Ch. 24 - Digestive System Flashcards
What are the 6 basic processes of digestion?
ingestion, secretion, motility, digestion, absorption, elimination
What are the 2 sets of nerves that innervate the GI tract?
- enteric nervous system (ENS - intrinsic)
2. ANS (extrinsic)
Describe the ENS
myenteric plexus: controls GI tract motility; between smooth muscle layers
submucosal plexus: controls secretions of GI tract organs; within mucosa
How does the ANS relate to the ENS activity?
the ANS regulates the neurons of the ENS
- parasympathetic (CN X, sacral nerves) increase activity
- sympathetic (thoracic/upper lumbar nerves) decrease activity
Where are sensory neurons located in the GI tract?
mucosal epithelium
Where are motor neurons located in the GI tract?
longitudinal and circular smooth muscle layers of the muscularis
What does the visceral peritoneum cover? The parietal peritoneum?
organs; walls of body cavity
What is the peritoneal cavity?
b/t visceral and parietal layers; contains serous fluid
What are the retroperitoneal organs?
located behind/external to peritoneal lining of abdominal cavity; pancreas, duodenum, ascending/descending colons
What is the function of peritoneal folds?
attach GI tract organs to each other and to abdominal cavity; route for blood, lymph vessels, nerves to and from GI tract
What are the 5 major peritoneal folds?
greater omentum, falciform ligament, lesser omentum, mesentery, mesocolon
What is the function of the greater omentum?
largest fold; covers transverse colon and small intestine
What is the function of the falciform ligament?
attaches liver to ant abd wall & diaphragm
What is the function of the lesser omentum?
connects stomach and duodenum to liver
What is the function of the mesentery?
attaches jejunum and ileum of SI to post abd wall
What is the function of the mesocolon?
2 folds; bind transverse and sigmoid colon to post abd wall
What are the 4 layers of the GI tract (deep to superficial) ?
mucosa, submucosa, muscularis, serosa
What are the 3 layers of the mucosa?
- epithelium (stratified sq, simple columnar)
- lamina propria (areolar CT, BV, nerves, glands, immune cells)
- muscularis mucosae (smooth muscle, folds to increase SA)
What organs are stratified squamous epithelium found in the mucosa?
mouth, pharynx, esophagus, anal canal
What organs are simple columnar epithelium found in the mucosa?
stomachs, intestines
What does the submucosa layer consist of?
areolar CT, BV, glands, lymphatic tissue, network of neurons
What does the muscularis layer consist of
- skeletal muscle - swallowing, defecation
2. smooth muscle (inner circular fibers and outer longitudinal fibers) - peristalsis
What does the serosa layer comprise of?
areolar CT covered with simple sq epithelium; forms part of the peritoneum
What does the mouth consist of?
oral cavity, hard palate, soft palate, oropharynx
What are the 3 types of salivary glands?
parotid, submandibular, sublingual
Where are parotid salivary glands found?
inf/ant to ears, between skin and masseter
Where are submandibular salivary glands found?
medial/inf to body of mandible
Where are sublingual salivary glands found?
deep to the tongue in floor of mouth
Describe the composition of saliva
99.5% water and 0.5% solutes; solutes consist of ions (Na+, K+, Cl-), dissolved gases, organic substances (mucus, IgA, lysozyme, salivary amylase)
What are some functions of saliva?
- dissolves food for tasting
- bicarbonate and phosphate ions buffer acidic foods
- chem digestion of starch by salivary amylase
- removal of metabolic wastes (urea, uric acid)
- immune functions: lysozyme helps destroy bacteria; IgA prevents attachment of microbes to E
What is the function of teeth?
mechanical breakdown of food
How does mechanical digestion occur in the mouth?
- breaks food into pieces
- mixes with saliva so enzymes can access food molec
- forms a bolus
How does chemical digestion occur in the mouth?
- salivary amylase begins starch digestion (pH 6.5-7) and inactivated by gastric juices (pH 2.5)
- lingual lipase begins digestion of triglycerides; activated in stomach by low pH
What is the pharynx?
tube of sk muscle lined by mucous membrane
What is the function of the esophagus?
secrete mucus and transport food to stomach
Describe the mucosa and submucosa of the esophagus
mucosa - stratified sq epithelium
submucosa - contains large mucous glands
Describe the muscularis of the esophagus
- upper 1/3 is sk muscle
- middle 1/3 is sk/sm muscle
- lower 1/3 is sm muscle
upper esophageal sphincter (sk) and lower (sm) are prominent circular muscles that control entry and exit of food
Does the esophagus have a serosa?
no, adventitia; areolar CT without epithelium
What is deglutition?
swallowing; moves a bolus from mouth to stomach
What are the 2 phases of swallowing
voluntary - where bolus is forced into oropharynx by tongue movement (mechanoreceptors stimulate brain)
involuntary - food moves from pharynx to esophagus; soft palate moves up to block nasal cavity and epiglottis blocks trachea; peristalsis moves bolus from esophagus to stomach
What does a bolus become in the stomach?
chyme
What does the stomach digest?
proteins, carbs, triglycerides
What is the function of rugae in the stomach?
allows large capacity to stretch
What are the 4 parts of the stomach?
cardia, fondus, body, pylorus
How does chyme exit the stomach?
via the pyloric sphincter
Describe the mucosa of the stomach?
layer of simple columnar epithelial cells that extend down into lamina propria forming gastric pits/glands
What are the 4 gastric glands in the stomach mucosa and what do they secrete?
- mucous neck cells; mucous
- parietal cells; HCl, intrinsic factor
- chief cells; pepsinogen, gastric lipase
- G cells; gastrin
What is the function of HCl in the stomach mucosa? Intrinsic factor?
converts pepsinogen into pepsin (pepsin breaks proteins into pp)
required for vitamin B12 absorption (needed to make RBCs)
What is the function of pepsinogen? Gastric lipase?
prot digestion; fat digestion
What is the function of gastrin?
stimulate release of gastric juice, increases gastric motility, relaxes pyloric sphincter, constricts esophageal sphincter
Describe the muscularis of the stomach
3 layers of smooth muscle (longitudinal, circular, inner oblique)
- third inner layer allows greater churning and mixing of food with gastric juice
How does mechanical digestion occur in the stomach?
gentle mixing waves to mix bolus with gastric juice to form chyme; emptying of stomach into small intestine occurs ~3mL at a time
How does chemical digestion in the stomach?
protein digestion begins: HCl denatures proteins and converts pepsinogen –> pepsin which breaks peptide bonds bt certain aa
fat digestion begins: gastric and lingual lipases split triglycerides
How does the pancreas connect to the duodenum of the SI?
via pancreatic and accessory ducts
What do the acini and islets of Langerhan produce?
pancreatic juice; hormones (glucagon & insulin)
Pancreatic juice contain enzymes that digest:
- starch (pancreatic amylase)
- fat (pancreatic lipase)
- nucleic acids (ribonuclease, deoxyribonuclease)
- proteins (trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase) –> inactive; get activated in duodenum
What is the function of sodium bicarbonate in the pancreas?
converts acidic chyme to slightly alkaline pH (7.1-8.2); inhibits stomach pepsin activity; promotes pancreatic enzyme activity
What organs produce and store bile?
liver; gallbladder
Describe liver histology
hepatocytes in hexagonal lobules (hepatic sinusoids)
Kupffer cells phagocytize microbes, foreign matter, dead blood cells
Describe the double blood supply to the liver
- oxygenated blood from hepatic artery
- deoxygenated blood from hepatic portal vein containing newly absorbed nutrients, drugs, microbes, toxins (which are then taken up by hepatocytes as they circulate through sinusoids)
After being taken up by the hepatocytes, where do the products go?
secreted back into blood via central vein, then eventually leaves liver via hepatic vein
What are some functions of the liver?
lipid homeostasis, fats emulsification, glucose homeostasis, detoxification, vit/min storage, phagocytosis
How does the lipid function in lipid homeostasis?
cholesterol synthesis, lipoprotein synthesis, breakdown of FA to generate ATP
What does bile contain?
water, bile salts, pigments (bilirubin from RBC breakdown), ions
What is the function of bile salts?
emulsification of fats to assist in lipid digestion and absorption in small intestine
What are the 3 sections of the SI?
duodenum, jejunum, ileum
What allows for a large surface in the SI?
circular folds, villi, microvilli
Describe circular folds in the SI
folds of mucosa and submucosa not stretchy)
Describe villi in the SI
finger-like projections of mucosa
- lamina propria contains blood and lymph capillaries, covered by simple columnar epithelium
Describe microvilli in the SI
finger-like projections on individual cells
What are the 4 types of epithelial cells in the SI?
- absorptive cells
- goblet cells
- enteroendocrine cells (secrete hormones that regulate digestion)
- paneth cells (phagocytic, secrete lysozyme)
What does the lamina propria of the ileum contain?
mucosa-associated lymphoid tissue (MALT) - Peyer’s patches with immune function
What does the submucosa of the SI contain?
duodenal glands that secrete alkaline mucus to neutralize gastric juice
Describe the muscularis of the SI
muscularis - 2 layers of smooth muscle (outer longitudinal, inner circular)
What is intestinal juice and its function?
alkaline substance made of H2O and mucus; provides liquid medium to aid in absorption; several enzymes secreted for digestion in lumen of SI
What kind of intestinal enzymes exist at the brush-border (microvilli) ?
carb-digesting, prot-digesting, nucleic acid-digesting enzymes
What are the 2 types of movement in the SI?
segmentation and peristalsis
What is segmentation?
localized contractions in areas containing chyme; mixes and absorbs food
What is peristalsis?
propels chyme onward through intestinal tract
What is required for complete chemical digestion?
pancreatic juice, bile, intestinal juice, brush border enzymes from the SI
Describe digestion of carbs
- salivary amylase in mouth (polysacc to disacc/oligosacc)
- pancreatic amylase in duodenum (to smaller oligosacc)
- maltase, alpha-dextrinase, sucrase, lactase in brush border of SI (act on oligosacc to produce monosacc)
Describe digestion of proteins
- HCl and pepsin in stomach
- digestive enzymes in pancreas
- brush border enzymes in SI
How does digestion of prot occur in stomach?
HCl denatures proteins; pepsin hydrolyzes peptide bonds
How does digestion of prot occur in pancreas?
trypsin, chymotrypsin, elastase, carboxypeptidase split peptide bonds bt specific aa
How does digestion of prot occur in SI?
aminopeptidase and dipeptidase break down peptides into single aa
Describe digestion of lipids
lingual lipase and gastric lipase digests triglycerides to FA/diglycerides/monoglycerides
- mostly occurs in small intestine
How does lipid digestion occur in SI?
- emulsification of triglycerides by bile
- pancreatic lipase splits triglycerides into FA and monoglycerides
Describe digestion of nucleic acids
- pancreatic juice contains ribonuclease and deoxyribonuclease
- Ntides are further digested by brush border enzymes (nucleosidase, phosphatase –> pentose sugars, phosphate, N bases)
How are monomers absorbed in the SI?
- carbs absorbed as monosacc
- prot absorbed as aa
- small FA enter/exit by simple diffusion
- larger lipids exist as micelles
Describe how larger lipids exit the SI?
- lipids enter cells by simple diffusion
- inside E cells, fats are rebuilt and coated with hydrophilic prot to form chylomicrons
- chylomicrons leave intestinal cells by exocytosis into a lacteal (does not enter blood!) and then removed by liver and adipose tissue
Where are the monomers transported to after leaving the SI?
blood containing absorbed monosacc/aa/short FA travel to hepatic portal vein
How is absorption of water done?
- SI absorbs 8 L
- LI absorbs 90% of last L
- done by osmosis through cell walls into vascular cap inside villi
Describe mucosa of LI
- smooth tube with no villi or folds
- intestinal glands are lined with simple columnar E (absorptive cells have microvilli, goblet cells produce mucus)
- lamina propria contain lymphatic nodules
Describe muscularis of LI
- external longitudinal and inner circular smooth muscle layers
- thickened long. fibers form teniae coli
- contractions form haustra (reason for pouch-like appearance)
What are the 3 types of movements in the LI?
haustral churning, peristalsis, mass peristalsis
What is haustral churning?
stretching of haustrum causes walls to contract and squeeze contents into next haustrum
What is mass peristalsis?
strong peristaltic wave from middle of transverse colon driving contents into rectum
How does chemical digestion occur in the LI?
no enzymes are secreted; only mucus!
- bacterial fermentation occurs
- bact produce vit. B/K in colon
What is bacterial fermentation?
undigested carbs are fermented, releasing CO2 and CH4
- breakdown of undigested protein –> odour
- breakdown of bilirubin –> brown colour
What does the large intestine absorb?
some ions and vitamins, water (chyme becomes semi solid)
What does feces consist of?
dead E cells, undigested food (e.g. cellulose), bacteria (living and dead)
What is defecation?
elimination of feces from rectum; reflex
Describe the reflex of defecation
- mass peristalsis causes filling of rectum
- stretching of rectal wall initiates reflex (includes mechanoreceptors & parasym NS)
- external anal sphincter can be voluntarily controlled to allow/postpone
- voluntary contractions of diaphragm/abs aid reflex
What is diarrhea?
- increase in freq, vol, fluid content of feces
- chyme passes too quickly through intestine
- H2O not reabsorbed
What is constipation?
- decreased intestinal motility
- too much H2O reabsorbed
