DIGESTIVE SYSTEM Five Basic Processes The Gastrointestinal tract (alimentary canal)

Transcription

1 DIGESTIVE SYSTEM Five Basic Processes 1. Ingestion - eating 2. Movement of the food along the G.I. tract. 3. Digestion- chemical and mechanical breakdown of food. 4. Absorption of the breakdown products into the lymphatic and circulatory systems. 5. Defecation - elimination of undigested substances. The Gastrointestinal tract (alimentary canal) - extends from the moth to the anus feet long (in the cadaver) - accessory structures - aid in digestion - example: teeth, liver, pancreas

2 Histology - 4 basic layers with subdivisions 1. Mucosa - mucous membrane - lining of a tract - has 3 sublayers (1) Epithelium - in contact with the food - varies from stratified squamous in esophagus and anal canal to simple columnar in the small intestine (2) Lamina Propria - areolar connective tissue (3) Muscularis mucosa - smooth muscle 2. Submucosa - binds mucosa to underlining smooth muscle layer - made of areolar connective tissue - contains ANS supply to muscularis mucosa called Submucosal Plexus (Plexus of Meissner) 3. Muscularis - in mouth, pharynx and esophagus is skeletal muscle - in the rest of the tract is smooth muscle. - usually contains 2 sheets (stomach has 3) - inner circular - outer longitudinal - contains the major nerve supply to the major nerve supply the Myenteric Plexus (Plexus of auerbach) controls motility 4. Serosa - serous membrane - made of connective tissue and epithelium - below the diaphragm known as visceral peritoneum 17

3 Mouth - oral or buccal cavity - anatomy - cheeks - help hold food in place between molars - labial frenulum - mucous membrane fold - attaches lips to gums - lingual frenulum - attaches tongue to base of oral cavity - fauces - opening between the oral cavity and the oropharynx (throat) - hard palate - made from: - palatine processes of the maxilla and horizontal plates of the palatine bones - soft palate - arch between oral pharynx and nasal pharynx 18

4 Tongue - holds food in place over molars (with cheeks) so food can be masticated - moves food to back of mouth for swallowing and is involved in swallowing - 2 sets of muscles 1. Extrinsic muscles the origin of the muscle is outside of the tongue. Functions; aids in chewing and swallowing and moves tongue in and out and from side to side. - 19

5 2. Intrinsic muscles - origin and insertion of muscles is inside the tongue itself - functions - alters the shape of the tongue - speech and swallowing - glands on the surface of the tongue secrete Lingual Lipase -digest triglycerides into fatty acids and monoglycerides Salivary Glands - buccal glands secrete some saliva - 3 pairs of major salivary glands 1. Parotid Salivary glands - located anterior to the ears - secrete through parotid duct into the oral cavity by upper second molar - secrete a watery mucous with salivary amylase 2. Submandibular Salivary glands - located under the mandible at sides of tongue - secrete a thicker saliva with mucous and salivary amylase - empty under the tongue 3. Sublingual Salivary gland - located under the tongue - secrete a thick mucous with little salivary amylase - secrete into mouth under the tongue - saliva is composed of: % water, 0.5% solutes like: ions, gases, urea/uric aid, serum proteins, lysozyme, salivary amylase 20

6 Secretion of Saliva to 1500 ml secreted daily from salivary glands - under nervous system parasympathetic stimulation from the salivary nuclei in the brain stem - secretion of the saliva can be brought about by: - food in the mouth - thought, smell, sound, sight of food. Teeth - accessory structures - aid in masticating food - two dentitions: 1. Deciduous teeth - appear at 6 months - 20 in all - formula: 2 incisors, 1 cuspid, 2 molars (per quadrant) 21

7 2. Permanent teeth - appear 6-12 years - 32 in all - formula: 2 incisors, 1 canine (cuspid), 2 bicuspids (premolars), 3 molars Chemical Digestion in the Mouth - Salivary amylase breaks down starches into disaccharides - Lingual Lipase - breaks down triglycerides into fatty acids and monoglycerides Deglutition 22

8 - the act of swallowing - 3 stages 1. Voluntary - the tongue moves the bolus into the pharynx 2. Pharyngeal stage - bolus stimulates receptors in the oropharynx causing the respiratory passageway to close and breathing to stop. This is initiated by the deglutition center in he medulla. 3. Esophageal stage - upper esophageal sphincter and the bolus enters the esophagus. Then peristalsis pushes the food toward the stomach. The lower esophageal sphincter opens so the food can enter the stomach. Stomach Histology - same four layers Mucosa has gastric pits with gastric glands with 4 types of cells 1. Chief (Zymogenic) cells - secrete pepsinogen (enzyme precursor) and gastric lipase. 2. Parietal (oxyntic) cells - secrete hydrochloric acid (HCl) which converts pepsinogen to pepsin and intrinsic factor that aids in the absorption of Vitamin B12 (necessary for red blood cell production) 3. Mucous cells - secrete mucous to protect the mucosal layer of the stomach from the acid and enzymes 4. G cells - secrete the hormone stomach gastrin - the combined secretions of chief cells, parietal cells, and mucous cells is called: 23

9 Submucosa - same as rest of G.I. tract - made of areolar connective tissue Muscularis - has three layers instead of two - inner oblique - middle circular - outer longitudinal Serosa - forms greater and lesser omentum Mechanical Digestion in the Stomach - peristaltic mixing waves occur every 15 to 25 seconds - food mixed with HCl and reduced to chyme - food is initially stored in the fundus, and ground down in the body and pylorus - each wave ejects chyme into the duodenum Chemical Digestion in the Stomach - HCl denatures proteins and converts pepsinogen to enzyme pepsin - pepsin breaks large polypeptides into peptides - gastric lipase digests buttermilk in infants Absorption in the Stomach - the stomach only absorbs water, some electrolytes, some drugs (like aspirin) and alcohol Pancreas 24

10 - produces 1200 to 1500 ml of pancreatic juice per day - juice consists of 2 major groups of compounds 1. Digestive enzymes. 2. Sodium Bicarbonate. - the sodium bicarbonate neutralizes the acidic chyme coming from the stomach - the digestive enzymes are; Enzyme Breaks Into Pancreatic amylase Starches Disaccharides Trypsin Proteins Peptides Chymotrypsin Proteins Peptides Carboxypeptidase Proteins Peptides Pancreatic Lipase Lipids Fatty acids and monoglycerides Ribonuclease RNA Nucleotides Deoxyribonuclease DNA Nucleotides The Liver 25

11 - heaviest organ in the body (3 lbs) - Histology - lobules - made up of hepatic cells lining sinusoids which empty into central veins - sinusoids - twisting capillaries - lined with hepatocytes - sinusoids are lined with hepatocytes 26

12 - sinusoids have fixed macrophages called Stellate Reticuloendothelial cells that phagocytize worn-out red blood cells - bile capillaries are found adjacent to the sinusoids - bile is recycled from the sinusoids by the hepatocytes and emptied into bile ducts to go back to the gall bladder - the hepatocytes store most of the nutrients coming from the digestive system via the hepatic portal vein Blood supply to the liver - from two sources - the hepatic portal vein brings blood with digested products from the G.I. tract - the hepatic portal vein empties into the sinusoids where nutrients in the blood coming from the digestive organs is removed (and stored in the hepatocytes). - the hepatic artery brings oxygenated blood to the liver Bile - secreted by hepatic cells of the liver to 1000 ml a day - is both a digestive and an excretory product - the digestive component is bile acids that emulsify fats - excretory products include bilirubin (that makes feces brown in color) Liver - functions 1. The liver can take most digested organic molecules and convert them into other molecules. 2. Metabolizes lipid molecules. 3. Deamination (removal of N) of proteins and synthesis of new proteins. 4. Removes drugs and hormones. 5. Excretion of bile. 6. Storage of glycogen and vitamins (A, B, D, E, K) 7. Synthesis of bile salts 8. Phagocytosis of worn-out red blood cells. 9. Activates vitamin D. Gall Bladder - concentrates bile by reabsorbing water from bile - ejects bile into duodenum (keyed by CCK) Small Intestine - 21 feet long 27

13 - 3 segments - duodenum - jejunum - ileum - Histology (1) Mucosa - secretes intestinal juice (brush border enzymes) - has three structures to increase surface area Villi - Microvilli - Plica circulares (2) Submucosa - Areolar tissue containing the nerves and blood vessels (3) Muscularis - two layers - Inner circular and Outer longitudinal (4) Serosa - forms the mesentery Intestinal Juice - 1 to 2 liters a day produced - some juice comes from the surface of the enterocytes and is called Brush Border Enzymes - these include: - alpha destrinase - maltase Breaks down disaccharides into monosaccharides - sucrase Breaks down disaccharides into monosaccharides - lactase Breaks down disaccharides into monosaccharides - peptidase Breaks down peptides into amino acids - nucleosidases Breaks down nucleotides into N bases, pentoses, P - phosphatases " Mechanical Digestion Segmentation - chyme is confined to one region of the small intestine 28

14 - washes back and forth to make contact with the mucosa - peristalsis then moves the chyme to the next part of the small intestine where segmentation occurs again Regulation of secretion in the Small Intestine - local reflexes due to the presence of chyme - vasoactive intestinal polypeptide stimulates secretion of intestinal juice - parasympathetic nervous system stimulates secretion Absorption in the small intestine - in order to be absorbed - carbohydrates must be broken down into monosaccharides - proteins must be broken down into amino acids - lipids must be broken down into fatty acids and monoglycerides - 90% of the absorption occurs in the small intestine - 10% in the rest of the G.I. tract Vitamin Absorption - fat-soluble ADEK pass with triglycerides - water-soluble pass by diffusion - vitamin B12 with intrinsic factor Large Intestine -5 feet long - attached to posterior abdominal wall by a double fold of the serosa called mesocolon 29

15 Gastroileal Reflex - ileocecal sphincter opens - chyme passes into the cecum by ileal peristalsis - initiated by hormone from the intestinal mucosa stomach gastrin - the fecal material is forced from one haustra to the next - called haustral churning - haustral churning continues until the second half of the transverse colon where mass peristalsis moves it into the rectum Chemical Digestion in the Colon - bacteria break down what's left of the chyme - bacteria make vitamins B2, B6, Pantothenic acid, and Vitamin K. Physiology of Defecation - mass peristalsis ejects fecal material from the sigmoid colon into the rectum - the stretching of the wall of he rectum initiates a defecation reflex - the defecation reflex is initiated by the sacral portion of the parasympathetic nervous system diarrhea is the discharge of liquid feces - this often results in the loss of ions and dehydration constipation is infrequent of difficult defecation. The cause is usually a decrease in the motility of the large intestine resulting in an increase in water reabsorption, and feces become hard and dry 30

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