Chapter 19 The Digestive System
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Functions of the Digestive System
Ingestion: eating Secretion: release of water, enzymes, buffers Mixing and propulsion: movement along GI tract Digestion: breakdown of foods
Mechanically: by movements of digestive organs Chemically: by enzymes
Absorption: moving products of digestion into the body Defecation: dumping waste products Copyright 2010, John Wiley & Sons, Inc.
Organs of the Digestive System
Gastrointestinal (GI) tract
A tube through which foods pass and where digestion and absorption occur. Includes: mouth, pharynx, esophagus, stomach, small intestine, large intestine
Accessory organs:
Organs that help in digestion but through which food never passes. Includes: teeth, tongue, salivary glands, liver, gallbladder, and pancreas
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Organs of the Digestive System
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Layers of the Gastrointestinal (GI) Wall
Four layers from lower esophagus to anus 1. Mucosa: epithelium in direct content with food; made of connective tissue, glands, and thin muscularis mucosae 2. Submucosa: connective tissue, blood vessels, lymphatic vessels, and enteric nervous system (ENS)
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Layers of the Gastrointestinal (GI) Wall 3. Muscularis: inner circular layer, outer longitudinal layer
Smooth muscle in most of GI tract Except skeletal (voluntary muscle) in mouth, pharynx, upper esophagus, and external anal sphincter
4. Serosa: visceral layer of peritoneum
Also forms extensions: greater omentum and mesentery
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Layers of the Gastrointestinal (GI) Wall
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Layers of the Gastrointestinal (GI) Wall
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Layers of the Gastrointestinal (GI) Wall
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Mouth (Oral Cavity)
Formed by
Uvula
U-shaped extension of soft palate posteriorly During swallowing, uvula blocks entry of food or drink into nasal cavity
Tongue: muscular accessory organ
Cheeks and tongue Hard palate anteriorly, soft palate posteriorly
Maneuvers food for chewing Adjusts shape for speech and swallowing
Lingual tonsils at base of tongue Copyright 2010, John Wiley & Sons, Inc.
Salivary Glands
Exocrine glands with ducts that empty into oral cavity Three pairs of salivary glands
Parotid
Submandibular
In floor of mouth; medial and inferior to mandible
Sublingual
Largest; inferior and anterior to ears
Inferior to tongue and superior to submandibular
Saliva: 99.5% water, salivary amylase, mucus and other solutes
Dissolves food and starts digestion of starches Copyright 2010, John Wiley & Sons, Inc.
Salivary Glands
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Teeth
Accessory organs in bony sockets of mandible and maxilla Three external regions
Crown: above gums Root: part(s) embedded in socket Neck: between crown and root near gum line
Three layers of material
Enamel: hardest substance in body; over crown Dentin: majority of interior of tooth Pulp cavity: nerve, blood vessel, and lymphatics
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Teeth
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Teeth Humans have two sets of teeth
The 20 deciduous teeth are replaced by the permanent teeth between ages 6 and 12 years. The 32 permanent teeth appear between 6 years and adulthood.
Four types of teeth
Incisors (8): used to cut food Cuspids (canines) (4): used to tear food Premolars (8): for crushing and grinding food Molars (12): used for crushing and grinding food Copyright 2010, John Wiley & Sons, Inc.
Digestion in the Mouth
Mechanical digestion
Chewing mixes food with saliva Rounds up food into a soft bolus for swallowing
Chemical digestion
Salivary amylase (enzyme) breaks down polysaccharides (starch) maltose and larger fragments Continues in the stomach for about an hour until acid inactivates amylase
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Pharynx and Esophagus
Food passages from mouth stomach Swallowing: 3 stages
Voluntary stage: bolus of food oropharynx Pharyngeal stage: oropharynx esophagus
Soft palate moves up and epiglottis moves down; prevent food from entering nasopharynx and larynx
Esophageal: food stomach by peristalsis
Esophageal sphincters:
Upper: controls entry esophagus Lower: controls entry stomach; GERD affects Copyright 2010, John Wiley & Sons, Inc.
Pharynx and Esophagus
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Pharynx and Esophagus
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Stomach
J- shaped enlargement of GI tract Mixing chamber and holding reservoir Very elastic/expandable and muscular Four regions
Cardia: surrounds upper opening Fundus: superior and to left of cardia Body: large central portion Pylorus: lower part leading to pyloric sphincter and duodenum
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Stomach
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Stomach
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Stomach Wall: Four Layers 1. Mucosa
Empty stomach lies in folds called rugae Epithelium: simple columnar; glands secrete mucus Gastric glands line gastric pits
2. Secretory cells
Mucous cells mucus Parietal cells HCl and intrinsic factor
These secretions collectively called gastric juice Intrinsic factor helps with vitamin B12 absorption needed for RBC formation. If missing anemia
Chief cells inactive enzyme pepsinogen G cells secrete gastrin (hormone) into blood Copyright 2010, John Wiley & Sons, Inc.
Stomach Wall: Four Layers 3. Muscularis: Three layers
Outer: longitudinal Middle: circular Inner: oblique (extra layer not in other organs) provides for efficient gastric contractions
4. Serous membrane (serosa)
Visceral peritoneum: covers organs Extensions of serosa
Greater omentum: hangs from curve of stomach Mesentery: attaches small intestine to posterior wall of abdomen and provides route for vessels
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Stomach Wall: Four Layers
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Digestion and Absorption
Digestion
Mechanical digestion
Chemical digestion
Pepsin (pepsinogen + HCl) digests protein peptides (small chains of amino acids)
Gastric emptying through pyloric sphincter
Stretching of stomach wall nerve impulses Secretion + mixing waves Food mixed with juice now called chyme
Carbohydrates fastest, proteins next, fats last Once in duodenum feedback inhibition of stomach
Little absorption: water, ions, some drugs
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Pancreas
Location: behind stomach
Produces pancreatic juice in acinar cells Passes into duodenum via pancreatic duct
Secretions that help digestion
Sodium bicarbonate (NaHCO3): pH 7.1-8.2)
Digestive enzymes: many
Pancreatic lipase: fat-digesting Pancreatic amylase: starch-digesting Proteases: made in inactivated form
Activated by enterokinase from small intestine Chymotrypsinogen, trypsinogen, carboxypeptidase RNAase and DNAase
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Liver and Gallbladder
Weighs 1.4 kg (3 lb): 2nd largest organ in the body; large right lobe + 3 smaller parts In right upper quadrant, below diaphragm Bile production and pathway
Hepatocytes (liver cells) make bile Bile canaliculi bile ducts hepatic duct Gallbladder (green, pear-shaped organ that stores bile) Cystic duct common bile duct duodenum
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Liver and Gall Bladder
Functional unit is lobule
Consists of hepatocytes in rows that radiate around central vein Sinusoids (permeable capillaries with phagocytic [Kuppfer] cells) are between cells Blood reaches liver lobules from
Hepatic artery (branch of celiac): blood high in O2 Hepatic portal vein (formed by veins from digestive organs and spleen): blood low in O2 but rich in nutrients from digestive organs
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Bile
Functions of bile
Formation and recycling of bile
Emulsification: breaking apart clusters of fats so they are more digestible Absorption of fats Bilirubin from heme when RBCs broken down Bile is digested stercobilin: gives feces brown color Bile salts reabsorbed into blood in small intestine (ileum) portal vein liver
Gallstones may form from bile
Obstruct bile ducts from gallbladder pain
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Liver, Gallbladder, Duodenum
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Liver
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Liver
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Liver Functions 1. Carbohydrate metabolism
Polysaccharide stored in liver as glycogen Converts glycogen, fructose, galactose, lactic acid, amino acids glucose to blood glucose
2. Lipid metabolism
Produces cholesterol, triglycerides; makes bile Makes lipoproteins for lipid transport
3. Protein metabolism
Remove NH2 from amino acids ammonia (NH3) urea to kidneys (urine) Synthesize most plasma proteins: albumin
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Liver Functions 4. Removes many harmful substances from blood
Detoxifies alcohol Inactivates steroid and thyroid hormones Eliminates some drugs (like penicillin) into bile
5. Excretion of bilirubin
From heme (in RBCs) to bile feces
6. Stores fat-soluble vitamins (ADEK) and minerals (Fe, Cu) 7. Activates vitamin D Copyright 2010, John Wiley & Sons, Inc.
Small Intestine
Length
Three major regions: duodenum, jejunum, ileum Functions
10 feet long in living person Extends from pylorus of stomach to cecum of large intestine
Site of most of digestion Essentially all nutrient absorption occurs here
Ends in ileocecal sphincter (in RLQ) Copyright 2010, John Wiley & Sons, Inc.
Small Intestine
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Small Intestine
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Intestinal Wall Structure
Same 4 layers but with modifications Epithelium in mucosa: simple columnar
Intestinal glands secrete
Absorptive cells with microvilli Goblet cells: secrete mucus Enzymes that complete digestion Secretin, cholecystokinin (CCK), glucosedependent insulinotropic peptide (GIP)
Lymphatic tissue within wall: defense
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Intestinal Wall Structure
Submucosa has duodenal glands
Circular folds
Alkaline mucus helps neutralize stomach acid In mucosa and submucosa; increase surface area
Villi: fingerlike projections of mucosa
Increase absorptive surface area
Microvilli on absorptive cells further enhance absorption
Contain vessels that absorb nutrients:
Arteriole, capillary, venule Lacteal (lymph capillary) for lipid absorption
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Intestinal Wall Structure
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Digestion in Small Intestine
Mechanical digestion
Segmentation activity: for mixing Peristalsis for movement of intestinal contents after most absorption completed: slow waves
Chemical digestion: 2 L/d of secretions
Alkaline chyme due to bicarbonate
From pancreas and alkaline mucus from small intestine
Enzymes produced by cells on villi
Peptidases: breaks small peptides Disaccharidases: sucrase, lactase, and galactase
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Absorption in the Small Intestine
Chyme enters small intestine carrying partially digested carbohydrates and proteins Intestinal juice (composed of bile, pancreatic juice, intestinal juice) completes digestion 90% of absorption of products of digestion occurs in the small intestine
Monosaccharides; amino acids Fatty acids and monoglycerides Phosphate sugar, and bases of DNA, RNA
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Summary: Carbohydrate Digestion
Amylases (salivary and pancreatic):
Starch and dextrin maltose
Disaccharidases (from small intestine):
Maltose: maltose glucose + glucose Lactase: lactose glucose + galactose Sucrase: sucrose glucose + fructose
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Protein and Fat Digestion
Pepsin, trypsin, chymotrypsin, and carboxypeptidase
Peptidases at surface:
Proteins small peptides Peptides amino acids, dipeptides, and tri-peptides
Lipase (pancreatic)
Triglyceridesfatty acids + monoglycerides
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Absorption of Products of Digestion
By diffusion, facilitated diffusion, osmosis and active transport Carbohydrates monosaccharides
Proteins (jejunum + ileum) amino acids
Via portal system (blood) to liver Via portal system (blood) to liver
Lipids
Short-chained fatty acids or monoglycerides or blood in villi Larger lipids coated by proteins in chlyomicrons lacteals lymphatics (lymph) then blood Copyright 2010, John Wiley & Sons, Inc.
Absorption of Products of Digestion
Water and salt
Primarily osmotic movement that accompanies other nutrients
Vitamins
Fat-soluble (A, D, E, K) absorbed with fat Water-soluble (B’s, C) with simple diffusion B12
Combines with intrinsic factor for transport through duodenum and jejunum Finally can be absorbed by active transport in ileum
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Absorption of Products of Digestion
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Absorption of Products of Digestion
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Large Intestine
Structure: 4 regions
Cecum Ileocecal sphincter Appendix attached Colon: ascending, transverse, descending and sigmoid Rectum Anal canal with sphincters
Wall: standard 4 layers
Mucosa: goblet cells secrete mucus Muscularis: incomplete longitudinal layer
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Large Intestine
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Large Intestine
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Large Intestine
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Digestion and Absorption
Ileocecal sphincter limits rate of emptying of ileum Slow peristalsis Mass peristalsis
Triggered by presence of food in stomach Wastes move from mid-colon rectum
Bacterial digestion
Produce some B-vitamins + vitamin K Produce gases: flatus Colon absorbs salt + water
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Defecation Reflex
Stretch of rectum wall neural reflex contraction of longitudinal muscle Combined pressure + parasympathetic activity relaxes internal anal sphincter External anal sphincter is voluntary Contraction of diaphragm and abdominal muscles aid defecation
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Control: Phases of Digestion
Rule: activate forward and inhibit behind Three phases: cephalic, gastric, intestinal 1. Cephalic: smell, sight, thought of food
Cranial nerves VII + IX stimulate salivary glands Cranial nerve X (vagus) stimulates gastric glands
2. Gastric: stretching, pH of stomach
Gastrin activates stomach and relaxes pyloric sphincter
3. Intestinal phase: intestinal hormones play key roles
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Control: Phases of Digestion
Secretin Released when acidic chyme enters intestine Stimulates release of pancreatic juice high in bicarbonate to buffer acidic chyme from stomach
Cholecystokinin (CCK) Released when chyme rich in amino acids and fatty acids enters intestine Stimulates release of pancreatic juice high in digestive enzymes Decreases gastric motility and secretion Causes gallbladder to contract and eject bile
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Aging
Decreased GI secretion, motility, strength of responses Loss of taste, increased risk for periodontal disease, difficulty swallowing, hiatal hernia, gastritis, peptic ulcer disease Increased risk for gallbladder problems, cirrhosis of liver, pancreatitis, constipation, hemorrhoids, diverticulitis
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End of Chapter 19
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