Digestive System PDF

Digestive system  Digestive System The digestive system in higher animals, including humans, consists of the digestive tract and its accessories. The digestive gastrointestinal tract (GIT): GIT is about 9 m long. It includes the ingestion region, which is the oral cav...

Digestive system  Digestive System The digestive system in higher animals, including humans, consists of the digestive tract and its accessories. The digestive gastrointestinal tract (GIT): GIT is about 9 m long. It includes the ingestion region, which is the oral cavity. The region that follows it, then the digestion and absorption regions, which include: the stomach and small intestine. While the egestion region which includes the large intestine and the rectum that ends at the anus.  DIGESTION AND DIGESTIVE ENZYMES ❖ Digestion means breakdown of …………. kinds of Digestion: 1-Mechanical Digestion: physical changes Biting, grinding and chewing. The action of bile on fats. 2-Chemical Digestion: the chemical changes By digestive or hydrolytic enzymes.  It is entirely under voluntary control. It breaks the food into smaller pieces. The muscles of the jaws, the cheeks and the tongue are all involved. Important for digestion of all foods: 1- It increases the surface of food particles exposed to digestive enzymes. 2- It breaks the indigestible cellulose membrane around the nutrient portions of most fruits, row vegetables and grains. 3- Grinding of the food into very fine particles prevents injury of the G.I.T. wall. 4- Mix the food with saliva and allow the food to be tasted.  Saliva is secreted by the salivary glands whose ducts open into the mouth. There are 3 pairs of salivary glands : 1. Parotid. 2. Submandibular (submaxillary). 3. Sublingual.  Types of salivary glands secretions : A- Serous secretion : containing (α- amylase) enzyme for digestion of starch. B- Mucous secretion : contain mucous for lubricating purpose. The parotid secrets mainly serous ( α- amylase), sublingual secrets only mucous, while the submandibular secrete both types. Composition of saliva : Volume : 1 - 1.5 L / day. PH : 6.5 - 7.0 (slightly alkaline). It contains 99.5 % water and 0.5 % solids ( 0.2 % inorganic such as bicarbonate and phosphate act as buffer & 0.3 % organic such as: amylase, mucous and lysozymes ). 1) moistens and lubricates the food thereby holding them together and by providing lubrication through the presence of mucous. 2) Aids speech by facilitating movements of the lips and tongue. 3) It keeps the mouth moist. 4) cleans mouth and teeth, (lysozymes enzyme) and its lysing effect on certain bacteria. 5) Buffers in the saliva in the form of bicarbonate which neutralizes acids in food as well as acids produced by bacteria in mouth thereby helping to prevent dental caries. The buffers in saliva helps to keep PH about 7. They help to neutralize gastric acidity. 6) Digestive function : salivary amylase stars digestion of starch. It requires neutral PH and Cl- for its optimal activity. amylase Starch + H2O maltose pH 6.7 7) It serves as solvent for molecules, which stimulate taste buds. 8) Excretion of drugs: fluoride and calcium phosphate ( incorporated in teeth), and mercury. 9) Heat loss: It helps heat loss in panting animals. Swallowing. Respiratory and digestive passages diverge into pharynx. During swallowing, the epiglottis covers the opening to the trachea and prevents food from entering this airway. Peristaltic Contractions Peristalsis is a series of wave-like muscle contractions that move food through the digestive tract. It starts in the esophagus where strong wave-like motions of the smooth muscle move balls of swallowed food to the stomach. Esophageal Stage : It conducts food from the pharynx to the stomach. It is involuntary peristaltic waves. consists of 3 steps: A-Relaxation of upper esophageal sphincter (pharyngeo-esophageal sphincter). B- Traveling of bolus of food along the esophagus. C- Relaxation of lower esophageal sphincter (gastro-esophageal sphincter).  Structure of the wall of the GIT (4 main layers) from inside to outside mucosa, submucosa, muscularis & serosa (adventitia). The structure of the wall of the GI Tract. THE STOMACH  Types of cells in gastric mucosa 1- Mucus cells: they secrete mucous to prevent stomach to eat itself. 2- Oxyntic cells: (Parietal cells) present in the body and fundus. They secret HCl and intrinsic factor (Glycoprotein). 3- Peptic cells: (Chief cells) which secrete pepsinogen (inactive pepsin). 4- G cells: present in pyloric secrete gastrin hormone stimulate gastric juice. cells in gastric mucosa 1- Storage of food: stomach can store large amount of food to be stored during the day. 2- Piecemeal evacuation: evacuation of food into the intestine occur slowly over a long time to allow intestinal digestion and absorption. 3- Digestion: partial digestion of proteins to proteoses. Partial digestion of fats by the enzyme lipase into fatty acids, mono and diglycerides. 4- Antibacterial action: by the high acidity to kill most of microorganisms before reaching the intestine. 5- Secretion of intrinsic factor: which is essential for vit. B12 absorption. 6- Absorption: of small amount of water and alcohol. 7- Iron absorption: is facilitated by HCl. 1- Kills many ingested bacteria. 2- Aids protein digestion by conversion of pepsinogen to pepsin PH 1-3 and providing the necessary PH for pepsin to start protein digestion. 3- It stimulates the flow of bile and pancreatic juice. 4- Helps absorption of Ca++, iron and other minerals.  Peptic ulcer How are ulcers produced? ❖ Damage of mucus barrier that protects wall of stomch and first part of duodenum. ❖ Acid can produce holes in wall. Then, pepsin can digest stomach or duodenum lining. ❖ Bacteria infection may be involved in ulcer form. Symptoms of Peptic ulcer  The Small Intestine It is approximately 3.7 m long in a living person. The first 20-30 cm is duodenum. The next two-fifths (2/5) is jejunum, and last three-fifths (3/5) is ileum.  Digestion in Small Intestine is achieved by the aid of:  BILE Secreted by the liver and stored in the gall bladder. The liver secretes 250-1,500 ml of bile/day. Component Percentage Water 97.o% Bile salts 0.7% Composition of Bile Bile pigments 0.2% Cholesterol 0.06% Inorganic salts 0.7% Fatty acids 0.15% Lecithin 0.1% Fat 0.1%  Functions of the liver 1-Metabolic processing: of nutrients (carbohydrates, proteins and lipids) after their absorption 2-Detoxification: of body waste and hormones as well as drugs. 3-Synthesis: of plasma proteins. 4-Storage: of glycogen, fats, iron, copper and vitamins 5-Activation: of vitamin D. 6-Protective role: removal of bacteria due to the presence of macrophages. 7-Excretion of cholesterol and bilirubin (bile): The main secretory product of the liver is bile. The bile is stored and concentrated in the gall bladder between meals. After meal, bile enters the duodenum. Metabolism  Jaundice ▪ Hemolytic: yellow coloration of skin and eyes. In newborn babies, caused by excessive break down of RBCs. Usually treated by photo therapy. ▪ In adults: it results when bile excretion is blocked by for example: gallstones. ▪ Physiological: when liver is diseased and is unable to deal with even the normal load of bilirubin. The Pancreas  Pancreatic Juice (P.J.) ❑ pancrease is a soft, lobulated, glandular organ. ❑ has both exocrine & endocrine functions. ❑ It secretes P.J. through pancreatic ducts into the duodenum. Composition of P.J. ❖ About 1500 ml / day. ❖ It is watery alkaline fluid of pH = 8.0. ❖ contains water, bicarbonate & enzymes: 1- Pancreatic amylase: completes the digestion of carbohydrates. p.amylase Starch (glycogen) maltose pH 7.1  Composition of P.J. (cont.) 2-Trypsin and chymotrypsin: powerful protein-splitting enzymes. secreted as inactive proenzymes. Trypsinogen is converted to active trypsin in intestine by enterokinase (enzyme secreted by duodenal mucosa). Trypsin converts chymotrypsinogens into chymotrypsins. 3- Carboxypeptaidase: It transforms peptides into dipeptides. Carboxypept. Peptides pH 8.0 dipeptides and amino acids 4-Pancreatic lipase: It hydrolyses the ester linkage ….. p. lipase Fats fatty acids + di- & monoglycerides + glycerol Bile salts, pH7.0  Digestion in small intestine By the pancreatic enzymes. Fats: by bile secretion and pancreatic enzymes, fats are completely reduced to monoglycerides and free fatty acids. Proteins: broken down into small peptide fragments and some amino acids. Carbohydrates: are reduced to disaccharides and some monosaccharides.  Large intestine  Large Intestine (L.I.) The parts of L.I. are caecum, appendix, colon, rectum, the anal canal and anus. The mucosa of L.I. has no villi. In humans, L.I. does not perform digestive function, but their two main functions are: 1- Absorption of water & electrolytes: mainly in the first half of the colon. 2- Storage of feces: mainly occurs in the distal half. Foodstuffs Enzy. & its sourc pH Products of digestion Carbohydrates: Starch Salivary amylase 6.7 Maltose+small polysac Polysaccharid Pancreatic amylase 7.1 Disaccharides Disaccharides Maltose Maltase (intestine) 8.0 Glucose Sucrose Sucrase (intestine) 8.0 Glucose + fructose Lactose Lactase (intestine) 8.0 Glucose + galactose Proteins Pepsin (stomach) 2.0 Polypeptides,proteoses, Peptones. Polypeptides Trypsin (pancreas) 8.0 Dipeptides Chymotrypsin (pancreas) 8.0 Dipeptides Carboxypeptidase (pancreas) Peptides Aminopeptidase (intestine) 8.0 Dipeptides + amino acids Dipeptidase (intestine) 8.0 Amino acids Dipeptides 8.0 Amino acids Fats (emulsified) Pancreatic lipase 8.0 Fatty acids + glycerol  ABSORPTION (abs.) The passage of digested food, water, salts & vitamins through the intestinal mucosa into blood or lymph. Sites for absorption: o The buccal cavity: no abs. takes place except for some drugs. o The wall of stomach: water, glucose, alcohol & certain drugs (aspirin) can be abs. o The small intestine: almost all products of digestion are absorbed. o The large intestine: more water & electrolytes.  Absorption of Carbohydrates ❖ Absorption of monosaccharides occurs through active transport or simple diffusion. ❖ It is actively transported into the intestinal cells utilizing a common carrier protein with Na+ transport. ❖ The carrier binds both glucose & Na+ at separate sites & transports them into the cell cytoplasm.  Absorption of amino acids through the intestinal mucosa into the blood capillaries is Absorption carrier-mediated as in case of of Proteins glucose, but It uses different carrier systems for different classes of amino acids. Mechanism for glucose and amino acids transport across intestinal epithilium  The free fatty acids and monoglycerol enter micelles. Micelles Cylindrical aggregates of lipid & bile salts. Having 3-10 nm diamter. Outer cylinder is polar (hydrophilic). Absorption Inner cylinder is non-polar (hydrophobic). of Lipids The hydrophobic centers contain fatty acids, monoglycerol and cholesterol. Absorption of Lipids (cont.)  Vitamins as B group are absorbed by simple diffusion. Absorption The fat-soluble vitamins as of vitamins, A,D,E, and K, depend on bile minerals and salts for their absorptions. H2O The majority of mineral salts and H2O are actively absorbed.