Gastroenterology Flashcards
What is the primary function of the digestive system?
Transfer nutrients, water, and electrolytes from ingested food into body’s internal environment
What organs are included in the digestive tract?
Mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus
What are 4 accessory organs of the digestive tract?
Teeth, salivary glands, liver, and pancreas
What are the 4 digestive processes?
1) Digestion
2) Absorption
3) Motility
4) Secretion
What is digestion?
The biochemical breakdown of structurally complex food into smaller, absorbable units
How is digestion accomplished?
By enzymatic hydrolysis
What can be absorbed from carbohydrates?
Monosaccharides
What can be absorbed from proteins?
Amino acids
What can be absorbed from fats?
Glycerol and fatty acids
What is absorption?
When small units resulting form digestion, along with water, vitamins, and electrolytes are transferred from digestive tract lumen to enterocytes of villi to blood or lymph
What is motility?
Muscular contractions that mix and move forward he contents of the digestive tract
What is peristalsis?
Propulsive movements that push contents forward through the digestive tract
What are segmentation and churning?
Mixing movements
What are the 2 functions of segmentation and churning?
1) Mixing food with digestive juices to promote digestion
2) Facilitate absorption by exposing all parts of intestinal contents to absorbing surfaces of digestive tract
What 3 substances are commonly secreted?
Water, electrolytes, and digestive juices
Where are secretions released?
Into digestive tract lumen
What causes secretion?
Appropriate neural or hormonal stimulation
True or false: most secretions are reabsorbed in one form or another back into blood after their participation in digestion
True
True or false: digestive system regulates uptake
False, it does not
What percent of ingested food is made available for the body to use?
95%
What are 4 regulators of digestive motility and secretion?
1) Autonomous smooth muscle function
2) Intrinsic nerve plexuses
3) Extrinsic nerves
4) Gastrointestinal hormones
What do autonomous smooth muscle produce?
Self-induced electrical activity (slow-wave potentials)
What are slow-wave potentials?
- Not action potentials
- Bring the membrane closer and further to threshold
True or false: when food is present in the digestive tract, membranes are further from threshold than normal
False, membranes are closer to threshold
What are the 2 networks of nerve fibres that lie within the digestive tract wall?
1) Submucosal plexus
2) Myenteric plexus
What is a collective term for the submucosal plexus and myenteric plexus?
Enteric nervous system
What does the enteric nervous system allow for?
Self-regulation
What are extrinsic nerves?
Nerve fibres from both branches of the ANS
What effect does the sympathetic nervous system have on digestion?
Inhibits digestion
What effect does the parasympathetic nervous system have on digestion?
Stimulates digestion
What are 3 functions of extrinsic nerves on digestion?
1) Influence digestive tract motility and secretion
2) Modify activity of the enteric nervous system
3) Alter the level of hormone secretion
What is another term for chewing?
Mastication
What is the first step in the digestive process?
Chewing
What is chewing?
Grinding and breaking food into smaller pieces to make swallowing easier and increase food surface area for salivary enzymes to act on
What produces saliva?
3 major pairs of salivary glands
What percent of saliva is water?
99.5%
What percent of saliva is electrolytes and protein?
0.5%
How much saliva is secreted per day?
1 - 1.5 L per day
What are 7 functions of saliva?
1) Salivary amylase begins digestion of carbohydrates
2) Facilitates swallowing by moistening food
3) Mucus provides lubrication
4) Antibacterial action
5) Solvent for molecules that stimulate taste buds
6) Aids speech by facilitating movement of lips and tongue
7) Rich in bicarbonate buffers
What are some antibacterial actions of saliva?
- Lysozyme destroys bacteria
- Saliva rinses away material that could serve as food source for bacteria
What is the parasympathetic effect on saliva?
Stimulates secretion of an abundant amount of thin, enzyme rich saliva
What is the sympathetic effect on saliva?
Decreases volume of saliva
What happens to the mouth when the SNS has a higher dominance?
Mouth is drier than usual
Why are saliva and gastric juice not essential for digesting and absorbing foods?
Because enzymes produced by the pancreas and small intestine can complete food digestion
What is the only digestive secretion that is entirely under neural control?
Salivary secretion
What are all digestive secretions (except salivary secretion) regulated by?
Nervous system reflexes and hormones
True or false: there is no absorption in the mouth
True
Swallowing occurs in __ phases
3
What are the 3 phases of swallowing?
1) Buccal phase
2) Pharyngeal phase
3) Esophageal phase
Is the buccal phase voluntary or involuntary?
Voluntary
What occurs in the buccal phase?
Food is compacted by the tongue into bolus and moves to the pharynx by tongue pressing on hard palate
Is the pharyngeal phase voluntary or involuntary?
Involuntary
Is the esophageal phase voluntary or involuntary?
Involuntary
True or false: the pharyngeal phase is an all-or-none reflex
True
What initiates the swallowing reflex?
Swallowing center in the medulla
What is the function of the pharyngoesophageal sphincter?
Keeps the entrance to the esophagus closed to prevent air entrance during respiration
What is the function of the gastroesophageal sphincter?
Maintains barrier between stomach and esophagus preventing gastric reflux
What occurs in the esophageal phase?
Food moves down esophagus by peristalsis
What type of neurons control the upper 1/3 of the esophagus?
Somatic motor neurons to skeletal muscle
What type of neurons control the lower 1/3 of the esophagus?
Parasympathetic NS (vagus nerve) to smooth muscle
What type of neurons control the middle 1/3 of the esophagus?
A mixture of somatic motor neurons and the vagus nerve
What happens if a large or sticky piece of food is swallowed and gets stuck?
- Esophagus distends stimulating pressure receptors
- A second, more forceful, wave is initiated by the intrinsic nerve plexus
What are the mechanisms in the different part of the mouth/esophagus that prevent food from entering while swallowing?
- Mouth – tongue on hard palate
- Nasopharynx – uvula, soft palate elevated
- Trachea – epiglottis
Esophageal (mucous) secretions are ______
Protective
What do esophageal secretions do?
Reduce the likelihood of damage by sharp edges
What is the stomach?
J-shaped sac-like chamber lying between esophagus and small intestine
What are the 3 sections of the stomach?
1) Fundus
2) Body
3) Antrum
What is the function of the pyloric sphincter?
Serves as barrier between stomach and upper part of small intestine
What are the 3 main functions of the stomach?
1) Store ingested food until it can be emptied into the small intestine
2) Secrete HCl and enzymes that begin protein digestion
3) Mixing movements convert pulverized food to chyme
What does gastric filling involve?
Receptive relaxation
What is receptive relaxation?
Smooth muscle relaxes to accommodate the extra volume of food
What triggers gastric filling?
Act of eating
Where does gastric storage take place?
In body of stomach
What occurs in gastric storage?
Carbs are broken down by salivary amylase and gastric mixing takes place in antrum
What controls gastric emptying?
Factors in the duodenum
What are 4 factors in the duodenum that delay the rate of gastric emptying?
1) Fat
2) Acid
3) Hypertonicity
4) Distension
How does fat delay the rate of gastric emptying?
- Fat digestion and absorption takes place only within lumen of small intestine
- When fat is already in duodenum, further gastric emptying of additional fatty stomach contents is prevented
How long will fatty meals remain in the stomach?
About 6 hours
How long will a meal of lean meat and potatoes remain in the stomach?
3 hours
How does acid delay the rate of gastric emptying?
Unneutralized acid in duodenum inhibits further emptying of acidic gastric contents until neutralization can be accomplished by sodium bicarbonate
Why does acid delay the rate of gastric emptying?
To prevent damage (duodenal peptic ulcers) and to allow pancreatic enzymes to function
How does hypertonicity delay the rate of gastric emptying?
Osmolarity of duodenal contents begins to rise as molecules of protein and starch are digested in the duodenal lumen, causing lots of amino acids and glucose to be released
What does osmolarity depend on?
The number of molecules present, not on their size
How does distension delay the rate of gastric emptying?
Too much chyme in duodenum inhibits emptying of gastric contents
What are 2 factors that regulate gastric motility and secretion?
1) Neural response
2) Hormonal response
How is gastric motility and secretion regulated neurally?
Through intrinsic nerve plexuses and autonomic nerves; collectively called enterogastric reflex
How is gastric motility and secretion regulated hormonally?
Through the release of hormones from duodenal mucosa collectively known as enterogastrones
What are the 2 most important hormones in the regulation of gastric motility and secretion?
1) Secretin
2) Cholecystokinin (CCK)
What is gastric dumping syndrome?
Rapid emptying
When does early gastric dumping syndrome occur and what are the symptoms?
- During or right after a meal
- Symptoms – nausea, vomiting, bloating, diarrhea, dizziness, fatigue
When does late gastric dumping syndrome occur and what are the symptoms?
- 1-3 hours post-meal
- Symptoms – weakness, sweating, and dizziness
True or false: it is uncommon for people to have early and late gastric dumping syndrome
False, it is very common
What are some treatments for gastric dumping syndrome?
- Adjust eating habits (avoid certain foods)
- Medications to slow digestion
- Surgery as a last resort
What are 3 types of gastric exocrine secretory cells?
1) Parietal cells
2) Chief cells
3) Mucous cells
What do parietal cells secrete?
HCl and intrinsic factor
What is the importance of the intrinsic factor of parietal cells?
Vitamin B12 can be absorbed only when bound to intrinsic factor; vitamin B12 is essential for formation of RBCs
What does HCl do?
Denatures proteins – causes them to lose their tertiary and secondary structure, allowing digestive enzymes o break down the amino acid chains
True or false: HCl kills most of the microorganisms ingested with food
True
What does HCl act on?
Pepsinogen
What do chief cells secrete?
Pepsinogen
What does HCl change pepsinogen into?
Pepsin
What does pepsin do?
Turns proteins in small amino acid chains
What do mucous cells secrete?
Mucous
What is the function of mucous?
Acts as a lubricant and prevents mechanical injury
Stomach is protected from self-digestion by ______
Alkaline mucous
True or false: mucous cells are replaced about every 3 months
False, they are replaced very rapidly and a new cell lining appears every 3 days
What happens if the mucous barrier is broken?
The gastric wall is injured, resulting in a gastric peptic ulcer
What does the cephalic phase refer to?
Increased secretion of HCl and pepsinogen that occurs in response to stimuli acting in the head before food reaches the stomach
When does the gastric phase begin?
When food actually reaches the stomach; presence of protein increases gastric secretions
The intestinal phase is the ______ phase
Inhibitory
What is the function of the intestinal phase?
Helps shut off flow of gastric juices as chyme begins to empty into small intestine
What are the 3 regulatory phases?
1) Cephalic phase
What occurs in the cephalic phase?
Stomach is prepared for food
What is the pH of the stomach when it is nearly empty?
3
What happens when the pH of the stomach dips below 3?
NS shuts off, causing decreased acid and gastrin decreases, causing decreased acid
Does gastrin secretion increase or decrease initially in the intestinal phase?
Increases
During the intestinal phase, what do inhibitory signals cause?
- Decreased gastric motility
- Decreased gastric secretion
What causes decreased gastric motility?
1) CCK that is released due to presence of fatty and amino acids
2) Enterogastric reflex
What triggers the enterogastric reflex?
- Amino acids/peptides
- Acid
- Duodenal stretch
- Hypertonicity
What causes decreased gastric secretion?
1) Secretin released due to acid
2) CCK released due to fatty and amino acids
Where can ethyl alcohol be absorbed the most quickly?
By the small intestine mucosa
What effect does gastric emptying have on absorption?
It slows it
What is the small intestine the site of?
Most digestion and absorption
What are the 3 segments of the small intestine?
1) Duodenum
2) Jejunum
3) Ileum
What does motility of the small intestine include?
- Segmentation
- Migrating motility complex
What is the primary method of motility in the small intestine?
Segmentation
What does segmentation consist of in the small intestine?
Ring-like contractions along the length of the small intestine
What does segmentation action do?
Mixes chyme throughout small intestine lumen
How is segmentation initiated?
By pacemaker cells in small intestine which produce basic electrical rhythm
How is segmentation controlled?
Circular smooth muscle responsiveness is influenced by distension of intestine, gastrin, and extrinsic nerve activity
What are 2 functions of segmentation?
1) Mixing chyme with digestive juices into small intestine lumen
2) Exposing all chyme to absorptive surfaces of small intestine mucosa
3) Slowly move chyme through digestive tract
The frequency of segmentation _______ along the length of the small intestine
Declines
How long does it take food to get through the small intestine?
3-5 hours
What happens when most of the meal has been absorbed?
Segmentation contractions cease and are replaced by migrating motility complex
What is migrating motility complex?
Slow, weak peristaltic waves
What is the function of migrating motility complex?
Sweeps intestines clean between meals
The pancreas is a mixture of ____ and ______ tissue
Exocrine and endocrine
Where is the pancreas located?
Behind and below the stomach
What is the endocrine function of the pancreas?
Islets of Langerhans found throughout the pancreas secrete insulin and glucagon
What are 2 exocrine functions of the pancreas?
1) 3 different pancreatic enzymes actively secreted by acinar cells that form the acini
2) Aqueous alkaline solution actively secreted by duct cells that line pancreatic ducts (contains sodium bicarbonate)
What are the 3 exocrine steps of digestion?
1) Neutralize acid
2) Pancreatic enzymes
3) Intestinal wall
Why must stomach acid be neutralized?
So pancreatic enzymes can work
How is stomach acid neutralized?
Secretin secretion is increased, which causes increased bicarbonate and decreased gastric acid secretion
Where is alkaline fluid secreted from?
Duct cells of pancreas and gall bladder
Describe the process of carbohydrate breakdown
Polysaccharides are broken down into disaccharies by pancreatic amylase
Is amylase secreted in an active or inactive form, and why?
Active form so that it doesn’t endanger the secretory cells
Describe the process of protein breakdown (3 pathways)
1) Trypsinogen is broken down into trypsin by enteropeptidase on intestinal wall
2) Chymotrypsinogen is broken into chymotrypsin
3) Procarboxypeptidase is broke into carboxypeptidase
Where are pancreatic enzymes secreted?
Into intestinal lumen from pancreatic acinar cells
What does proteases do?
Breaks down dietary protein into peptides and amino acids
What is special about pancreatic lipase?
Only enzyme secreted throughout the entire digestive system that can digest fat
What does pancreatic lipase do?
Breaks triglycerides into fatty acids and monoglycerides
What happens when pancreatic enzymes are deficient?
Serious maldigestion of fats
What does the presence of fatty acids and amino acids in the duodenum cause?
Increased cholecystokinin (CCK)
True or false: carbs do not have any direct influence on pancreatic digestive enzyme secretion
True
What are 3 effects of CCK with respect to digestion?
1) Increases pancreatic enzyme secretion
2) Stimulates gallbladder contractions, causing release of bile
3) Decreases gastric motility (slows emptying) and decrease gastric secretions
True or false: small intestine secretions do not contain any digestive enzymes, they are mostly salt and mucous solutions
True
What occurs in the intestinal wall during digestion?
Synthesized enzymes act within brush-border membrane of epithelial cells to complete digestion of carbs and proteins
Which 3 synthesized enzymes act on the intestinal wall during digestion?
1) Disaccharidases
2) Enterokinase
3) Aminopeptidases
What does disaccharidases do?
Breaks down disaccharides into monosaccharides
What breaks down glucose and what does it produce?
- Sucrase
- Produces glucose and fructose
What breaks down maltose and what does it produce?
- Maltase
- Produces 2 glucose molecules
What breaks down lactose and what does it produce?
- Lactase
- Produces glucose and galactose
What causes lactose deficiency?
Lactose intolerance – lactose remains undigested and therefore bacteria in large intestine ferment the lactose into gas, which is why cramping and diarrhea are symptoms
What does aminopeptidases do?
Breaks peptides into smaller peptides and amino acids
True or false: there is no digestion of lipids in the large intestine
True
Where is fat digested and how?
Entirely in small intestine lumen by pancreatic lipase
What does the small intestine absorb?
- All products of carb, protein, and fat digestion
- Most ingested electrolytes, vitamins, and water
Where does most absorption of the small intestine occur?
In duodenum and jejunum
True or false: the more food consumed the less that will be digested and absorbed
False, more food consumed = more digested and absorbed
Absorption of ___ and ____ are adjusted to the body’s needs
Calcium and iron
True or false: the small intestine absorbs almost everything presented to it
True
Why is the small intestine the main site of absorption?
1) Large surface area
2) Nutrients are small molecules
Why does the small intestine have such a large surface area?
Because of folds, villi and microvilli
How often does the lining of the small intestine replace itself and why?
- About every 3 days
- Because of the harsh environment
True or false: small intestine cells can withstand radiation and anti-cancer drugs
False, they are very sensitive to it
What can cause malabsorption in the small intestine?
Damage to or reduction of the surface area
What does absorption of the broken down carbs and proteins involve?
Carrier-mediated secondary active transport mechanisms
What are end products of carb and protein digestion absorbed into?
Blood
Are fats absorbed into the blood and why?
No because they aren’t water soluble
How do glucose and galactose enter enterocytes?
Secondary active transport
How do glucose and galactose leave enterocytes?
Facilitated transport
How does fructose enter enterocytes?
Facilitated transport
How does fructose leave enterocytes?
Facilitated transport
How do amino acids enter enterocytes?
Secondary active transport
How do amino acids leave enterocytes?
Facilitated transport
How do di and tripeptides enter enterocytes?
Secondary active transport
What happens to di and tripeptides when they enter enterocytes?
Converted to amino acids by cytosolic enzymes
How do monoglycerides and long fatty acids enter enterocytes?
Diffusion
How do monoglycerides and long fatty acids leave enterocytes?
Exocytosis from enterocyte, diffusion into lymph lacteal
How do short fatty acids enter enterocytes?
Diffusion
How do short fatty acids leave enterocytes?
Diffusion into blood
How does water enter enterocytes?
Osmosis
How does water leave enterocytes?
Osmosis into blood and lymph
How much water is ingested from food and drink per day?
About 2000 mL
How much water is absorbed in the small intestine from secretions per day?
About 7000 mL
Where do absorbed nutrients go?
1) Liver
2) Lacteals (lymph)
How do absorbed nutrients enter the liver?
Via hepatic portal vein
How do absorbed nutrients enter lacteals?
Via thoracic duct which goes into left subclavian vein
What does fat digestion and absorption require and where does this come from?
Requires bile from liver and gall bladder
The liver _____ bile
Synthesizes
The gall bladder _____ bile
Stores
What is the most important metabolic organ in the body?
Liver
Why is the liver important to digestive system?
Secretes bile salts
What are 6 functions of the liver that ARE NOT related to digestion?
1) Metabolic processing of nutrients
2) Detoxifying or degrading body wastes and foreign compounds
3) Synthesizes plasma proteins
4) Stores glycogen, fats, iron, copper, and many vitamins
5) Activates vitamin D
6) Excretes cholesterol and bilirubin
What performs the functions of the liver that are not related to digestion?
Hepatocytes
What function is not accomplished by hepatocytes, and what cell performs this function?
- Removal of bacteria and worn-out RBCs
- Kupffer cells
What does bile consist of?
Bile salts, cholesterol, lecithin and bilirubin, detoxified drugs, toxins, and hormones in an aqueous alkaline fluid
What does bile do AFTER a meal?
Enters duodenum
Describe the enterohepatic circulation
Bile salts in liver –> common bile duct –> stomach or gall bladder –> in stomach, sphincter of Oddi –> duodenum –> terminal ileum –> hepatic portal vein –> liver
Bile salts are derivatives of _____
Cholesterol
What are 2 functions of bile salts?
1) Convert large fat globules into a liquid emulsion
2) Micelle formation
True or false: after bile salts participate in fat digestion and absorption, most are excreted
False, most are reabsorbed into the blood
What does CCK do with respect to bile?
Stimulates contraction of the gall bladder and relaxation of the hepatopancreatic sphincter
How do bile salts form micelles?
Bile salts and lecithin aggreagate in small clusters with their fat-soluble parts huddled together in the middle to form a hydrophobic core while their water-soluble parts form an out hydrophilic shell
What is found within the core of bile salt micelles?
Products of fat digestion and fat-soluble vitamins
With respect to micelle formation, where does cholesterol dissolve?
In the core
What is the function of bilirubin?
Gives bile and urine their colour
True or false: some bilirubin is reabsorbed into the blood
True
What happens to bilirubin in the intestinal tract?
It is modified by bacteria, giving feces it’s brown colour
What occurs in the lumen with respect to fat digestion and absorption?
- Lipid globule is converted to fatty acids and monoglycerides by emulsification and lipase
- Fatty acids and monoglycerides diffuse from micelle into enterocytes
What occurs in the enterocyte with respect to fat digestion and absorption?
- Monoglycerides and fatty acids are converted to diglycerides and fatty acid
- Diglycerides are converted to triglycerides
- Triglycerides are converted to a chylomicron with cholesterol, proteins, and phospholipids
- Chylomicron moves to interstitial by exocytosis, and then to lymph by diffusion
What primarily happens in the large intestine?
Drying and storage
What does the large intestine consist of?
- Colon
- Cecum
- Appendix
- Rectum
What does the large intestine receive from the small intestine?
Indigestible food residues, unabsorbed biliary components, and remaining fluid
Where is the ileococcal valve found?
Between ileum and large intestine
What surrounds the ileococcoal valve?
Thickened smooth muscle, the ileocecal sphincter
What does pressure on the cecal side do to the ileococcal valve?
Pushes it closed and contracts the sphincter, preventing the bacteria-laden colonic contents from contaminating the nutrient-rich small intestine
What does pressure on the ideal side do to the ileococcal valve?
Opens the valve/sphincter, allowing ileal contents to enter the large intestine and gastrin to be secreted
What causes the motility of the large intestine?
1) Haustral contractions
2) Mass movements
What are Haustral contractions?
- Main motility
- Slow and weak contractions initiated by autonomous rhythmicity of colonic smooth muscle cells
What are mass movements?
Massive contractions mediated from stomach to colon by gastrin and autonomic nerves
What causes mass movements?
Food in the stomach (gastrocolic reflex)
What is the gastrocolic reflex?
Powerful waves of contractions from transverse colon to rectum
What stimulates the defecation reflex?
Feces in rectum
What part of the CNS is responsible for the defecation reflex?
Sacral segment of spinal cord (para NS)
What is the effect of the defecation reflex?
Smooth muscle of rectum contracts and internal anal sphincter relaxes
True or false: external anal sphincter is under voluntary control
True
True or false: no digestive enzymes are secreted into the large intestine
True
What is secreted into the large intestine?
- Alkaline mucous secretions
- K and bicarbonate
What occurs in the large intestine?
Bacterial fermentation of undigested nutrients
How does the large intestine absorb water?
Osmosis
What 3 hormones are secreted from the stomach and duodenum?
1) Gastrin
2) Secretin
3) CCK
What is the function of gastrin?
Increases acid and gastric enzyme secretion
What is the function of secretin?
- Increases alkaline fluid secretion
- Decrease gastric secretion
