General functions of digestive system. Ch. 15 The Digestive System. General histology of the wall of the digestive tract. Overview of digestive organs

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1 Overall idea: obtain nutrients from food (for energy and raw materials for synthesis), and defecate the leftover waste 2 types of organs involved: 1. Parts of the digestive tract (= a long muscular tube from mouth to anus) A.k.a. gastrointestinal (GI) tract Made up of mouth (oral cavity), pharynx, esophagus, stomach, small intestine, large intestine Food passes through these 2. Accessory organs teeth, tongue, salivary glands, liver, pancreas Food does not pass through these Ch. 15 The Digestive System General functions of digestive system 1. Ingestion eating 2. Mechanical digestion (e.g. teeth, stomach) physically breaking, crushing, and tearing food into smaller pieces Increase surface area for attack by enzymes Smaller pieces easier to move 3. Chemical digestion breaking of chemical bonds (usually by enzymes) Proteins amino acids Triglycerides glycerol + fatty acids Polysaccharides monosaccharides 4. Absorption of nutrients across digestive tract epithelium, eventually into blood 5. Secretion of water, acids, enzymes, buffers, ions 6. Excretion removal of wastes (defecation) General histology of the wall of the digestive tract (hollow space) Epithelium varies by location (e.g. mouth stratified squamous to resist abrasion; small intestine simple columnar for secretion, absorption) Loose (areolar) CT, N.A.V.a.L., glands, autonomic nerve plexus ( brain of gut) For mixing and propulsion of food along tract Overview of digestive organs 1

2 Example of the histology of the digestive tract wall : duodenum of small intestine Lumen (interior hollow space) Serosa not shown Functions that occur here: Begin mechanical digestion (teeth, tongue) Begin chemical digestion (salivary enzymes) Monitor food quality/taste (tongue) Moisten and manipulate food to prepare for swallowing (saliva, tongue) Mouth (oral cavity) Function: mechanical digestion (chewing) Periodontal ligament Dense CT that, along with cementum, anchors tooth in socket Tooth composition: 1. Dentin = bulk of tooth Calcified CT Similar to, but harder than bone ( [Ca 2+ ]) 2. Pulp cavity within crown Contains pulp (N.A.V.a.L.) Exit via root canal 3. Enamel hardest biological substance Covers dentin of crown Crystalline calcium phosphate But dissolved by acids (e.g. from bacteria)! Periodontal ligament Teeth Types of teeth in an adult 2

3 3 pairs: Parotid (~25% of saliva) produces serous (watery) secretion that contains the enzyme salivary amylase (which begins chemical digestion of starches) Sublingual (~ 5%) produces mucous secretion Submandibular (~70%) secretion a mix of serous and mucous Salivary glands Composition of saliva 99.4% water, rest is solutes Some of the solutes: Ions (e.g. chloride activates salivary amylase) Salivary amylase (begins digestion of starches/carbohydrates) Bicarbonate and phosphate buffers Urea and uric acid (nitrogen-containing waste products from protein metabolism) Mucus (lubricates food) About 1 liter produced every day = skeletal muscle studded with taste buds Functions: Manipulates food for chewing, swallowing Moves for speech Sensory (taste, touch, temperature) Secretes mucins (= mucus glycoproteins) Secretes lingual lipase: Initiates digestion of triglycerides (fats) to fatty acids and monoglycerides Tongue Pharynx (throat) and swallowing Pharynx = common passageway for solid food, liquids, and air Underlying skeletal muscle for swallowing Bolus = soft mass of chewed and moistened food 3

4 Esophagus and peristalsis Esophagus = hollow muscular tube that conveys solid food and liquids from pharynx to stomach Muscle tone keeps upper and lower ends closed (except when bolus passes through), acting as sphincters (not true valves) Passes through diaphragm at esophageal hiatus; diaphragm reinforces lower esophageal sphincter Peristalsis: waves of rhythmic contractions that propel bolus forward through tract from mouth to anus Anatomy of the stomach Lower esophageal Pyloric Cardia Fundus Body Pylorus Rugae (folds) 1. Mucosa - folded into gastric pits with deeper gastric glands that secrete gastric juice; the glands are made up of Mucous cells secrete alkaline mucus Chief cells secrete pepsinogen Converted by HCl to pepsin, which begins protein digestion Parietal cells secrete HCl and intrinsic factor 2. Submucosa 3. Muscularis externa has 3 layers of smooth muscle Adds inner oblique to circular and longitudinal layers 4. Serosa Histology of stomach lining Functions of stomach Temporary storage (3-5 hours) of ingested food Mechanical digestion churning and mixing until food is soupy liquid called chyme Chemical digestion pepsin performs limited chemical digestion of proteins Secretes HCl (keeps stomach contents at ph = ) Kills bacteria Denatures proteins, enzymes in food Helps break down plant materials and CT in meat Helps convert inactive pepsinogen to active pepsin Secretes intrinsic factor needed for intestinal absorption of vitamin B 12 (which is needed for red blood cell production) Absorption: No real nutrient absorption Some absorption of alcohol, aspirin and some other lipid-soluble drugs 4

5 Small intestine Site of most of chemical digestion and nutrient absorption in digestive tract ~ 21 feet long in cadaver; ~ 10 feet long in living person 3 segments: 1. Duodenum (~ 10 in. long in cadaver) Has glands that secrete an alkaline mucus to neutralize acidic chyme from stomach 2. Jejunum (~ 8 ft) 3. Ileum (~ 12 ft) Has lots of lymphoid tissue to battle bacteria that may travel up from large intestine Exits into cecum of large intestine at ileocecal valve 2. Villi small (microscopic) folds of mucosa Contain blood capillaries that absorb most nutrients and lacteals (= lymphatic capillaries) that absorb fats and transport them to blood via lymphatic system 3. Microvilli very microscopic folds of epithelial cell membrane that contain cytoplasm Form brush border and contain brush border (digestive) enzymes Total increase in surface area (for digestion and absorption) compared to a simple, unmodified tube is 600X! (~ 2200 ft2 total) Small intestine histology Small intestine histology The intestinal wall is highly modified on 3 different levels to increase surface area for secretion and absorption: 1. Circular folds large (macroscopic) folds of submucosa 2. Villi small (microscopic) folds of mucosa Some accessory organs that aid digestion and absorption within the small intestine 5

6 A mixed gland: both exocrine and endocrine Exocrine portion secretes (into the duodenum) pancreatic juice, which contains: Water Lots of digestive enzymes Sodium bicarbonate (a buffer that neutralizes the acidic chyme from stomach) Endocrine portion secretes (into the bloodstream) the hormones glucagon and insulin that regulate blood glucose levels Pancreas Largest internal organ in body Main digestive function: production of bile Main ingredients of bile: mostly water, minor amounts of bile salts (derived from cholesterol) Main digestive function of bile: emulsification of fats Bile salts have hydrophilic and -phobic ends, so they surround large fat globules and break into small lipid droplets in a watery environment Increases surface area for digestion by enzymes, aids absorption Liver Hepato- = liver Other liver functions Hundreds of them! Here are just a few 1. Metabolic regulation A. Carbohydrate metabolism e.g. glycogenolysis, gluconeogenesis B. Lipid metabolism regulate blood lipid concentration C. Amino acid metabolism protein synthesis, conversion to lipids/glucose) D. Waste removal (e.g. ammonia converted less toxic urea) E. Vitamin storage (B 12 and fat-soluble vitamins A, D, E, K) F. Mineral storage (e.g. iron) G. Drug/toxin inactivation (by smooth ER in hepatocytes) 2. Blood regulation A. Phagocytosis of pathogens and initiation of immune response B. Synthesize plasma proteins C. Remove circulating hormones D. Remove circulating antibodies (= immune system proteins that attack viruses and other pathogens) E. Remove (or store) toxins Hepatic portal system 6

7 Functions: 1. Bile storage 2. Bile modification (reabsorbs water, concentrating bile) 3. Bile release: Presence of lipids in duodenum of small intestine releases a hormone that causes smooth muscle in wall of gallbladder to contract, and bile enters duodenum Gallbladder Large intestine About 5 feet long Ileocecal valve (a sphincter) connects with small intestine Functions: 1. Absorption of water forming feces 2. Absorption of ions and vitamins, some produced by bacteria (e.g. vitamin K) 3. Storage of feces prior to defecation Large intestine features Produces lots of mucus for lubrication Absorbs most of remaining water and a few vitamins and ions Does NOT produce digestive enzymes (any remaining digestion performed by resident bacteria) Has lots of lymphoid tissue to prevent bacteria from crossing wall Muscularis has thick longitudinal bands called taeniae coli Contractions produce haustra (pouches) Haustra Taeniae coli Large intestine defecation Peristalsis typically slow, but powerful mass movements several times a day move and compact feces into rectum, triggering defecation reflex: Internal anal sphincter (not shown at right) smooth muscle involuntarily relaxes Stretch alerts brain, initiates urge to defecate External anal sphincter skeletal muscle must be voluntarily relaxed 7

8 Some digestive system problems Summary of digestive tract organs Summary of accessory organs of digestion 8