Lecture Digestive System Part I
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What are the two groups of the digestive system?
- Alimentary Canal (GI tract)
- -digests and absorbs food
- -mouth, pharynz, esophagus, stomach, small intestine, and large intestine
- Accessory Organs
- -teeth, tongue, gallbladder
- -digestive glands: liver, salivary glands, pancreas
What are the 6 essential activities of the digestive processes?
- 1. Ingestion- occurs at the mouth
- 2. Propulsion-swallowing, peristalis
- 3. Mechanical digestion-chewing, churning,segmentation
- 4. Chemical digestion-polymers into monomers
- 5. Absorption-into blood and lymph
- 6. Defecation- elimated undigested food stuffs
What are the regulatory mechanisms of the digestive tract and how do they work?
- 1. Mechanoreceptors and chemoreceptors
- -respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion
- -they intiate reflexes that:
- *activate or inhibit digestive glands and juices
- *stimulate smooth muscle to mix and move lumen contents
- 2. Intrinsic and Extrinsic controls
- -enteric nerve plexus (gut brain) initiate short reflexes in response to stimuli in the Gi tract
- -long reflexes in response to stimuli inside or outside the Gi tract involve the CNS centers and autonomic nerves
- -hormones from cells in the stomach and small intestine stimulate target cells in the same or different organs
What is the serous tissue of the digestive system like?
covers the exernal surface of most digestive organs
lines the body wall
: between 2 layers, fluid lubricates mobile organs
- Mesentary is a double layer of peritoneum
- -routes for blood vessels, lymphatics, and nerves
- -holds organs in place and stores fat
- Retroperitoneal organs lie posterior to the peritonium
- -part of Large intestine
- Intraperitoneal organs are surrounded by the peritoneum
What is Splanchnic Circulation?
Blood to the digestive system
- -hepatic, splenic, and left gastric
- -superior and inferior mesenteric
- Hepatic Portal Ciruclation (venous)
- -drains nutrient rich bloo from the digestive organs
- -delivers it to the liver for processing
What are the four tunics of the alimentary canal?
- -lines the lumen
- *secretes mucus, digestive enzymes and hormones
- *absorbs end products of digestion
- *protects against infectious disease
- 3 sublayers:
- 1. Epithelium
- -simple columnar epithelium and mucus secreting cells
- -this mucus protects digestive organs from enzymes and eases food passage
- -may secrete hormones nad enzymes like in the stomach or SI
- 2. Lamina Propria
- -loose areolar connective tissue
- -capillaries for nourishment and absorption
- -lymphoid follicles (part of MALT)
- 3. Muscularis mucosae
- -smooth muscle that produces local movements of mucosa
- -dense connective tissue
- -blood and lymphatic vessels, lymphoid folicels, and submucosal nerve plexus
- Muscularis externa
- -responsible for segmentation and peristalsis
- -inner circular and outer longitudunal layers
- -myenteric nerve plexus
- -sphincters in some regions
- -Visceral Peritonium of areolar connective tissue
- -replaced by fibrous adventitia in the esophagus
- -retroperitoneal organs have both serosa and adventitia
What is the enteric nervous system?
Instrinic nerve supply of the alimentary canal
- submucosa nerve plexus
- -regulated glands and smooth muscle in the mucosa
- myenteric nerve plexus
- -controls the GI tract motility
Name the parts of the mouth and oral cavity
- lined with stratified squamous epithelium
- soft palate and uvula close nasal cavity during swallowing
What are the functions of tongue and what are the different papillae?
- Functions include:
- -repositioning and mixing of foor during chewing
- -formation of the bolus
- -initiation of awallowing, speech, and taste
- instrinsic muscles change the shape of the tongue
- extrinsic muscles alter the tongue's position
- lingual frenulum: attachment to the floor of the mouth
- Papillae: (1st 3 house taste buds)
- 1. Filiform-whitish, give th tongue roughness and provide friction
- 2. Fungiform-reddish, scattered over the tongue
- 3. Circumvallate (Vallate)- V-shaped row in the back of tongue
- 4. Foliate- on the lateral spects of the posterior tongue
What are the intrinsic and extrinsic salivary glands?
- Extrinsic glands: parotid, submandibular, and subligual
- Instrinsic (buccal) glands: scattered in the oral mucosa
- Parotid Gland
- -parotid duct opens into the vestibule next to the second upper molar
- Submandibular gland
- -duct opens at the base of the lingual frenulum
- -opens via 10-12 ducts into the floor of the mouth
What is saliva made of?
- 97-99.5% water, slightly acidic solution containing...
- -electrolytes: Na+, K+, Cl-, PO42-, HCO3-
- -salivary amylase and lingual lipase
- -metabolic wastes: urea and uric acid
- -lysozyme, IgA, defensins, and a cyanice compound to protect against microorganisms
What controls salivation?
intrinsic glands continuously keep the mouth moist
- Extrinsic glands produce saliva when...
- -ingested food stimulated chemoreceptors and mechanoreceptors in the mouth
- -Salivatory nucleio in the brain stem send impulses along the parasympathetic fibers in the CN VII and IX
- *parasympathetic system is the main control
strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia)
Whats the different between desiduous and permanent teeth? Decribe the characteristics of teeth
- Primary dentitions are desiduous teeth --> 20
- permanent dentitions are adult teeth-->32
- incisors: chisel for cutting
- canines: fanglike teeth that tear or pierce
- premolars (bicuspids) and molars: grinding and crushing
- periodontal ligament forms fibrous joint called a gomphosis
What are cavities, gingivitis, and periodontitis?
These are tooth and gum diseases!
- Cavity: gradual demineralization of enamel and dentin
- -dental plaque adhere to teeth
- -acid from bacteria dissolves the calcium salts
- -proteolytic enzymes digest organic matter
- -prevention: daily brushing and flossing
- -plaque calcified to form calculus (tartar)
- -calculus disrupts the seal between the gingivae and the teeth
- -anaerobic bacteria infect gums
- -infection reversible if calculus removed
- -eventually causes Periodontitis
- -immune cells attack intruders and body tissues
- -these destroy the periodontal ligament and activate osteoclasts
- -possible tooth decay, promotion of atherosclerosis and clot formation in coronary and cerebral arteries from bacteria getting into blood
Describe the Pharynx
Allows passage for food, fluids, air
stratified squamous epithelial lining
skeletal muscle layers: inner longitudinal, outer pharyngeal constrictors
Describe the Esophagus
flat muscular tube from laryngopharynx to stomach
pierces the diaphram at the esophageal hiatus
joins the stomach at the cardiac sphincter
Esophageal mucosa contains stratified squamous epithelium which changes to simple columnnar at the stomach
Esophageal glands in submucosa secrete mucus to aid in bolus movement
muscularis: skeletal superiorly; smooth inferiorly
- *adventitia instead of serosa
What role does the mouth play in the digestion process?
Both mechanical and chemical digestion begin in the mouth
- -mastication is partyly voluntary, partly reflexive
- -salivary amylase and lingual lipase
- -deglutition (swallowing)
What is the process of swallowing?
- -involves the tongue, solft pallate, pharynx, esophagus, and 22 muscle groups
- Two phases:
- 1. Buccal Phase- voluntary contraction of the tongue
- 2. Pharyngeal-esophageal phase: involuntary, control center in the medulla and lower pons
- 1. Upper esophageal sphincter is contracted, During the bucca phase, the tongue presses against the hard palate, forching the food bolus into the oropharynx where the involuntary phase begins
2. The uvula and soft palate rise while the epiglottis closes to prevent food from entering the respiratory passages. The tongue blocks off the mouth. The upper esophageal sphincter relaxes, allowing food to enter the esophogus.
3. The contrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after entry.
4. Food is noved through the esophagus to the stromach by persitalsis
5. The gastroesophageal sphincter (cardiac sphincter) opens and food enters the stomach
Name the regions and characteristics of the stomach
How are nerves supplied to the stomach and how does the stomach get its blood supply?
- ANS nerve supply:
- -sympathetic via splanchic nerves and celiac plexus
- -parasympathetic via vagus nerve
- -celiac trunk
- -veins of the hepatic portal system
Name the tunics of the stomach
Four tunics: mucosa, submucosa, musculara externis, serosa
- - forms ridges called rugae
- -simple columnar epithelium with mucous cells that trap bicarbonate rich fluid beneath teh mucus
- -gastric pits lead to gastric glands
- -three layers of smooth muscle (longitudal, circular, oblique)
- -inner oblique later allows the stomach to mix, churn, move, and physically break down food
What is the role of the gastric glands?
- produce the gastric juice that begins to break the food down
- *the stomach is the first to break down protiens
- *gands in the fundus and body produc the most gastric juice
- Cell types:
- 1. Mucous neck cells
- -secrete thin, acidic mucus
- 2. Parietal cells
- -Secrete Hydrochloric Acid (HCl)
- *breaks down protien
- *activates pepsin
- *kills bacteria
- -Intrinsic Factor
- *glycoprotien required for absorption of B12 in small intestine
- 3. Chief cells
- -Secrete inactive enzyme pepsinogenn
- -activated to pepsin by HCl and by pepsin itself (positive feedback cycle)
- 4. Enteroendocrine cells
- -secrete chemical messengers into the lamina propria
- *paracrines: serotonin and histamine
- *hormones: somatostatin and gastrin
What is the mucosal barrier?
Stops self digestion in stomach
- 1. layer of bicarbonate-rich mucus
- 2. tight junctions between epithelial cells
- 3. damaged epithelial cells are wuickly replaced by division of stem cells (completely renews every 3-6 days)
What is gastritis?
inflammation caused by anything that breaches the mucosal barrier
Peptic or gastric ulcers: erosion of the stomach wall
most are caused by Heliobactor pylori bacteria
What regulated gastric secretion?
Neural and hormonal mechanisms
- stimulatory and inhibutory events occur in 3 phases
- 1. cephalic (refllex phase): few minutes to propr to good entry (hypothalamus)
- 2. gastric phase: 3-4 hours after food enters the stomach (gastrin)
- 3. Intestinal phase: brief stimulatory effect as partially digested food enters the duodenum, followed by inhibtory effecs (enterogastric reflex and enterogastrones)
- Three chemicals (ACh, histamine, and gastrin) stimulate parietal cells through secondary messengers (all three go through cyclical AMP and increase Ca2+ levels)
- -All three are necessary for maximum HCl secretion
- -Antihistamines block H2 receptors and decrease HCl release (like for ulcers)
What is the response to stomach filling?
Stretches to accomodate incoming food:
- 1. Reflex-mediatde receptive relaxation--> both in anticipation and response to eating
- -coordinated by the swallowing center in the brain
- 2. Gastric accomodation
- -plasticity (Stress-relaxation response) of smooth muscle
- -this allows stretching without greatly increasing tension
How does the stromach contract to digest food?
- Peristaltic waves move from fundus and body to pyloris at rate of 3/min
- -basic electrical rythm (BER) initiated by pacemaker cells in longitudinal muscle (cells of Cajal)
- -Distension and gastrin increase the force of contraction
- Grinding is vigorous in the pylorus
- -chyme is either delivered in ~3ml spurts into the duodenum or forced backward into the stomach
- As Chyme enters the duodenum...
- -receptors respond to stretcg and chemical signals
- -enterogatric reflex and enterogastrones inhibit gastric secretion and duodenal filling
- -carb rich chyme moves quickly through duodenum
- -fatty chyme remains in the duodenum 6 hours or more
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