Organs of the Digestive System
-Alimentary canal, gastrointestinal tract, guts
-Accessory digestive organs
Alimentary Canal ~30 ft
-Continuous muscular digestive tube
-It digests food and absorbs products of digestion
-Comprised of:
1. mouth
2. pharynx
3. esophagus
4. stomach
5. small intestine
6. large intestine
7. anus
-GI tract is outside body
Accessory Digestive Organs
-Teeth
-Tongue
-Gall bladder
-Digestive glands
-Salivary glands
-Liver
-Pancreas
Digestive Process
1. Ingestion
2. Propulsion
3. Mechanical digestion
4. Chemical digestion
5. Absorption
6. Defecation
Ingestion
taking food into digestive tract
Propulsion
swallowing; peristalsis (alternate waves of contraction and relaxation of muscles)
Mechanical digestion
chewing; churning food in the stomach; segmentation (rhythmic local constrictions of intestine) mixes food with digestive juices
Chemical digestion
food molecules are broken down by enzymes secreted by various glands (begins in the mouth, completed in the small intestine)
Absorption
passage of digested products into blood or lymph; occurs mostly in small intestine
Defecation
elimination of indigestible substances from the body
Organ Structure
-Most organs of digestive system reside in the abdominopelvic cavity
-Organs and cavity are covered in serous membranes
1. visceral peritoneum: covers external surface of digestive organs-is continuous with the...
2. parietal peritoneum: lines abdominopel
A mesentery
-Is a double layer of peritoneum
-In most places the mesentery is dorsal
-Provides routes for blood vessels, lymphatics, nerves
-Holds organs in place
-Stores fat
Peritonitis
-Inflammation of peritoneum
-From wound piercing abdomen
-From perforating ulcer
-Burst appendix
-Very painful
-Friction between peritonea due to lack of serous fluid
Histology
-From esophagus to anus are 4 layers
1. Mucosa
2. Submucosa
3. Muscularis Externa
4. Serosa
Mucosa
-Innermost layer
-Lines lumen
-Functions: secretion of mucous, enzymes, hormones, absorption, protection against infection
-Mucosa has 3 sub layers: epithelial lining, lamina propria, muscularis mucosae
Epithelial lining
mucus secreting
Lamina propria
loose connective tissue
Muscularis mucosae
thin layer of smooth muscle cells-twitches to dislodge food
Submucosa
-Dense connective tissue with blood and lymphatics
-Rich supply of elastic fibers
-Provides the vascular network to surrounding tissues
Muscularis Externa
-Inner circular layer of smooth muscle cells
-Outer longitudinal layer of smooth muscle cells
-When circular layer thickens it forms a sphincter
Serosa
-Outermost protective layer-is the visceral peritoneum
-In the esophagus the serosa is replaced by the adventitia
Alimentary canal has 2 nerve plexii of enteric neurons
-Submucosal nerve plexus
-Myenteric nerve plexus
Submucosal nerve plexus
-In Submucosa
-Controls activity of glands and smooth muscle in mucosa
Myenteric nerve plexus
-Between circular and longitudinal smooth muscle
-Controls GI tract motility
Enteric systems are linked to CNS by:
-Afferent visceral nerves
-Efferent sym. and parasym. ANS
Oral or buccal cavity
-Lined with stratified squamous epithelia-due to friction
-Continuous with oropharynx
Tongue
-Grips food
-Mixes food with saliva
-Compacts food into bolus
-Pushes bolus into pharynx
-Held in place by lingual frenulum
Salivary glands
secrete saliva
Extrinsic salivary glands
-Produce majority of saliva
1. parotid
2. submandibular
3. sublingual
4. Located outside oral cavity-secretes saliva into mouth
Intrinsic salivary glands
scattered throughout oral mucosa
Salivary Gland Cells
-Serous-watery fluid full of enzymes
-Mucous-stingy, viscous fluid (mucous)
-Parotid gland-serous cells
-Submandibular-serous and mucous
-Intrinsic glands-serous and mucus
-Sublingual-mucus
Saliva
-Mostly water-98%
-Slightly acidic
-Contains:
1. ions
2. Amylase: digestive enzymes-begins breakdown of starch
3. Lysozymes: bacterioside
4. IgA
5. Metabolic wastes
-Produce ~ 1 liter/day
-Salivation controlled by ANS
Pharynx
-From mouth food-oropharynx-laryngopharynx
-Stratified squamous epithelium
-Mucus producing glands
-Constrictor muscles propel food into esophagus
Esophagus
-Normally collapsed
-Food propelled from pharynx into esophagus when epiglottis closes of larynx
-Pierces diaphragm at esophagus hiatus
-Joins stomach at cardiac orifice, surrounded by the cardiac sphincter-also called, gastroesophageal sphincter
-Sphinct
Stomach
-Expansion of GI tract Is a temporary storage tank
-Is a temporary storage tank
-Food-creamy paste chime
-Empty-holds 50mls
-Maximum-hold 4L or 1gallon
-When empty mucosa and Submucosa fold-rugae
-Fundus-superior-dome shape
-Body-midportion
-Pyloric regio
Lesser Omentum
a mesentery connection liver to lesser curvature
Greater Omentum
-A mesentery running from greater curvature to the coils of small intestines
-Muscularis Externa has additional layer of smooth muscle-oblique layer-allows churning and mixing of food
Digestive process in stomach
-Physically
-Chemically-only chemical digestion is of proteins by pepsin
-Protein digestion only initiated in stomach
-In children-renin secreted by glands. Breaks down milk protein casein
-Absorption of lipid-soluble substances-alcohol, aspirin, and drug
Gastric Secretions
-Mucosa makes 3L (0.76) gastric juice/day
-Stimulated by Vagus nerve (parasym.)-increase gland activity
-Sympathetic nervous system-decrease in gland secretion
Stomach lining
-Simple columnar epithelium-are goblet cells
*produce protective alkaline mucus
-Millions of gastric pits
*Lead to gastric glands
*Produce gastric juice
*Lined with goblet cells
Gastric Glands
-Glands in cardiac and pylorus secrete mucus
-Glands in pyloric antrum secrete mucus and the hormone gastrin
-Glands in fundus and body (are larger) secrete mucus, HCL, enzymes, gastrin-this is where most digestion occurs
Cell Types of Gastric Glands
-Mucous neck cells
-Parietal cells
-Chief cells
-Enteroendocrine cells
Mucous neck cells
produce acidic mucus
Parietal cells
-Look like fuzzy pitch forks
-Secrete HCL acid and intrinsic factor IF
-Responsible for extreme acidity (pH 1.5-3.5) of stomach
*activates pepsin
*denatures proteins
*breaks down cell wall of plant foods
*kills many bacteria
Chief cells
-Secrete:
*pepsinogen (inactive)
pepsin (active)
digests proteins
Enteroendocrine cells
-Secrete:
*Gastrin
*Serotonin
*Histamine
*Somatostatin...
Into the Lamina Propria-->blood system-->digestive organs
-Stomach epithelium is protected by mucus barrier and replaced every 3-6 days as cells are frequently damaged - acid and enzymes are corros
Gastrin
-Increases gastric gland activity esp. HCL production
-Increases intestinal motility
-Relaxes the ileocecal valves
-Stimulates mass movements
Serotonin
causes contraction of smooth muscle
Histamine
activates parietal cells
Somatostatin
-Inhibits gastric secretion
-Inhibits gastric motility
-Inhibits GI blood flow and absorption
-Inhibits gall bladder and pancreatic activity
3 Phases of Gastric Secretion
-Cephalic (reflex) phase
-Gastric phase
-Intestinal phase