Chapter 17 - Digestive System Flashcards
What is the order in which a food molecule will encounter the digestive tract?
- Pharynx
- Esophagus
- stomach
- duodenum
- cecum
- transverse colon
what are the main 4 tunics of the digestive tract?
- serosa
- muscularis externa
- submucosa
- mucosa
Muscularis externa tunic
- muscle tissue
- outer smooth muscle for length
- inner layer controls radius
- enteric nervous system & myenteric plexus
Submucosa tunic
- connective tissue
- submucosal plexus
- located within muscularis externa
- contains nerves, circulatory and lymphatics
Mucosa Tunic
- forms lumen
- three sublayers:
1. muscularis mucosae (smooth muscle; thin)
2. lamina propria (loose connective)
3. epithelial tissue forming lumen
Serosa tunic
- contiguous with mesenteries
- contains blood vessels, nerves, lymphatics
Peristalsis
- ** wavelike movements** through coordinated contraction of circular smooth muscle (controls radius) behind a bolus, and relaxation of circular sooth muscle in front of it.
- bolus moves forward
- contraction of longitudinal smooth muscle (length) which sends bolus forward (inchworm)
-** short distance** waves
segmentation
- simultaneous muscle contractions of circular smooth muscle in front of AND behind bolus
- back and forth movements
- bolus gets mixed with digestive secretions
Mechanical digestion
- begins in oral cavity
- does NOT break chemical bonds
- goal is to increase surface area for chemical digestion
Chemical digestion
- begins in oral cavity but peaks in stomach and small intestine
- carried out by enzymes
- breaks chemical bonds to generate small molecules from large molecules
____________and ____________ are the last of the major digestive processes.
secretion; absorption
Secretion
- movement of substance from cells into the lumen
- e.g. secretion of HCl into stomach to start digestion
- PRODUCES ACID IN STOMACH
- highly selective
Absorption
- movement of a substance from the lumen to cells
- highly selective process by specific region
Functions of the oral cavity
- protects mouth/digestive system against physical and chemical abrasions and food-borne pathogens
- Increasing surface area of food
- Coating food with saliva
- Initiation of swallowing to deliver food to the stomach
Mastication
Chewing
Features of mastication
- mechanical digestion
- increases surface area
- mixes food with saliva
- under control of medulla oblongata
- done w/ teeth
What teeth are for cutting and tearing food?
incisors (8) and canines (4)
What teeth are for crushing and grinding?
premolars (8) and molars (12)
What are all the components of saliva?
- water
- electrolytes
- mucous
- leukocytes
- epithelial cells
- glycoproteins
- enzymes
- IgA
- lysozymes
Main features/functions of saliva
- pH between 6.5-7.5
- moistens epithelia and liquefies food
- 5% of polysaccharide breakdown (salivary amylase)
- innate immunity due to antimicrobial lysozyme and IgA
- secretion is stimulated by facial and glossopharyngeal nerves(CN VII & CN IX) in response to varied stimuli
Saliva is secreted into oral cavity by 3 glands:
- parotid gland
- submandibular gland
- sublingual gland
What are the 3 phases of deglutition?
-
swallowing = deglutition
1. voluntary phase
2. pharyngeal phase
3. esophageal phase
Voluntary phase of swallowing
- mouth, tongue, teeth, and secretions make bolus
- tongue pushes bolus against hard palate and toward back of oral cavity into oropharynx and detected by receptors in pharyngeal wall
Pharyngeal phase of swallowing
- contact of food w/ sensory receptors in posterior wall of oropharynx —> afferent info through trigeminal (CN 5) and glossopharyngeal (CN IX) nerves to swallowing center in medulla
- motor info travels —> trigeminal (CN V), glossopharyngeal (CN IX), vagus, and accessory nerves to soft palate and pharynx
- upper esophageal sphincter relaxes (involuntary)
Esophageal phase of swallowing
- ends swallowing process
- lower esophagus = smooth muscle (unconsciously controlled)
- upper esophagus = skeletal muscle (conscious control to initiate swallowing)
- food travels through esophagus –> stim. stretch receptors –> signal enteric nerve plexus to smooth muscle –> stim. contraction and peristalsis waves
- activates efferent neurons - food enters stomach
Stomach
- short-term storage reservoir
- mix and grind stomach contents w/ HCl and pepsin –> chyme
- cont. chemical and enzymatic digestion (proteins)
- move chyme into small intestine
- connects to esophagus in cardiac region
What are gastric pits?
openings for secretions into the stomach
- parietal cells
- chief cells
- G cells (gastrin)
- D cells (somatostatin)
- Enterochromaffin cells
chyme
an acidic fluid containing partially digested food
chief cells
secrete and synthesize pepsins (major proteases active in stomach)
- contain hormone receptors that stimulate pepsin release
- preproenzyme –> pepsinogen –> pepsin
what are the pepsins synthesized by chief cells?
- endopeptidases that recognize A.A. sequences embedded within proteins
responsible for bulk of protein digestion
Exopeptidases
cleave peptides from the ends of proteins into single amino acids
includes carboxypeptidase
Parietal cells
- secrete HCl to a concentration of ~160 mM (0.8 pH) using active transport
- regulated by paracrine signaling
- secrete bicarbonate on basolateral side
- secrete intrinsic factor
- use enzyme, carbonic anhydrase to combine CO2 and H2O into carbonic acid (H2CO3)
- secrete H+ via active transport w/ H+/K+ ATPase proton pump
Enterochromaffin (ECL) cells
- secrete histamine to stimulate acid secretion
- Binds H2 receptors on parietal cells to stimulate their HCl secretion
D cells
- secrete somatostatin
- inhibits gastrin and histamine secretion
G cells
- secrete gastrin in response to food in lumen and stomach distension
- binds receptors on basolateral side of parietal cells to stimulate HCl release
- also stimulates pepsin (chief cells) and histamine (ECL cells) secretion
Proton Pump
- parietal cells secrete H+ into stomach lumen via H+/K+ - ATPase
- Cl- ions are transported into lumen via conductance channels
- HCl is formed in the lumen
Gastric/duodenal ulcer
- break in normal tissue lining stomach or small intestine
- benign ulcers caused by imbalance between defenses of stomach mucosal lining and the secretion of acid and pepsin
- 90% of all cases are bc of the bacteria H. pylori
- stress/spicy foods do NOT cause/worsen ulcers
- most receptive inhibitors: H2 receptor antagonists and proton pump inhibitors
- can also be treated w/ antacids, antibiotics, and dietary changes
Interstitial cells of Cajal
within greater curvature of stomach; act as pacemaker cells
- stimulate tonic (w/o outside stimulation) contractions
Pacemaker potentials
reduce the threshold for contraction of smooth muscle and can lead to burst of action potential/smooth muscle contraction
Steps of stomach motility contractions
- food mixes with stomach secretions to form chyme
- tonic smooth muscle contractions
- phasic (stim. by nervous system/hormones/food) smooth muscle contraction
- upper stomach expands, keeps constant pressure
- mixing waves push chyme towards pyloric sphincter
- stomach distension stimulates gastrin release
- pyloric sphincter relaxes, food is pushed into duodenum
Functions of small intestine
- mix liver and pancreatic secretions w/ chyme
- to continue digesting carbohydrates and proteins, and to initiate fat digestion
- absorb nutrients
- move chyme to large intestine
- produce hormones that regulate digestive system
- produce increased numbers of immune cells that protect against pathogens associated with food and microbiome
How much digestion occurs in small intestine?
90%
Three parts of small intestine
- duodenum
- jejunum
- ileum
Duodenum
- shortest segment (4%)
- duodenal papillae: common bile duct and pancreatic duct join w/ hepatopancreatic ampulla (allows small intestine to receive secretion from liver, pancreas, and gallbladder)
Duodenum
- shortest segment (4%)
- duodenal papillae: common bile duct and pancreatic duct join w/ hepatopancreatic ampulla (allows small intestine to receive secretion from liver, pancreas, and gallbladder)
Plicae
series of large projections formed by mucosa and submucosal layers