Chapter 16 - Digestive System Flashcards
What is the primary function of the digestive system?
transfer nutrients, water, and electrolytes from ingested food into the body’s internal environment
How long is the digestive tract?
4.5m
What is food broken down into?
molecular forms
What are the 4 functions of the digestive system?
motility, secretion, digestion, absorption
Motility
the muscular contractions that mix and move forward the contents
What kind of muscle makes up the digestive tract?
smooth muscle
Tone
-the constant low level of contraction maintained by smooth muscles
What is tone used for?
-maintains a steady pressure
-prevents permanent stretching
Propulsive Movements
-a type of motility
-used to push contents forward through the tract
Mixing Movements
-a type of motility
-two functions: 1) mixing food with juices 2) absorption
Where is skeletal muscle used in digestion?
-mouth and anus
-chewing, swallowing, defecation are voluntary
What secretes digestive juices?
exocrine glands
What makes up digestive secretions
water, electrolytes, specific organic constituents (enzymes, bile salts, mucus)
What monitors the release of secretions?
neural or hormonal stimulation
When are digestive secretions normally reabsorbed?
after their participation in digestion
Digestion
the biochemical breakdown of structurally complex food into smaller, absorbable units
What accomplishes digestion?
enzymes
What is the absorbable unit of carbohydrates?
Monosaccharides (ie. glucose and fructose)
What must be done to disaccharides and polysaccharides before they can be absorbed?
they must be broken down into monosaccharides
What are some polysaccharide examples?
starch, glycogen, cellulose
What are some disaccharide examples?
sucrose, maltose, and lactose
What are some monosaccharide examples?
glucose, fructose, galactose
Disaccharide
2 monosaccharides linked together
Polysaccharide
multiple monosaccharides (glucose) linked together
What is the organic composition of single sugars?
C₆H₁₂O₆
Sucrose = Glucose + __________
fructose
Lactose = Glucose + _________
galactose
Maltose = Glucose + _________
glucose
What are proteins broken down into?
peptide fragments
What are peptide fragments further digested into?
amino acids
Where do amino acids go?
epithelial cells
Can short amino acid chains be absorbed?
yes
What form are dietary fats in?
triglycerides
What does fat digestion produce?
2 free fatty acids and monoglyceride
Hydrolysis
how the enzymes break bonds to digest contents into smaller molecules
Where does most absorption occur?
the small intestine
Absorption
of small units resulting from digestion, vitamins, electrolytes, and water into blood or lymph
Digestive Tract Organs
-mouth
-pharynx
-esophagus
-stomach
-sm. intestine (duodenum, jejunum, ileum)
-lg. intestine (cecum, appendix, colon, rectum)
-anus
Accessory Digestive Organs
-salivary glands
-pancreas (exocrine)
-liver
-gall bladder
What are the functions of the mouth and pharynx?
-chewing
-swallowing
What are the functions of salivary glands?
-moisten and dissolve food
-lubrication
-polysaccharide digesting enzyme (amylase)
What do the salivary glands secrete?
-salt
-water
-mucus
-amylase
What does the stomach do?
-store, mix, dissolve
-continue digestion
-regulate emptying into sm. intestine
What does the stomach secrete?
-HCl (kill microbes, pepsinogen to pepsin)
-Pepsin (protein digesting enzyme)
-Mucus (lubrication and protection)
What does the pancreas do?
-secrete enzyme and bicarbonate
What does the pancreas secrete?
-enzymes (digestion)
-bicarbonate (neutralize HCl entering sm. intestine)
What does the liver do?
-secrete bile
What does the liver secrete?
-bile salts (solubilize fats)
-bicarbonate (neutralize HCl)
What does the gallbladder do?
store and concentrate bile between meals
What does the small intestine do?
-digestion and absorption of most substances
-mixing and propulsion
What does the small intestine secrete?
-enzymes (digestion)
-salt and water (fluid balance)
-mucus (lubrication)
What does the large intestine do?
-storage
-absorption of salt and water
-mixing and propulsion
-defecation
What does the large intestine secrete?
mucus for lubrication
What are the 4 major tissue layers of the digestive tract?
from inward to outward: mucosa, submucosa, muscularis externa, serosa
Mucosa
-lines luminal surface
-divided into 3 layers: mucous membrane, lamina propria, muscularis mucosa
What do the folds of the mucosa do?
increase absorptive surface area
Mucous Membrane
-most inward
-exposed to lumen
-serves as a protective surface
-secretion and absorption in certain areas
Mucous Membrane: Exocrine Gland Cells
secrete digestive juices
Mucous Membrane: Endocrine Gland Cells
secrete blood-borne GI hormones
Mucous Membrane: Epithelial Cells
-specialized
-absorb digestive nutrients
Lamina Propria
-thin middle layer
-house GALT
Gut-Associated Lymphoid Tissue (GALT)
defence against pathogenic bacteria
Muscularis Mucosa
-outermost mucosal layer
-sparse layer of smooth muscle
Submucosa
-second layer inward
-thick connective tissue layer
What is the function of the submucosa?
provides distensibility and elasticity
What does the submucosa have?
-larger blood and lymph vessels
-submucosal plexus
Submucosal Plexus
nerve network in the submucosa
Muscularis Externa
-third layer inward
-the major smooth muscle coat
-2 layers: circular and longitudinal
-myenteric plexus
Muscularis Externa: Circular Layer
-inner muscular layer
-controls diameter of lumen
Muscularis Externa: Longitudinal Layer
-outer layer
-controls length of the tube
What is the function of the muscularis externa?
propulsion and mixing
Interstitial Cells of Cajal
-cells between the two muscularis externa layers
-the origin of BER
Basic Electrical Rhythm (BER)
-rhythmic electrical activity
-pacemaker cells
-control slow wave cycle activity
-not action potentials, controlled by calcium
Myenteric Plexus
-lies btween the two muscularis externa layers
-extensive network of nerve cells
What regulates local gut activity?
-submucosal and myenteric plexuses
-hormones
-local chemical mediators
Serosa
-outer of the 4 layers
-connective tissue covering
What does the serosa secrete?
serous fluid to lubricate and prevent friction with surrounding organs
What is the serosa continuous with? Why?
-the mesentery
-suspends GI organs like a sling
-provides fixation and support
What regulates motility and secretion?
-autonomic smooth muscle
-BER
-intrinsic nerves plexuses
-extrinsic nerves plexuses
-GI hormones
What are the two intrinsic nerve plexuses?
ENS (myentric and submucosal)
Enteric Nervous System
-the GI tracts own NS
-mostly self-regulated but receives some ANS input
The ENS has ________ and ________ functions
sensory and motor
Extrinsic Nerve
-from both ANS branches
-originate outside GI tract
Sympathetic NS
-inhibit GI contraction and secretion
-digestion not a priority in an emergency
Parasympathetic NS
-dominates when regular activity needed
Gastrointestinal Hormones
-carried through blood and exhibit functions
-ie. Gastrin stimulates gastric juice release in stomach
Lips
-opening to the digestive tract
-procure, guide, maintain food in mouth
-speech
-tactile sensation
Palate
arched roof of the oral cavity
Uvula
seals off nasal passages during swallowing
Tongue
-floor of oral cavity
-chewing
-swallowing
-speech
-taste
What type of muscle is the tongue made of?
skeletal
Pharynx
-the cavity at the back of the throat
-passageway for air and food
Tonsils
-side walls of pharynx
-lymphoid tissue for immune support
Teeth
-functions of chewing
-mechanical digestion
-increases surface area for enzymes to act on
-mix food and saliva and stimulate taste buds
What are the 3 pairs of salivary glands?
-parotid
-sublingual
-submandibular
How is saliva secretion accomplished?
by a large increase in blood flow
Which exocrine has the largest secretion per gram?
salivary glands
What is the composition of saliva?
99.5% water and 0.5% electrolytes and proteins (amylase, mucus, lysozyme)
Salivary Amylase
-begins the digestion of carbohydrates
-breaks polysaccharides into maltose
Antibacterial Action of Saliva
-lysozyme breaks down the cell walls of certain bacteria
-rinse away food that could feed bacteria
Lubrication at Oral Cavity
-saliva moistens food
How does saliva act as a solvent?
-taste buds need moisture to work
Bicarbonate in Saliva
-neutralizes acids to prevent cavities and tooth decay
Do sympathetic or parasympathetic NS control saliva release?
both
Where is amylase broken down?
stomach
Is digestion at the oral cavity essential?
no, most of it is done later on
Parasympathetic Salivary Secretion
-low-level
-keeps throat and mouth always moist
Simple Salivary Reflex
-controlled by chemoreceptors and pressure receptors responding to presence of food
Salivary centre
-located in the medulla
-controls digestive activities
Conditioned Salivary Reflex
-occurs without oral stimulation
-thinking about, seeing, smelling, hearing food can activate
Is parasympathetic and sympathetic activity at salivary glands antagonistic?
no
What motility is associated with the pharynx and esophagus?
swallowing
Swallowing is an ______ or _____ reflex.
all or none
When is swallowing initiated?
when bolus is forced by tongue into pharynx
What is the most complicated reflex in the body?
swallowing
Can swallowing be stopped once begun?
no
Is swallowing voluntary?
yes
Where is the swallowing centre located?
medulla
Epiglottis
-small cartilage flap
-protects airways from food
Esophagus
-straight muscular tube between pharynx and stomach
The esophagus has ________ at each end.
sphincters
Pharyngoesophageal Sphincter
-upper esophageal sphincter
-prevents large volumes of air from entering digestive tract during breathing
Gastroesophageal Sphincter
-lower esophageal sphincter
-prevents reflux of gastric contents
Peristaltic Waves
push food through the esophagus
Esophageal secretions are entirely _________
protective (mucus lubricates)
What are the 3 sections of the stomach?
- fundus
- body
- antrum
Fundus
-lies above esophageal opening
-thin smooth muscle
Body (Stomach)
-the middle/main part
-thin smooth muscle
Antrum
-lower part
-heavier smooth muscle
Pyloric Sphincter
-barrier between stomach and duodenum (sm. intestine)
Where are the mucosa located in the stomach?
- Oxyntic (in the body and fundus)
- Pyloric Gland Area (in the antrum)
What are the 3 main stomach functions?
- Store ingested food
- Secrete HCl and enzymes for protein digestion
- Mixing to convert food to chyme
What are the 4 aspects of gastric motility?
- filling
- storage
- mixing
- emptying
Gastric Filling
-stomach has a 50mL capacity at empty but can hold up to 1L
Receptive Relaxation
-an aspect of filling
-how the stomach accommodates volume change
-stomach folds relax and get smaller as it fills
-without increasing pressure
Where does gastric storage take place?
the body of the stomach
What moves contents into the storage area?
-peristalsis
-BER
Where does gastric mixing take place?
antrum of the stomach
How is food mixed? What does it turn into?
food is mixed by strong peristaltic waves and is turned into chyme
Factors that influence gastric emptying: Stomach
-amount of chyme
-Gastrin
-fluidity of the chyme
Factors that influence gastric emptying: Duodenum
-*primary controls
-duodenum must be ready
-presence of fat, acid, hypertonicity, distention
How does ACID affect emptying in duodenum?
-acid in duodenum inhibits further emptying of acidic contents
How does HYPERTONICITY affect emptying in duodenum?
-emptying is inhibited when osmolarity starts to rise
How does DISTENSION affect emptying in duodenum?
-too much chyme inhibits emptying
How does FAT affect emptying in duodenum?
-fat digestion and absorption only occur in sm. intestine
-when fat is already present, emptying is inhibited
Emptying: Neural Response (Enterogastric Reflex)
-mediated by intrinsic nerve plexuses (short) and autonomic nerves (long)
Emptying: Hormonal Response (Enterogastrones)
-released from duodenal mucosa
-secretin and cholecystokinin
Effect of emotions and pain on emptying:
-decrease motility - sadness, fear, pain (SNS)
-increase motility - anger, aggressioon
What are the 3 types of secretory cells in oxyntic mucosa?
- Parietal (oxyntic) Cells
- Chief Cells
- Mucous Cells
Parietal (Oxyntic) Cells
-secrete HCl and intrinsic factor
Intrinsic Factor
used for vitamin B₁₂ absorption
Chief Cells
secrete pepsinogen, an enzyme precursor
Mucous Cells
-line gastric pits and gland entrances
-secrete watery mucus for lubrication and HCl and pepsin protection
HCl Functions
-activates pepsinogen into pepsin
-provides acid medium for optimal pepsin activity
-breakdown of connective tissue and muscle fibres
-denatures proteins
-kills microorganisms
Gastric Pits
invaginations in the stomach wall
Gastric Glands
lie at the base of gastric pits
The entire stomach mucosa is replaced every ___ days by means of _________ cells.
3 days; stem cells
Surface Epithelial Cells
-cover gastric mucosa
-secrete thick alkaline mucus
-forms visible layer for protection
Pepsinogen (inactive)
the major digestive constituent of gastric secretion
Pepsin (active)
splits certain amino acid chains into smaller chains
Where is pepsinogen stored and how is it activated?
-stored in secretory vesicles called zymogen granules
-HCl activates pepsinogen
Autocatalytic (“self-activating”) Process of Pepsin
-process by which pepsin activates other pepsinogen molecules to create more pepsin
Why is pepsin stored in its inactive form?
so it doesn’t digest the proteins of the cells that create it
What secretes ‘gastrin’ into the blood?
-endocrine ‘G cells’ found in gastric PGA pits
What secretes ‘histamine’ into the blood?
-Enterochromaffin-like (ECL) cells
-dispersed among parietal and chief cells
What secretes ‘somatostatin’?
-D cells
-in glands near pylorus and duodenum
What 4 substances control secretion of gastric juices?
-gastrin
-histamine
-ACh
-somatostatin
What does ACh do?
-stimulates parietal, chief, G cells, and ECL cells to secrete
What does Gastrin do?
-stimulates parietal and chief cells
-stimulates ECL cells to release histamine to bring about HCl
What does histamine do?
-acts locally to speed up HCl secretion
What does somatostatin do?
-acts as a paracrine gland locally
-negative feedback fashion on G cells and ECL cells to inhibit HCl secretion
What does the pyloric gland area (PGA) secrete?
gastrin and somatostatin
What are the 3 phases of gastric secretion?
- cephalic
- gastric
- intestinal
Cephalic Phase
-acts in the head before food reaches the stomach
What happens during cephalic phase?
-increased HCl and pepsinogen secretion
-a) thinking about food increases ACh action on these cells
-b) stimulation of G cells enhances HCl and pepsinogen secretion
Gastric Phase
-begins when food reaches stomach (ie. proteins)
What happens during gastric phase?
-proteins stimulate nerve pathways and gastrin release
Intestinal Phase
-the inhibitory phase
What happens during the intestinal phase?
when chyme begins to empty the flow of gastric juices is shut off
Which nerve controls all these actions?
vagus
What 3 things reduce gastric secretion?
- as the food empties into duodenum
- fall in pH triggers somatostatin release
- enterogastric reflex and enterogastrones suppress secretory cells
Tight Junctions
line mucosa to prevent leaking acid
Gastric Mucosal Barrier
-lubricant, prevention of mechanical injury, protects from self digestion
Peptic Ulcers
-when barrier is broken and gastric wall is injured by acid
Where does alcohol and aspirin absorption occur?
-stomach
Where does 95% of absorption occur?
small intestine
Pancreas
-below stomach and above duodenum
-has endocrine and exocrine functions
Islets of Langerhans
secrete insulin and glucagon in pancreas
Acini
-secretory cells in the pancreas
-connect to ducts and eventually empty into duodenum
Exocrine Pancreas
-secretes pancreatic juice that consists of 1) pancreatic enzymes secreted by acinar cells 2) aqueous alkaline solution secreted by duct cells
Where are pancreatic enzymes stored?
zymogen granules (inactive precursors)
What is the alkaline solution rich in?
sodium bicarbonate NaHCO3
What do proteolytic enzymes do?
digest proteins
What are the 3 proteolytic enzymes?
trypsin, chymotrypsin, carboxypeptidase
Trypsinogen is converted to ______
trypsin
Chymotrypsinogen is converted to ______
chymotrypsin
Procarboxypeptidase is converted to _______
carboxypeptidase
What 3 pancreatic enzymes are secreted by acinar cells?
- Proteolytic enzymes
- Pancreatic amylase
- Pancreatic lipase
Pancreatic Amylase
-converts polysaccharides into maltose (disaccharide)
Pancreatic Lipase
-only enzyme that can digest fat
What activated trypsinogen?
enterokinase in duodenum
Secretin acts on the ____ cells of the pancreas.
duct
CCK acts on the ______ cells of the pancreas.
acinar
What is the purpose of the alkaline fluid secreted?
-duodenum can be damaged by acid
-pancreatic enzymes don’t work in acidic conditions
Action of Secretin
-release is stimulated by acid in duodenum
-secretin carried by blood to pancreas
-stimulates duct cells to make alkaline fluid
-acid in duodenum is neutralized
Action of CCK
-presence of fat and protein in duodenum stimulate CCK release
-CCK carried by blood to pancreas
-acts of acinar cells to increase digestive enzyme secretion
-fat and proteins are digested
How are digestive enzymes released?
all of them are grouped together in zymogen granules and release together
What is the largest and most important metabolic organ in the body?
liver
Where are bile salts secreted from?
the liver via gallbladder and ducts
What are bile salts used for?
fat digestion and absorption
Where is bile stored?
gallbladder
What makes up bile?
-bile salts
-cholesterol
-lecithin
-bilirubin (yellow)
When does bile enter the duodenum?
after a meal
What does the hepatic portal vein carry? What does it connect?
-carries newly absorbed nutrients to liver for processing
-from sm. intestine to liver
What is bile salt?
-derivatives of cholesterol
What does bile salt do?
convert large fat globules into a liquid emulsion
What happens to bile salts after participating in fat digestion and absorption?
it is reabsorbed into the blood
Where are bile salts reabsorbed?
terminal ileum
Secreted bile salts are ____% recycled, ___% new.
95; 5
What % of bile salts are lost in feces?
5%
How do bile salts help lipase?
increase surface area for the enzyme to act on
Enterohepatic Circulation
mechanism that recycles bile salts
Micelle
-water soluble structures critical component for fat absorption
What do micelles do?
-transport digested free fatty acids, monoglycerides, fat soluble vitamins, cholesterol in their hydrophobic centre
Structure of a Micelle
-hydrophilic shell (water loving)
-hydrophobic centre (water fearing)
Bilirubin
-no role in digestion
-waste product excreted in bile
-pigment derived from breakdown of RBCs
How is bile concentration controlled?
-salt actively transported out of gallbladder and water osmotically follows
After luminal contents pass beyond the sm. intestine, _____ and _____ no longer occur
digestion and absorption (except salt and water)
What are the motility components of the sm. intestine?
-segmentation
-migrating motility complex
Segmentation
-the sm. intestines primary method of motility
-ring-like contractions
What does segmentation do?
-mix chyme
-expose chyme to absorptive surfaces in mucosa
-move chyme along lumen
What initiates segmentation?
-pacemaker cells that produce BER
Is segmentation frequency constant?
no, it is slower in the ileum
When do segmentation contractions cease?
when most of the meal has been absorbed
What replaces segmentation?
migrating motility complex (MMC)
When does the MMC act?
between meals
What does the MMC do?
-uses peristaltic waves to sweep from the stomach to the intestine
-get rid of meal debris and bacteria and push towards colon
Which hormone regulates MMC?
motilin, secreted by intestinal mucosa
Ileocecal Valve
-keeps ileal contents moving forward
The ileocecal juncture function
barrier between the sm. and lg. intestines
Ileocecal Sphincter
-remains mostly constricted
-mediated by intrinsic plexus and gastrin secretion
Does the juice secreted by sm. intestine contain digestive enzymes?
no
Succus Entericus
-aqueous salt and mucous solution secreted by sm. intestine
-lubricates and protects
-provides water to help with digestion (hydrolysis)
Where do synthesized enzymes in sm. intestine act?
within brush-border membrane of epithelial cells
What forms the brush border?
microvilli that line the intestinal lumen
What are the 3 brush border enzymes?
- enterokinase
- disaccharidases
- aminopeptidases
Enterokinase
activates trypsinogen to trypsin
Disaccharidases
-maltase, sucrase, or lactase convert disaccharides to monosaccharides
Aminopeptidases
convert peptides to amino acids
What continues carb and protein digestion in the sm. intestine?
pancreatic enzymes
What completes carb and protein digestion in sm. intestine?
brush border enzymes
Where and by what is fat ENTIRELY digested by?
-within small intestine by pancreatic lipase
What 2 enzymes digest carbohydrates?
-amylase (1st by salivary glands and in stomach: poly to di)
-disaccharidases (2nd by pancreas and sm. intestine: di to mono)
What are the absorbable nutrients of carbohydrates?
monosaccharides (esp. glucose)
What 3 enzymes digest proteins?
-pepsin (stomach antrum: peptide fragments)
-Trypsin, chymotrypsin, carboxypeptidase (pancreas: attack different fragments)
-Aminopeptidases (sm. intestine: peptide fragments to amino acids)
What are the absorbable nutrients of proteins?
amino acids (+ some small peptides)
What are the 2 enzymes for digesting fat?
-bile salts - *not an enzyme (liver: emulsification for attack by lipase)
-lipase (pancreas: triglycerides to fatty acids & monoglycerides)
What are the absorbable nutrients of fat?
fatty acids and monoglycerides
What does the small intestine absorb?
almost everything presented to it
Where does most absorption occur at the sm. intestine?
duodenum and jejunum
Where is absorption of bile salts and vit. B12 done?
ileum
Specialized Absorptive Sm. Intestine Surfaces
-folds to increase surface area
-microvilli
-600x greater than if it was flat
Villus Structure
-epithelial cells cover surface
-connective tissue core
-capillary netwrok
-terminal lymphatic vessel (central lacteal)
Crypts of Lueberkühn
-invaginations between villi
-don’t secrete digestive enzymes
-secrete water and electrolytes
-stem cells for new epithelial cells
The lining of the sm. intestine is replaced every ___ days
3
Protein and carb end products need _______ to cross the membrane during absorption.
cell-mediated transport systems
How are the products of fat absorbed?
broken down and passively absorbed then enter lymph
Carbohydrate Absorption (Glucose and Galactose)
-active transport by a) Na+ monosaccharide co transport or b) Na+ independent facilitated diffusion
Carbohydrate Absorption (Fructose)
facilitated diffusion
Carbs and proteins _____ into blood
diffuse
How are proteins absorbed?
Na+ depended transport
Free Ride
glucose, galactose, amino acids get a free ride on the energy expended for Na+ transport
How do the Micelle contents enter epithelial cells?
passive transport
Once entered epithelial cells, monoglycerides and fatty acids are…
resynthesized into triglycerides and packaged into chylomicrons
Chylomicrons
-packaged triglycerides coated in a lipoprotein that makes them water soluble
Do chylomicrons enter the blood?
no, they go to the central lacteal in the villus (lymph vessel)
The large intestine is primarily a ______ and _______ organ.
drying (salt and water absorption) and storage (of feces)
What does the lg. intestine receive from the sm. intestine?
-indigestible food residue
-unabsorbed biliary components
-remaining fluid
Taeniae Coli
-longitudinal bands of muscle in the lg. intestine
Haustra
-pouches or sacs on the lg. intestine
-actively change location due to smooth muscle contraction
What is the main motility of the colon?
haustral contractions
How fast are haustral contractions? What regulates them?
-slow
-the autonomic rhythmicity of sm. muscle cells
Mass Movements
-massive contractions of the colon to move contents into distal part
-stored until defecation
Gastrocolic Reflex
-from stomach to colon
-regulated by ANS and gastrin
-followed by urge to defecate
-evident after 1st meal of the day
-used to make room for new meals
What initiates the defecation reflex?
stretch receptors in the rectal wall
Where are the sensory receptors sent?
sacral portion of the spinal cord
Does the parasympathetic or sympathetic NS initiate the reflex?
parasympatheic
What type of muscle is the internal anal sphincter?
smooth muscle
What type of muscle is the external anal sphincter?
skeletal muscle
Does the lg. intestine secrete digestive enzymes?
no
What does the lg. intestine secrete?
-alkaline mucous solution
-lubrication and neutralizing acid produced by bacteria
Gastrin Summary
-stimulated by presence of proteins in the stomach
-inhibited by acid accumulation in stomach
-increase HCl and pepsinogen secretion
-enhances motility, mass movements
-GI tract lining maintenance
Secretin Summary
-stimulated by presence of acid is duodenum
-inhibits emptying into sm. intestine when acid already present
-stimulates duct cells to excrete alkaline solution
-stimulates bile secretion
-trophic to exocrine pancreas
CCK Summary
-inhibits motility and secretion
-stimulates acinar cells to secrete pancreatic enzymes
-gallbladder contraction
-trophic to exocrine pancreas
