Digestive System Gastrointestinal Tract 1 Mouth 2 Pharynx

Digestive System Gastrointestinal Tract 1. Mouth 2. Pharynx 3. Esophagus 4. Stomach 5. Small Intestine 6. Large Intestine Accessory Structures 1. Teeth 2. Tongue 3. Salivary Glands 4. Liver 5. Gallbladder 6. Pancreas

The Processes of Digestion 1. Ingestion taking food into the mouth 2. Secretion GI tract and accessory cells secrete water, acid, buffers, and enzymes into lumen 3. Mixing and Propulsion alternating contraction and relaxation of smooth muscles in the walls of the GI tract 4. Digestion Breaking down of larger food particles into smaller molecules Mechanical Digestion Chemical Digestion 5. Absorption small molecules pass from the lumen into the blood and lymph 6. Defecation elimination of indigestible substances and bacteria from the GI tract

Mouth (Oral Cavity) Mechanical Digestion mastication (chewing) tongue manipulates food teeth grind food saliva mixes so that food is reduced to a soft flexible mass called a bolus that is easily swallowed

Salivary Glands Chemical Digestion salivary amylase initiates the breakdown of starch ingested disaccharides and starches must be broken down into monosaccharides lingual lipase initiates digestion of triglycerides into fatty acids and monoglycerides

Esophagus and Swallowing

Stomach connects the esophagus to the duodenum (first part of the small intestine) Functions: 1. serves as a mixing area for saliva, food and gastric juice 2. serves as a reservoir for holding food before release into the SI 3. secretes gastric juice (HCL, pepsin, intrinsic factor, gastric lipase) HCL kills bacteria, denatures protein pepsin begins digestion of proteins intrinsic factor aids absorption of vitamin B 12 gastric lipase aids digestion of triglycerides 4. secretes gastrin (digestive hormone) into the blood

Physiology of Digestion and Absorption in the Stomach Mechanical Digestion after food enters the stomach, gentle mixing waves pass over the stomach every 15 -25 seconds it macerates the food and mixes it with gastric juice, forming chyme the chyme moves back and forth and keeps mixing until it is sent through the pyloric sphincter into the duodenum Chemical Digestion lingual lipase and salivary amylase continue to break down fats and carbohydrates respectively pepsin and HCL break down proteins gastric lipase splits triglycerides into fatty acids and monoglycerides Peristalsis

Segmentation

Regulation of Gastric Secretion and Motility • Cephalic Phase: – Sight, smell, taste, and thought initiate reflex • • Cortex and hypothalamus send impulses to medulla Parasympathetic (Vagus nerve) impulses from medulla promote: – – • Gastric Phase: – – Stretch receptors detect distention of stomach Chemoreceptors detect increased p. H of gastric juice • – Stimulates stomach churning and secreting ACH from parasympathetic activity stimulates gastrin secretion • • • Peristalsis in stomach Gastric glands to secrete: » Pepsinogen » HCL » Mucus » gastrin Stimulates gastric juice secretion Stimulates stomach motility Intestinal Phase – – Stretch receptors detect distention of duodenum Chemoreceptors detect fatty acids and glucose in duodenum • • Impulses from small intestine inhibit stomach secretions and motility Stimulates release of: – – – Gastric inhibitory peptide (GIP) » Inhibits gastric secretions and gastric motility in stomach Secretin » Decreases gastric secretions Cholecystokinin (CCK) » Inhibits stomach emptying » Inhibits gastric secretions and gastric motility in stomach

Stomach Cell Layers

Stomach Cells

Small Intestines source of the major events of digestion and absorption its long length (over 10 feet) is to increase the surface area it has many circular folds, villi and microvilli Functions: 1. mixes chyme with digestive juices and brings food in contact with surface mucosa for absorption 2. completes digestion of carbohydrates, proteins, & lipids begins & completes digestion of nucleic acids 3. is responsible for the absorption of about 90% of all nutrients

Small Intestines Physiology Digestion chyme remains in SI for 3 -4 hours Mechanical Digestion mixes chyme with digestive enzymes undergoes peristalsis Chemical Digestion pancreatic juice assists in digestion Absorption food products pass through mucosa absorptive cells passage of these molecules from the GI tract to the blood or lymph is called absorption

Small Intestine Cell Layers

Small Intestine Cells

Liver and Gall Bladder

Brush Border Enzymes • Carbohydrate digesting: – – Alpha-dextrinase Maltase Sucrase Lactase • Protein digesting: – Aminopeptidase – Dipeptidase • Nucleic Acid digesting – Nucleosidase – Phosphatase

Glucose Absorption Carbohydrates are broken down into monosaccharides like glucose Glucose passes through SI Absorptive cells via facilitative diffusion, actually with the help of a Na+ co-transporter Glucose then passively diffuses into the blood

Protein Absorption Proteins are broken down into amino acids Amino acids and the slightly larger di-peptides passively diffuse across the membranes of the SI Absorptive cells Amino acids then passively diffuse into the blood

Pancreatic Juice Enzymes • Carbohydrate digesting: – Pancreatic amylase • Protein digesting: – – Trypsin Chymotrypsin Carboxypeptidase Elastace • Lipid digesting – Pancreatic lipase • Nucleic Acid digesting – Ribonuclease – Deoxyribonuclease

Fat Absorption Large fat droplets are emulsified into monoglycerides and fatty acids by bile salts from the liver and are packaged into micelles Micelles merge with SI Absorptive cells and via a type of endocytosis deposit the fat fragments in the cells The fat fragments are then reassembled into triglycerides in the cell and packaged in chylomicrons Chylomicrons then exit the cell and enter the lymph on their way to the liver for final digestion and storage

Large Intestine (Colon) Functions: 1. completion of absorption 2. manufactures vitamins (some B’s & K) 3. formation of the feces 4. expulsion of the feces Structures: cecum ascending colon transverse colon descending colon sigmoid colon rectum anal canal anus anal sphincters

Rectum and Anus Internal Anal Sphincter External Anal Sphincter

Larger Intestine Cell Layers

Large Intestine Cells