Lecture 9 the digestive system

Copyright 2010, John Wiley & Sons, Inc.

Chapter 19

The Digestive System


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


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


Organs of the Digestive System


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)


Layers of the Gastrointestinal (GI) Wall3. 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


Layers of the Gastrointestinal (GI) Wall






Mouth (Oral Cavity) Formed by

Cheeks and tongue Hard palate anteriorly, soft palate posteriorly

Uvula U-shaped extension of soft palate posteriorly During swallowing, uvula blocks entry of food or

drink into nasal cavity Tongue: muscular accessory organ

Maneuvers food for chewing Adjusts shape for speech and swallowing

Lingual tonsils at base of tongue


Salivary Glands Exocrine glands with ducts that empty into

oral cavity Three pairs of salivary glands

Parotid Largest; inferior and anterior to ears

Submandibular In floor of mouth; medial and inferior to mandible

Sublingual Inferior to tongue and superior to submandibular

Saliva: 99.5% water, salivary amylase, mucus and other solutes Dissolves food and starts digestion of starches


Salivary Glands


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


Teeth


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


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


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


Pharynx and Esophagus


Pharynx and Esophagus


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


Stomach


Stomach


Stomach Wall: Four Layers1. 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


Stomach Wall: Four Layers3. 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


Stomach Wall: Four Layers


Digestion and Absorption Digestion

Mechanical digestion Stretching of stomach wall nerve impulses Secretion + mixing waves Food mixed with juice now called chyme

Chemical digestion Pepsin (pepsinogen + HCl) digests protein peptides

(small chains of amino acids) Gastric emptying through pyloric sphincter

Carbohydrates fastest, proteins next, fats last Once in duodenum feedback inhibition of stomach

Little absorption: water, ions, some drugs


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


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


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


Bile Functions of bile

Emulsification: breaking apart clusters of fats so they are more digestible

Absorption of fats Formation and recycling of bile

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


Liver, Gallbladder, Duodenum


Liver


Liver


Liver Functions1. Carbohydrate metabolism

Polysaccharide stored in liver as glycogen Converts glycogen, fructose, galactose, lactic

acid, amino acids glucose to blood glucose2. 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


Liver Functions4. 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


Small Intestine Length

10 feet long in living person Extends from pylorus of stomach to cecum of

large intestine Three major regions: duodenum, jejunum,

ileum Functions

Site of most of digestion Essentially all nutrient absorption occurs here

Ends in ileocecal sphincter (in RLQ)


Small Intestine


Small Intestine


Intestinal Wall Structure Same 4 layers but with modifications Epithelium in mucosa: simple columnar

Absorptive cells with microvilli Goblet cells: secrete mucus

Intestinal glands secrete Enzymes that complete digestion Secretin, cholecystokinin (CCK), glucose-

dependent insulinotropic peptide (GIP) Lymphatic tissue within wall: defense


Intestinal Wall Structure Submucosa has duodenal glands

Alkaline mucus helps neutralize stomach acid Circular folds

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


Intestinal Wall Structure


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


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


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


Protein and Fat Digestion Pepsin, trypsin, chymotrypsin, and

carboxypeptidase Proteins small peptides

Peptidases at surface: Peptides amino acids, dipeptides, and

tri-peptides Lipase (pancreatic)

Triglyceridesfatty acids + monoglycerides


Absorption of Products of Digestion By diffusion, facilitated diffusion, osmosis and

active transport Carbohydrates monosaccharides

Via portal system (blood) to liver Proteins (jejunum + ileum) amino acids

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


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


Absorption of Products of Digestion


Absorption of Products of Digestion


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


Large Intestine


Large Intestine


Large Intestine


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


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


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


Control: Phases of Digestion Secretin

Released when acidic chyme enters intestine Stimulates release of pancreatic juice high in

bicarbonate to buffer acidic chyme from stomachCholecystokinin (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


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


End of Chapter 19

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Lecture 9 the digestive system