Gastro - Physiology Flashcards
What are the functions of gastrin?
Gastrin increases acid secretion, promotes growth of the gastric mucosa, and increases gastric motility
What are stimuli for the release of gastrin?
Distension, amino acids, vagal stimulation
What serves as negative feedback for gastrin release?
Acid secretion; a pH < 1.5 will inhibit gastrin secretion
A patient presents with peptic ulcer disease refractory to medical treatment. On endoscopy, he is found to have multiple ulcers in the stomach and a few in the jejunum. Fasting gastrin levels are markedly elevated. What is the likely diagnosis?
Zollinger-Ellison syndrome due to ectopic production of gastrin
What two amino acids are especially potent stimulators of gastrin release?
Tryptophan and phenylalanine
Which cells in the gastrointestinal tract make cholecystokinin?
I cells of the duodenum and jejunum
What are the actions of cholecystokinin?
Stimulation of gallbladder contraction and pancreatic enzyme secretion; slowing of gastric emptying
What stimulates cholecystokinin release?
The presence of fatty acids and amino acids in the duodenum
In cholelithiasis, pain worsens after the ingestion of what type of foods?
Fatty foods; due to stimulation of cholecystokinin release, which causes gallbladder contraction
Secretin is produced by which cells?
S cells of the duodenum
What are the actions of secretin?
Increases bicarbonate secretion, increases bile acid secretion, decreases gastric acid secretion
Which two substances stimulate secretin release within the duodenum?
Acid and fatty acids
Secretin-stimulated pancreatic bicarbonate functions to neutralize _____ _____ within the _____.
Gastric acid; duodenum
Why is it important that secretin-stimulated bicarbonate neutralize gastric acid within the duodenum?
Pancreatic enzymes would otherwise be denatured and nonfunctional in the acidic environment created by unopposed gastric acid
Somatostatin is made by which cells?
D cells of pancreatic islets and gastrointestinal mucosa
Does somatostatin lead to increased or decreased pepsinogen secretion? Increased or decreased gastric acid secretion?
Decreased; decreased
Does somatostatin increase or decrease pancreatic secretions?
Decrease
Does somatostatin increase or decrease fluid secretions in the small intestine?
Decrease
What effect does somatostatin have on the gallbladder?
Somatostatin decreases gallbladder contraction
The presence of what substance in the gut lumen causes increased somatostatin release?
Acid
What functions to inhibit somatostatin release?
Vagal stimulation
Given the functions of somatostatin, why is it classified as an antigrowth hormone?
Somatostatin inhibits digestion and absorption of nutrient, thereby preventing the body from receiving nutrient needed for growth
What are two tumors that can be treated with somatostatin?
Vasoactive intestinal peptide tumors and carcinoid tumors; somatostatin reduces symptoms by inhibiting secretions from the tumors
Glucose-dependent insulinotropic peptide is made by which cells?
K cells of the duodenum and jejunum
What is the exocrine regulatory effect of glucose-dependent insulinotropic peptide?
Decreased secretion of gastric acid
What is the endocrine regulatory effect of glucose-dependent insulinotropic peptide?
Increased release of insulin
What increases glucose-dependent insulinotropic peptide secretion?
Fatty acids, amino acids, and oral glucose load
Why is an oral glucose load used more rapidly by the body than an equivalent load that is given intravenously?
Because oral (but not intravenous) glucose stimulates glucose-dependent insulinotropic peptide, which stimulates insulin release
Where is vasoactive intestinal polypeptide secreted within the gastrointestinal tract?
Parasympathetic ganglia in sphincters, gallbladder, small intestine
Vasoactive intestinal polypeptide _____ (increases/decreases) intestinal water and electrolyte secretion.
Increases
What effect does vasoactive intestinal peptide have on intestinal smooth muscle and sphincters?
Relaxation of these structures
What stimuli increase secretion of vasoactive intestinal peptide?
Distention, vagal stimulation
What is a negative regulator of vasoactive intestinal peptide release?
Adrenergic input
What is the major symptom of a vasoactive intestinal peptide-secreting tumor?
Profuse, watery diarrhea
Which small messenger molecule causes an increase in smooth muscle relaxation in the gut?
Nitric oxide
What is the pathophysiology of achalasia?
Increase in lower esophageal tone secondary to loss of nitric oxide secretion
What is the function of motilin?
Production of migrating motor complexes in the small intestine, thereby promoting peristalsis
Motilin secretion is _____ (increased/decreased) while in a fasting state.
Increased
What are the functions of ghrelin?
Regulation of hunger and meal initiation
Secretion of which hormones is stimulated by ghrelin?
Growth hormone, adrenocorticotropic hormone, cortisol, and prolactin
A patient undergoes gastric bypass surgery for weight loss. He reports feeling less hungry. What is a possible explanation for this finding?
Decreased ghrelin secretion by the stomach after bypass surgery
A young, obese male presents to the office with increased aggression and hyperphagia. He has an IQ of 60 and small testicles. What is his diagnosis and what hormone is implicated in his hyperphagia?
Prader-Willi; ghrelin
Intrinsic factor is a product of what cells?
Parietal cells of the stomach
What is the function of intrinsic factor?
Vitamin B12 binding and B12 absorption in the ileum
The destruction of parietal cells is seen in which two conditions?
Chronic gastritis and pernicious anemia
What cells make gastric acid?
Parietal cells
What is the main action of gastric acid?
To decrease stomach pH
Which substances increase gastric acid secretion?
Acetylcholine (from the vagus nerve), histamine, and gastrin
Which substances decrease gastric acid secretion?
Somatostatin, glucose-dependent insulinotropic peptide, prostaglandin, and secretin
A patient presents with chronic abdominal pain and nonhealing gastric ulcers despite treatment. What is a likely cause?
Gastrinoma
Which cells make pepsin?
Chief cells
What is the action of pepsin?
Protein digestion
What stimulates pepsin release?
Acid in the stomach and vagal stimulation
Inactive pepsinogen is converted to pepsin by _____ _____.
Low pH
Which cells secrete bicarbonate?
Mucosal cells throughout the gastrointestinal tract and Brunner& glands in the duodenum
By secreting bicarbonate within the stomach and the duodenum, mucosal cells prevent what harmful process from occurring?
Autodigestion
Bicarbonate secretion within the gastrointestinal tract is increased by what substance?
Secretin
HCl and intrinsic factor are released by _____ cells in the _____ of the stomach.
Parietal; body
_____ is produced by chief cells in the _____ of the stomach.
Pepsinogen; body
Name three different types of secretory cells found in the duodenum.
I cells (cholecystokinin), K cells (glucose-dependent insulinotropic peptide), S cells (secretin)
Somatostatin is made by _____ cells in what organ?
D; pancreas
What influence does atropine have on the gastrointestinal secretory cells?
Blocks vagal stimulation of parietal cells only
Does atropine affect vagal stimulation of the G cells?
No, the G cells are stimulated by gastrin-releasing peptide
Gastrin stimulates the production of which two substances?
Histamine (causes acid secretion) and intrinsic factor
What are two stimuli that cause increased acid secretion by parietal cells?
Histamine and acetylcholine secretion by the vagus nerve
What substances are secreted directly into the lumen of the stomach?
Mucus, pepsinogen, intrinsic factor, and HCl
Does gastrin stimulate the basal or apical surface of the parietal cells?
Basal surface; gastrin is secreted into circulation to stimulate enterochromaffin-like cells and parietal cells
What substances are secreted into the lumen of the duodenum?
Cholecystokinin, secretin, glucose-dependent insulinotropic peptide, and somatostatin
What are the sources of salivary secretions?
Parotid, submandibular, submaxillary, and sublingual glands
Which salivary gland produces the most mucinous saliva? The most serous?
Sublingual; parotid
The main function of α-amylase, which is found in salivary secretions, is to begin what process?
Starch digestion
α-Amylase, which is found in salivary secretions, is inactivated by what?
A low pH, as is found in the stomach
Which constituent of salivary secretions is important for maintaining dental health?
Bicarbonate
The mucins found in salivary secretions are composed of what category of biochemical compounds?
Glycoproteins
What is the role of growth factors in salivary products?
Epithelial renewal of the oral mucosae
Which branch of the autonomic system regulates salivary secretion?
Both sympathetic and parasympathetic
Which of the sympathetic nerves stimulate salivary secretion?
The T1-T3 superior cervical ganglion
Which two nerves carry the parasympathetic control of salivary secretions?
The facial and the glossopharyngeal nerves
Why does a low flow rate result in hypotonic salivary secretion?
There is more time to reabsorb electrolytes from the saliva
If salivary secretions flow at a high flow rate, do they then tend to be isotonic or hypotonic to the blood?
If salivary secretions flow at a high flow rate, do they then tend to be isotonic or hypotonic to the blood?
A patient comes to the clinic complaining of unilateral facial weakness after a parotidectomy. Which nerve was most likely damaged during surgery?
CN VII (facial nerve)
Which cells make gastric acid?
Parietal cells
The proton pump of parietal cells can be found on which side of the cell?
The luminal side
The proton pump of the parietal cells pumps _____ into the cell and sends _____ out of the cell and into the lumen.
Potassium; hydrogen
Which common class of drugs inhibit the parietal cells proton pump? Give an example of one
Proton pump inhibitors such as omeprazole
How does acetylcholine stimulate acid production by parietal cells?
I-t activates an M3 receptor on the cells that directly stimulates acid secretion via a G protein cascade
In what two ways does gastrin stimulate acid production by parietal cells?
Gastrin directly binds cholecystokinin receptors on parietal cells and stimulates a G protein cascade; gastrin also stimulates histamine production by enterochromaffin-like cells
What is the second messenger that is used in histamine-induced acid secretion?
cAMP, stimulated by the H2receptor
What substances inhibit acid secretion by the parietal cells?
Prostaglandins, somatostatin
Is the mucus secreted by Brunners glands acidic, alkaline, or neutral?
Alkaline
What is the function of Brunners gland secretions?
To neutralize acid contents entering the duodenum from the stomach
Brunners glands are located in what portion of the gastrointestinal tract?
The duodenum
Brunners glands are located in what layer of the gastrointestinal tract?
The submucosa
Which disease results in hypertrophy of Brunners glands?
Peptic ulcer disease
What is the function of α-amylase?
To digest starch
Lipase, phospholipase A, and colipase function primarily in what process?
Fat digestion
Which pancreatic enzymes are involved in protein digestion?
Trypsin, chymotrypsin, elastase, and carboxypeptidase
Are proteases secreted in active or inactive forms?
Inactive forms called proenzymes or zymogens
What enzyme converts trypsinogen to trypsin?
Enterokinase/enteropeptidase
Where is enterokinase (a protein that activates trypsin) located?
In the duodenal brush border
After being converted to its active form in the intestinal lumen, trypsin then does what?
It activates the other proenzymes as well as more trypsin molecules
Which pancreatic enzyme is also present in saliva?
Amylase
In addition to enterokinase, what other enzyme can convert trypsinogen to the active form of trypsin?
Trypsin itself
What are the three basic enzyme groups that digest carbohydrates?
Salivary amylase, pancreatic amylase, and oligosaccharide hydrolases
What are the functions of salivary amylase?
Starts digestion by hydrolyzing α-1,4 linkages in starches
What are the products of carbohydrate digestion by salivary amylase?
Disaccharides such as α-limit dextrin, maltose, and maltotriose
Pancreatic amylase is at its highest concentration in what area of the gastrointestinal tract?
The duodenal lumen
Pancreatic amylase hydrolyzes starch to what two types of substances?
Oligosaccharides and disaccharides
Where are oligosaccharide hydrolases found?
At the brush border of the intestine
What are the products of the reactions catalyzed by oligosaccharide hydrolases?
Monosaccharides
True or False? Only monosaccharides are absorbed by enterocytes.
True; this includes glucose, galactose, and fructose
Glucose and galactose are taken up by what transporter in the gastrointestinal tract?
Sodium-glucose linked transporter 1, which is sodium dependent
Fructose is taken up by what transporter in the gastrointestinal tract?
Glucose transporter 5, which works by facilitated diffusion
All monosaccharides are transported from the enterocytes to the blood by what transporter?
Glucose transporter 2
Can sucrose be directly absorbed by enterocytes?
No, sucrose is a disaccharide; only monosaccharides (glucose, galactose, fructose) are absorbed directly by enterocytes
Name the 3 monosaccharides that can be absorbed by enterocytes.
Glucose, galactose, fructose
Where is iron absorbed in the gastrointestinal tract?
Duodenum; iron must be absorbed as a divalent cation, which is the reduced form of the ion and favored in an acidic environment
Where is folate absorbed in the gastrointestinal tract?
Jejunum
Where is vitamin B12 absorbed in the gastrointestinal tract?
Ileum; B12 must be bound by intrinsic factor to be effectively absorbed
Peyers patches are what kind of tissue?
Unencapsulated lymphoid tissue
In what two layers of the gastrointestinal tract are Peyers patches found?
The lamina propria and the submucosa
What is the main action of M cells?
M cells take up antigens from the gut lumen for presentation to B cells and eventual creation of immunoglobulin A antibodies
After stimulated B cells from Peyers patches have differentiated into plasma cells, what do they then secrete?
Immunoglobulin A; (remember: Intra-gut Antibody)
After being secreted, immunoglobulin A functions to bind what?
Intraluminal antigens
What are the six components of bile?
Bile salts, phospholipids, cholesterol, bilirubin, water, and ions
What is the only means of cholesterol excretion from the body?
Via bile
Are bile salts water soluble?
Yes, they are conjugated to glycine and taurine (bile acids are not water soluble)
Bile acids are composed of bile acids conjugated to which two organic acids?
Glycine or taurine
What is the function of bile?
Bile salts aid in triglyceride absorption and micelle formation
Bilirubin is a breakdown product made during _____ metabolism.
Haem
What is the essential structural difference between direct and indirect bilirubin?
Direct bilirubin is conjugated; indirect bilirubin is not
Direct bilirubin is conjugated with what?
Glucuronic acid
What type of bilirubin is water soluble, direct or indirect?
Direct
What type of bilirubin is water insoluble, direct or indirect?
Indirect
What is jaundice?
Yellow skin and sclerae due to elevated levels of bilirubin
Unconjugated bilirubin travels through the circulation bound to _____. This complex can be measured as _____ _____.
Albumin; indirect bilirubin
The liver excretes what form of bilirubin into the bile?
Conjugated (direct) bilirubin
The conjugated bilirubin that gets excreted into the bile by the liver can be measured as _____ _____.
Direct bilirubin
The form of bilirubin that gets excreted renally is called _____.
Urobilin
Within the colon, conjugated bilirubin gets converted to _____ by _____.
Urobilinogen; bacteria
The form of bilirubin that gets excreted in the feces is called _____.
Stercobilin
Stercobilin gives feces what characteristic?
Its dark color
From the gut, urobilinogen that does not get excreted fecally enters the _____ _____, which takes it back to the _____.
Enterohepatic circulation; liver
Which enzyme catalyzes bilirubin conjugation?
Glucuronyl transferase
Can unconjugated bilirubin be excreted by the kidneys?
No, it is bound to albumin and therefore cannot pass through the glomerular basement membrane
