Digestive System Functions

Transcription

1 Digestive System Functions A. Gastrointestinal Processes 1. Ingestion: placing food in mouth (voluntary) 2. Propulsion: moving food through GI tract a. Peristalsis: alternating waves of contraction and relaxation of muscles in organ wall i. One-way movement of food Example: swallowing b. Segmentation: single segments of the small intestine contract and relax i. Moves food back and forth across internal walls of intestine --Mostly mixes digestive juices (mechanical digestion) --Small amount of propulsion 3. Food Breakdown: Mechanical Digestion a. Prepares food for chemical digestion i. By breaking down into smaller pieces (increase surface area) b. Examples: i. Mixing food with mouth and tongue ii. Churning of stomach iii. Segmentation by small intestine 4. Food Breakdown: Chemical Digestion a. Large food molecules are broken down by enzymes into their building blocks b. Carbohydrates: composed of monosaccharides (simple sugars) i. Simple Sugars: Glucose, Fructose, and Galactose --Glucose: the sugar in blood sugar 1

2 --Fructose: most abundant sugar found in fruits --Galactose: sugar found in milks ii. Main sugars broken down by digestive system: --Sucrose: Table sugar --Maltose: Malt sugar --Lactose: Milk sugar --Starch iii. Disaccharides: double sugars Examples: sucrose, maltose, lactose iv. Polysaccharides: formed by many sugar units Examples: starch v. Fiber: indigestible polysaccharides that do not provide nutrients --Provide bulk for material passing through tract c. Protein: composed of amino acids i. Polypeptides: small amino acids chains d. Lipids: composed of fatty acids and glycerol AKA: Fats 5. Absorption: transport of digested end products from lumen of GI tract to blood or lymph a. Small intestine is the major location of absorption 6. Defecation: elimination of indigestible residues from GI attract a. Via the Anus b. Called Feces 2

3 B. Activities of Mouth, Pharynx, and Esophagus 1. Mechanical and Chemical Digestion begin in the mouth a. Mechanical digestions begins by mastication (chewing) b. Chemical digestion of starch begins by salivary amylase 2. Essentially no food absorption occurs in the mouth a. Some drugs can be absorbed here Example: nitroglycerine 3. The pharynx and esophagus have no digestive functions a. They only transport food 4. Food Propulsion a. Deglutition: swallowing i. Involves coordination of several structures b. Two Phases: --Tongue, soft palate, pharynx, and esophagus i. Buccal Phase: voluntary, occurs in mouth --Forms bolus --Once enters pharynx, becomes reflex (out of our control) ii. Pharyngeal-Esophageal Phase: transports food through pharynx and esophagus --Controlled by parasympathetic division of the autonomic nervous system c. Food can only follow desired path i. Tongue blocks mouth ii. Soft palate blocks nasal passage iii. Epiglottis blocks trachea 3

4 d. Gravity play no part in transporting food e. When food reaches distal end of esophagus it pushes against cardioesophageal sphincter i. It opens so food can enter stomach C. Activities of Stomach 1. Food Breakdown a. Secretion of gastric juice is regulated by neural and hormonal factors i. Sight, smell, taste stimulate parasympathetic nervous system ii. Presence of food or falling ph stimulate stomach cells to release gastrin --Gastrin: (hormone) stimulates production of pepsinogens, mucus, HCl b. HCl makes stomach very acidic i. Can cause ulcers (a hole in the stomach) ii. Mucus of stomach usually protects the stomach c. Acid activates pesinogen i. Turns it into pepsin --A protein-digesting enzyme d. Rennin: enzyme that digests milk protein i. Produced in infants (not believed to be in adults) 2. Absorption a. Almost no food is absorbed in stomach i. Except for aspirin and alcohol b. Stomach mostly begins chemical digestion of proteins 3. When stomach fills it stretches 4

5 a. This actives three muscles layers that begin mechanical mixing 4. Food Propulsion a. Peristalsis of lower half of stomach moves chime b. Pylorus stores about 30 ml of chyme c. Pyloric Sphincter only allows about 3 ml of chyme into small intestine at once d. When duodenum is filled, it stretches i. Causes Enterogastric Reflex --Inhibits vagus nerves and tightens pyloric sphincter e. It takes about 4 hours to empty a full stomach --6 hours if high in fat D. Activities of Small Intestine 1. Food Breakdown a. Chemical digestion of carbohydrates and proteins has begun b. Lipid digestion begins here c. Chemical digestion and nearly all absorption is completed here i. Takes between 3-6 hours d. Brush Border Enzymes: break down double sugars into simple sugars and complete protein digestion i. Cells of the Microvilli (sometimes called brush border) ii. Intestinal Juice is enzyme-poor --Mostly forms protective mucus e. Foods enter small intestine get lots of enzymes from pancreatic juice and bile f. Pancreatic Juice contains enzymes that 5

6 i. Complete digestion of starch --Pancreatic amylase ii. Carryout half of protein digestions --Trypsin, Chymotrypsin, carboxypeptidase, etc iii. Digests all lipids (fats) --Lipases iv. Digests Nucleic Acids --Nucelases g. Pancreatic Juice contains bicarbonate i. Makes contents more basic --About ph of 8 ii. Give proper ph for the enzymes here 2. Hormones a. Chyme entering small intestine stimulates release of hormones b. Secretin i. Influence release of bile c. Cholecystokinin i. Causes gall bladder to contract --Causes bile and pancreatic juice to enter together 3. Absorption a. Most substances are absorbed through the intestinal cell plasma membranes i. By active transport b. Then enter capillary beds in villi 6

7 i. Then transported in blood to liver --By hepatic portal vein c. Lipids are absorbed by diffusion i. Enter capillaries and lacteals --Reach liver by hepatic portal vein and lymphatic system d. Remains of digestion are at end of ileum i. Contains water, indigestible material, bacteria 4. Food Propulsion a. Peristalsis moves food through intestine b. Some movement is done by segmentation E. Activities of Large Intestine 1. Food Breakdown a. Contents have few nutrients i. Spend another hours in large intestine b. Produces no digestive enzymes c. Bacteria living in colon may: i. Metabolize some remaining nutrients ii. Release gases --Methane and Hydrogen Sulfide --About 500 ml per day (called flatus) --More produced from carbohydrates (like beans) --Contributes to odor of feces iii. Make vitamins 7

8 2. Propulsion --Examples: Vitamin K and B d. Absorbs vitamins, ions, and most of the remaining water i. Feces: solid product delivered to rectum at end of large intestine a. Peristalsis and mass movements propel remaining materials in large intestine b. Mass Movements: long, slow-moving, powerful contractile waves that move over large area of the colon i. Occur 3-4 times daily ii. Bulk (fiber) increases strength of colon contraction c. Rectum is usually empty i. When rectum receives feces by mass movements it is stretched ii. Defecation Reflex: causes walls of the sigmoid colon to contract and the anal sphincters to relax --Sends signal to brain --Allows time to decide whether external voluntary sphincter should remain open or be constricted 8