The Digestive System
Very detailed lecture Presentation by:Patricia Zuk, Santa Monica College
The Digestive System 
    * Mouth---bite, chew, swallow
    * Pharynx and esophagus----transport
    * Stomach----mechanical disruption;  absorption of water  & alcohol
    * Small intestine--chemical & mechanical digestion & absorption
    * Large intestine----absorb electrolytes & vitamins (B and K)
    * Rectum and anus---defecation
Layers of the GI Tract 
1. Mucosal layer
2. Submucosal layer
3. Muscularis layer
4. Serosa layer 
Mucosa 
    * Epithelium
    * Lamina propria
    * Muscularis mucosae---thin layer of smooth muscle
Submucosa 
    * Loose connective tissue
    * Meissner’s plexus
Muscularis  
    * Skeletal muscle = voluntary control
    * Smooth muscle = involuntary control
    * Auerbach’s plexus (myenteric)
Serosa 
    * An example of a serous membrane
    * Covers all organs and walls of cavities not open to the outside of the body
    * Secretes a serous fluid
    * Consists of connective tissue covered with simple squamous epithelium
Peritoneum 
    * Peritoneum
    * Peritoneal cavity
    * Mesentery – small intestines
    * Mesocolon – large intestine
    * Lesser omentum
    * Greater omentum
    * Peritonitis = inflammation
Greater Omentum, Mesentery &  Mesocolon
Lesser Omentum
The path of food: 
oral cavity/teeth/salivary glands 
oropharynx/epiglottis 
esophagus 
stomach 
small intestine: duodenum 
small intestine: ileum 
small intestine: jejunum 
large intestine: ascending colon 
large intestine: transverse colon 
large intestine: descending colon 
sigmoid colon 
rectum 
anus
Mouth 
    * Oral cavity proper---the roof  = hard, soft palate and uvula
    * Lined with an oral mucosa (stratified squamous epithelium & lamina propria)
    * Landmarks: lingual frenulum, labial frenulum, uvula
Pharyngeal Arches 
    * Two arches skeletal muscles that elevate the soft palate when we swallow
    * Palatoglossal muscle
    * Palatopharyngeal muscle
Structure and Function of the Tongue 
    * Muscular structure covered with an oral mucosa
    * Muscle of tongue is attached to hyoid, mandible, hard palate and styloid process
    * Two groups of muscles
    * 1. Intrinsic muscless
    * 2. Extrinsic muscles
Salivary Glands 
    * Parotid below your ear and over the masseter
    * Submandibular is under lower edge of mandible
    * Sublingual is deep to the tongue in floor of mouth
    * All have ducts that empty into the oral cavity (exocrine glands)
    * Wet food for easier swallowing
    * Dissolves food for tasting
    * Bicarbonate ions buffer acidic foods
    * Helps build stronger enamel
    * Chemical digestion begins with enzyme salivary
amylase & lingual lipase
    * Also contains lysozyme ---helps destroy bacteria
    * Protects mouth from infection with its rinsing action---1 to 1 and 1/2qts/day
    * Cells in acini (clusters)
    * Serous glands - cells secrete a watery fluid - parotid
    * Mucous glands - cells (pale staining) secrete a slimy, mucus secretion
    * Mixed glands secrete both mucus and serous fluids – submandibular & sublingual
Salivation 
    * Increase salivation
    * Stop salivation
Primary and Secondary Dentition 
Teeth:
-grinding, tearing and shearing of food 
-two main divisions: crown and root
      -crown: above gumline/gingiva
      -root: entry of nerves and blood vessels via the apical foramen
      -neck – where crown and root meet
      -gingiva forms a seal at this area
      -innermost layer - pulp (nerves/blood vessels)
      -nerves and BVs enter the root and travel
      through root canals to enter the pulp cavity
-outer covering of calcified connective tissue – dentin
-outermost layer - enamel
-dentin and enamel – made of calcium phosphate (similar to bone)
Pharynx 
    * Funnel-shaped tube extending from internal nares to the esophagus (posteriorly) and larynx (anteriorly)
    * Skeletal muscle lined by mucous membrane
    * Deglutition or swallowing is facilitated by saliva and mucus
Esophagus 
    * Collapsed muscular tube
    * In front of vertebrae
    * Posterior to trachea
    * Posterior to the heart
    * Pierces the diaphragm at hiatus
    * Mucosa = stratified squamous
    * Submucosa = large mucous glands
    * Muscularis = upper 1/3 is skeletal, middle is mixed, lower 1/3 is smooth
Physiology of the Esophagus - Swallowing 
    * Voluntary phase---tongue pushes food to back of oral cavity
    * Involuntary phase----pharyngeal stage
    * Peristalsis pushes food down
    * Travel time is 4-8 seconds for solids and 1 sec for liquids
    * Lower sphincter relaxes as food approaches
Anatomy of Stomach 
    * Size when empty
    * Muscularis – three layers of smooth muscle
    * Parts of stomach
    * Empties as small squirts of chyme leave the stomach through the pyloric valve
Histology of the Stomach - Mucosa 
    * simple columnar epithelium with embedded surface mucus cells
    * lamina propria layer under the epithelium (areolar connective tissue) + muscularis mucosae (smooth muscle)
    * along the mucosa – will find columns of secretory cells = gastric glands that open into the stomach lumen through gastric pits
    * Hydrochloric acid (parietal cells) converts pepsinogen (from chief cells) to the enzyme pepsin = protein digestion
    * Intrinsic factor (parietal cells)
    * Gastrin hormone (G cell)
Anatomy of the Small Intestine 
Small Intestine 
    * Structures that increase surface area
Small intestine - Mucosa 
    * Absorptive cells
    * epithelial cells at the bottom of the villus form a gland = Intestinal gland
    * Goblet cells – mucus production
    * Enteroendocrine cells
    * Paneth cells
    * Submucosal layer has duodenal glands
Anatomy of Large Intestine 
    * 5 feet long by 2½ inches in diameter
    * Ascending & descending colon are retroperitoneal
    * Cecum & appendix
    * Rectum = last 8 inches of GI tract anterior to the sacrum & coccyx
    * Anal canal = last 1 inch of GI tract
Histology of Large Intestine 
    * Muscular layer
    * Serosa = visceral peritoneum
    * Appendix
Histology of Large Intestine 
    * Mucosa
    * Submucosal & mucosa contain lymphatic nodules
Defecation  
    * Gastrocolic reflex moves feces into rectum
    * Stretch receptors signal sacral spinal cord
    * Parasympathetic nerves contract muscles of rectum & relax internal anal sphincter
    * External sphincter is voluntarily controlled
Anatomy of the Pancreas 
    * 5" long by 1" thick
    * Head close to curve in C-shaped duodenum
    * pancreatic duct joins common bile duct from liver
    * Opens 4" below pyloric sphincter
Histology of the Pancreas 
    * Acini- dark clusters
    * Islets of Langerhans
Composition and Functions of Pancreatic Juice 
    * 1 + 1/2 Quarts/day at pH of 7.1 to 8.2
    * Contains water, enzymes & sodium bicarbonate
    * Digestive enzymes
          o pancreatic amylase, pancreatic lipase, proteases
                      # trypsinogen---activated by enterokinase (a brush border enzyme)
                      # chymotrypsinogen----activated by trypsin
                      # procarboxypeptidase---activated by trypsin
                      # proelastase---activated by trypsin
                      # trypsin inhibitor---combines with any trypsin produced inside pancreas
          o ribonuclease----to digest nucleic acids
          o deoxyribonuclease
Anatomy of the Liver and Gallbladder 
    * Liver
    * Gallbladder
Blood Supply to the Liver 
    * Hepatic portal vein
    * Hepatic artery from branch off the aorta (Common hepatic artery from the celiac trunk)
Histology of the Liver 
    * Hepatocytes arranged in lobules
    * Sinusoids in between hepatocytes are blood-filled spaces
    * Kupffer cells phagocytize microbes & foreign matter
Gallbladder 
    * Simple columnar epithelium
    * No submucosa
    * Three layers of smooth muscle
    * Serosa or visceral peritoneum
Bile Production 
    * One quart of bile/day is secreted by the liver
    * Components
Flow of Bile
    * Bile capillaries
    * Hepatic ducts connect to form common hepatic duct
    * Cystic duct from gallbladder & common hepatic duct join to form common bile duct
    * Common bile duct & pancreatic duct empty into duodenum
Liver Functions--Carbohydrate Metabolism 
    * Turn proteins into glucose
    * Turn triglycerides into glucose
    * Turn excess glucose into glycogen & store in the liver
    * Turn glycogen back into glucose as needed
Liver Functions --Lipid Metabolism 
    * Synthesize cholesterol
    * Synthesize lipoproteins----HDL and LDL (used to transport fatty acids in bloodstream)
    * Stores some fat
    * Breaks down some fatty acids
Liver Functions--Protein Metabolism 
    * Deamination = removes NH2 (amine group) from amino acids
    * Converts resulting toxic ammonia (NH3) into urea for excretion by the kidney
    * Synthesizes plasma proteins utilized in the clotting mechanism and immune system
    * Convert one amino acid into another
Other Liver Functions 
    * Detoxifies the blood by removing or altering drugs & hormones (thyroid & estrogen)
    * Releases bile salts help digestion by emulsification
    * Stores fat soluble vitamins-----A, B12, D, E, K
    * Stores iron and copper
    * Phagocytizes worn out blood cells & bacteria
    * Activates vitamin D (the skin can also do this with 1 hr of sunlight a week)
Types of Digestion  
    * Mechanical – mouth, stomach, LI
    * Chemical – mouth, stomach, SI
Chemical Digestion in GI tract 
Digestion of Carbohydrates 
    * Mouth---salivary amylase
    * Esophagus & stomach---nothing happens
    * Duodenum----pancreatic amylase
    * Brush border enzymes (maltase, sucrase & lactase) act on disaccharides
Digestion of Proteins 
    * Stomach
    * Pancreas
    * Intestines
Digestion of Lipids 
    * Mouth----lingual lipase
    * Small intestine
Digestion of Nucleic Acids 
    * Pancreatic juice contains 2 nucleases
    * Nucleotides produced are further digested by brush border enzymes (nucleosidease and phosphatase)
Digestion in the Mouth 
    * Mechanical digestion (mastication or chewing)
    * Chemical digestion
Stomach--Mechanical Digestion 
    * Gentle mixing waves
    * More vigorous waves
    * Intense waves near the pylorus
Stomach--Chemical Digestion 
    * Protein digestion begins
    * Fat digestion continues
    * HCl kills microbes in food
    * Mucous cells protect stomach walls from being digested with 1-3mm thick layer of mucous
Absorption of Nutrients by the Stomach 
    * Water especially if it is cold
    * Electrolytes
    * Some drugs (especially aspirin) & alcohol
    * Fat content in the stomach slows the passage of alcohol to the intestine where absorption is more rapid
    * Gastric mucosal cells contain alcohol dehydrogenase that converts some alcohol to acetaldehyde-----more of this enzyme found in males than females
    * Females have less total body fluid that same size male so end up with higher blood alcohol levels with same intake of alcohol
Mechanical Digestion in the Small Intestine 
    * 1. Weak peristalsis in comparison to the stomach---chyme remains for 3 to 5 hours
    * 2. Segmentation---local mixing of chyme with digestive juices in the SI
Small Intestine-Chemical Digestion
Digestive Hormones 
    * Gastrin
    * Gastric inhibitory peptide--GIP
    * Secretin
    * Cholecystokinin--CCK
Mechanical Digestion in Large Intestine 
    * Smooth muscle = mechanical digestion
    * Peristaltic waves (3 to 12 contractions/minute)
Chemical Digestion in Large Intestine 
    * No enzymes are secreted only mucous – the goblet cells in the intestinal glands
    * chyme is prepared by the action of bacteria
    * Bacteria ferment
    * Bacteria produce vitamin K and B in colon
Absorption & Feces Formation in the Large Intestine 
    * food has now been in the GI tract for 3 to 10 hours
    * solid or semisolid due to water reaborption = feces
    * feces – water, salts, sloughed-off epithelial cells, bacteria, products of bacterial decomposition, unabsorbed and undigested materials
    * 90% of all water absorption takes place in the SI – 10% in the LI
    * but the LI is very important in maintaining water balance
    * also absorbs some electrolytes---Na+ and  Cl-  and vitamins
    * dietary fiber = indigestible plant carbohydrates (cellulose, lignin and pectin)
    * soluble fiber – dissolves in water (beans, barley, broccoli, prunes, apples and citrus)
    * insoluble fiber – woody or structural parts of the plant (skins of fruits and vegetables, coatings around bran and corn)
Where will the absorbed nutrients go?
Absorption of Water  
    * 9 liters of fluid dumped into GI tract each day
    * Small intestine reabsorbs 8 liters
    * Large intestine reabsorbs 90% of that last liter
    * Absorption is by osmosis through cell walls into vascular capillaries inside villi
The Digestive System.ppt
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