The Digestive System

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