Guyton Physiology Flashcards
What are the layers of the intestinal wall?
serosa
muscularis
submucosa
mucosa
What are the two types of electrical waves in the intestinal smooth muscles?
slow wave
spike potentials
What is the function of the interstitial cells of Cajal
produce the slow waves - electrical pacemakers
What is the threshold potential of intestinal smooth muscle cells?
- 40
List 3 factors that make the intestinal smooth muscle membranes more excitable
stretch
ACh
several GI hormones (from food e.g.,)
What protein mediates the intestinal muscle activation caused by IC Ca++ entry
calmodulin
What are the two plexuses of the enteric nervous system? What are their main functions?
submucosal plexus - Auerbach’s
myenteric plexus - Meissner’s plexus
What is the function of the inhibitory effects of the myenteric plexus?
inhibit sphincter muscles
What are the divisions of the intestinal parasympathetic enteric nervous system?
cranial and sacral divisions
Explain the gastrocolic reflex
food enters the stomach –> stretch –> prevertebral sympathetic ganglia –> colon –> evacuation
Explain the enterogastric reflex
duodenum stretched, filled with acid/food –> prevertebral sympathetic ganglia –> stomach emptying inhibited
Explain the colonoileal reflex
colon filled –> prevertebral sympathetic ganglia –> inhibit ileal emptying
What are the three types of gastrointestinal reflexed (in terms of how high they travel up and back down the nervous system)
- reflexes that travel just within the enteric nervous system within the gut wall
- reflexes that travel from the enteric system to the prevertebral ganglia and back to the GI tract
- reflexes that travel from the GI tract to the spinal cord or brain stem –> back to the GI tract (e.g., defecation reflex)
For gastrin: Where is it produced, what is its stimulus, and what is its effect?
G-cells
leads to gastric acid secretions
stimulated by meal ingestion (stomach distention, protein products)
For cholecystokinin (CKK): Where is it produced, what is its stimulus, and what is its effect?
produced by I-cells (duodenum, jejunum)
stimulus: food entering the duodenum, fat and fatty acids
effect: gall bladder emptying, inhibits stomach contractions
For secretin: Where is it produced, what is its stimulus, and what is its effect?
produced in S-cells
stimulus: gastric acid entering the duodenum
effect: induces pancreatic bicarbonate secretion
For glucose-dependent insulinotropic peptide: Where is it produced, what is its stimulus, and what is its effect?
produced by the mucosa of the upper SI
stimulus: fatty acids in the duodenum
effect: slows gastric emptying
For motilin: Where is it produced, what is its stimulus, and what is its effect?
produced by the stomach and upper duodenum
stimulus: fasting
effect: interdigestive myoelectric complexes
What are the two functional movements of the GI tract?
- propulsive
- mixing
What is the main stimulus for peristalsis
stretching/distention
How far does each peristaltic contraction usually travel?
5-10 cm
List all organs included in the splanchnic circulation
GI tract, pancreas, spleen, liver
List mechanisms by which GI gut flow increases after a meal
- vasodilatory substances released after meal (cholecystokinin, vasoactive intestinal peptide, gastrin, secretin)
- kinins (e.g., bradykinin)
- decreased O2 from increased metabolism triggering increased blood flow
- adenosine
Why are the intestinal villi especially at risk for ischemic death from decreased local blood flow or circulatory shock?
Counter arterial and venous blood flow exchanges O2, I.e., arteriole give O2 to venules, so most O2 doesn’t even reach the tip of the villi
already lower O2 environment –> if circulation drops further –> ischemic injuries
How does the musculature of the esophagus differ between dogs and cats?
dog: all striated
cat: 2/3 striated, bottom 1/3 smooth
What are the triggers that slow stomach emptying
- stretch of duodenum
- fat, protein products and gastric acid in the duodenum
- osmolality of the chyme
- irritants in the duodenum
Name 3 hormones, slowing gastric emptying
- cholecystokinin
- secretin
- gastric inhibitory peptide (=glucose-dependent insulinotropic polypeptide)
How fast do peristaltic waves travel through the intestines, and how far do they typically reach?
0.5-2cm/second
3-5 cm, cannot travel further than 10cm
Describe the difference between the gastroenteric and the enterogastric reflexes
gastroenteric –> full stomach triggers duodenal peristaltic activity
enterogastric –> overfilled duodenum inhibits further gastric emptying
Describe the gastroileal reflex
ileocecal valve usually blocks chyme from moving forward - opening of the valve is triggered by an additional meal
Describe how small intestinal diarrhea develops
excessive SI peristaltic movements caused by irritation from infection or chemicals (“peristaltic rush”)
What are the 2 reflexes initiating mass reflexes of the colon
- intrinsic reflex - opens the internal anal sphincter when feces reaches the rectum
- parasympathetic defecation reflex - more powerfully opens the internal anal sphincter
note: external anal sphincter must be opened voluntarily
Explain how peritonitis, kidney, or urinary bladder disease may cause ileus
inflammation of the peritoneum –> peritoneointestinal reflex –> intestinal paralyzsis
renointestinal and viscerointestinal reflexes also cause inhibition
How does the saliva’s electrolyte/ionic composition differ from plasma?
high K, high bicarbonate, low Na and Cl
Describe the secretory cells of the oxyntic gland and what they secrete
peptic cells/chief cells - pepsinogen
parietal cells/oxyntic cells- HCl + intrinsic factor
enterochromaffin-like cells - histamine
What is the pH of concentrated hydrochloric acid?
0.8
Describe how parietal cells secrete hydrochloric acid
CO2 and H2O via carbonic anhydrase become H+ and HCO3-
H+ is transported out of the cell with countercurrent with K+ (KHATPase pump)
HCO3- exits on basolateral membrane in exchange for Cl- –> Cl- then enters lumen through Cl- channel –> HCl formed
What is intrinsic factor?
essential factor for vitamin B12 absorption
secreted also by parietal cells
How is pepsinogen activated?
in the presence of HCl
What do the pyloric glands produce?
mucous
gastrin (G cells)
How does gastrin affect gastric acid secretion?
gastrin stimulates acid secretion
gastrin stimulates histamine release by the enterochromaffin-like cells in the oxyntic glands –> histamine is an important trigger for parietal cells’ HCl production
What inhibits gastric secretions?
- distention of the SI
- acid in the SI
- food products (protein, fat) in SI
- hyper or hypoosmolar chyme in SI
–> releases secretin –> opposes acid secretion
