline in solution. •When acid stomach contents enter the duodenum they are mixed with pancreatic juice and bile and the pH is raised to between 6 and 8. This is the pH at which the pancreatic enzymes, amylase, and lipase, act most effectively. •Digestion of proteins. Trypsinogen and chymotrypsinogen are inactive enzyme precursors activated by enterokinase (enteropeptidase), an enzyme in the microvilli, which converts them into the active proteolytic enzymes trypsin and chymotrypsin. These enzymes convert polypeptides to tripeptides, dipeptides, and amino acids. •Digestion of carbohydrates. Pancreatic amylase converts all digestible polysaccharides (starches) not acted upon by salivary amylase to disaccharides. •Digestion of fats. Lipase converts fats into fatty acids and glycerol. To aid the action of lipase, bile salts emulsify fats.
• 14.
  Digestion by BileJuice •Bile, secreted by the liver has a pH of 8 and between 500 and 1000 ml are secreted daily. It consists of water, mineral salts, mucus, bile salts, bile pigments (mainly bilirubin), and cholesterol. •The bile salts, sodium taurocholate, and sodium glycocholate emulsify fats in the small intestine. The breakdown of fat globules in the duodenum into tiny droplets, which provides a larger surface area on which the enzyme pancreatic lipase can act to digest the fats into fatty acids and glycerol is called bile emulsification. •Fatty acids are insoluble in water, which makes them very difficult to absorb through the intestinal wall. Bile salts also make fatty acids soluble, enabling both these and fat-soluble vitamins (e.g. vitamin K) to be readily absorbed. Digestion by Intestinal juice •Alkaline intestinal juice (pH 7.8 to 8.0) assists in raising the pH of the intestinal contents to between 6.5 and 7.5. •Enterokinase activates pancreatic peptidases such as trypsin which convert some polypeptides to amino acids and some to smaller peptides. The final stage of breakdown to amino acids of all peptides occurs inside the enterocytes. •Lipase completes the digestion of emulsified fats to fatty acids and glycerol partly in the intestine and partly in the enterocytes. •Sucrase, maltase, and lactase complete the digestion of carbohydrates by converting disaccharides such as sucrose, maltose, and lactose to monosaccharides inside the enterocytes. •The intestinal glands are simple tubular glands situated below the surface between the villi. The cells of the glands migrate upwards to form the walls of the villi replacing those at the tips as they are rubbed off by the intestinal contents. •During migration, the cells form digestive enzymes that lodge in the microvilli and, together with intestinal juice, complete the chemical digestion of carbohydrates, protein, and fats. •Thus, in the small intestine the digestion of all the nutrients is completed: • carbohydrates are broken down to monosaccharides
• 15.
  The Large Intestine •Itis about 1.5 meters long, beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep in the pelvis. •Its lumen is larger than that of the small intestine. It forms an arch around the coiled-up small intestine. •The colon is divided into the caecum, ascending colon, transverse colon, descending colon, sigmoid colon rectum, and anal canal. The caecum- This is the first part of the colon. The vermiform appendix is a fine tube, closed at one end, which leads from the caecum. It is usually about 13 cm long and has the same structure as the walls of the colon but contains more lymphoid tissue. The ascending colon- upwards from the caecum to the level of the liver and curves acutely to become the transverse colon. The transverse colon- This is a loop of the colon which extends across the abdominal. The descending colon- This passes down the left side of the abdominal cavity and After it enters the true pelvis it is known as the sigmoid colon. The sigmoid colon- This part describes an S-shaped curve in the pelvis then continues downwards to become the rectum. The Rectum and the Anal Canal •It is a slightly dilated section of the colon about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal. •The anal canal is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior.
• 16.
  Functions of thelarge intestine, rectum and anal canal Absorption •The contents of the ileum which pass through the ileocaecal valve into the caecum are fluid, even though some water has been absorbed in the small intestine. •In the large intestine absorption of water continues until the familiar semisolid consistency of feces is achieved. •Mineral salts, vitamins, and some drugs are also absorbed into the blood capillaries from the large intestine. Microbial activity •The large intestine is heavily colonized by certain types of bacteria, which synthesize vitamin K and folic acid. They include Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis, and Clostridium perfringens (welchii). Defaecation •Defaecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter.