Lecture 5: Response of the Gut to a Meal Flashcards
Major function of the GI tract
extract water, nutrients, and electrolytes from food and to expel the undigested residue as feces
what is motility?
Mechanism by which gut contents are moved from proximal to distal
Describe the integrated response to a meal
Food is ingested–>digested–>absorbed–>eliminated. Motor and secretory behaviors aid in this process.
4 major functions of the gut
1) secretion
2) digestion
3) absorption
4) motility
Function of the Upper Esophageal Sphincter (UES)
closes off the pharynx and prevents air from entering into the esophagus during breathing, and prevents reflex of esophageal contents into the pharynx
fx of the Lower Esophageal Sphincter (LES)*
protects esophagus from gastric reflux, allows entry of food into the stomach.
fx of esophagus
transports bolus from pharynx to stomach, clears material refluxed from stomach.
What does “basal tone” refer to?
The pressure that is always present in a normal state
How does swallowing effect the intraluminal pressure of the esophagus, LES, and proximal stomach?
induces relaxation of LES and proximal stomach, and a series of contractions in the esophagus.
Receptive relaxation in the proximal stomach in response to meal is dependent on which nerve?
vagus nerve, mediated by VIP and NO. vagus is very important for motility, etc. in all parts of the GI system
luminal secretions from LES and cardia
mucus, bicarbonate
luminal secretions from the fundus and body of the stomach
H+, mucus, pepsinogens, bicarb, lipase, intrinsic factor. First 3 are the most important
In which part of the stomach does mixing/grinding/sieving/emptying occur?
Antrum
luminal secretions from the antrum and pylorus
mucus, bicarb.
main fx. of the fundus and body of the stomach
STORE food
main fx of the pylorus
acts as a sphincter to prevent particles that are too large or in the wrong chemical composition from entering the Small Intestine
What do parietal cells produce?
acid
What do chief cells produce?
pepsinogen
What do G cells produce?
gastrin
What is gastrin?
An endocrine hormone that stimulates chief and parietal cells
3 phases of gastric acid secretion in response to a meal
1) cephalic
2) gastric (largest)
3) intestinal
Describe the cephalic phase of gastric secretion
Vagal afferents go from brain to GI tract and stimulate chief, parietal, ECL, and G cells to function. Ach is the NT to stimulate chief, parietal, and ECL cells, whereas GRP (gastrin releasing peptide) is the NT to stimulate G cells
What converts pepsinogen to pepsin?
acid
what does pepsin do?
breakdown protein
what are oligopeptides?
protein breakdown products
Explain why gastrin has endocrine properties
When it is produced by G cells, it feeds back on chief and parietal cells to upregulate their production of pepsinogen and acid
3 mechs. of communication mediated responses in the GI tract
1) endocrine (hormone released from sensor cell, which enters into circulation and eventually activates target cell)
2) neurocrine (sensory neuron activates target cell)
3) paracrine (paracrine cells directly activate neighboring target cells via diffusion of paracrine mediators)
ECL cells release:
histamines
3 things that can stimulate a parietal cell
1) Ach
2) gastrin
3) histamine
parietal cell will still produce acid if 1-2 of the receptors for these molecules are blocked. There is no drug that blocks all 3 receptors
describe pathway of acid secretion from a parietal cell
acid flows from cell into the intestinal lumen via pathway created by canaliculi and mitochondria
What is the function of mucus and bicarbonate secretions at the luminal surface of the stomach lining?
Raises the pH to near 7 near the cell surface and protects the lining from the harsh low pH in the rest of the stomach
Role of retropulsion in the stomach
pylorus closes and food is forced back into proximal stomach. Ensures breakdown of food particles
fun fact
composition of food itself influences how much gastrin is produced
Neg. feedback mech. for gastrin release
Acid produced by parietal cells inhibits gastrin release from G cells
acid in the antrum –> somatostatin and gastrin secretion
stimulates somatostatin release from D cells, which inhibits gastrin secretion from nearby G cells. This is PARACRINE INHIBITION.
What type of cells produce somatostatin?
D cells
Under what conditions does the stomach empty?
only when intragastric pressure exceeds duodenal pressure and pyloric resistance. Influenced by the physical and chemical composition of a meal and by food type, pH, and osmolarity
Order protein, pectin, and lipid by how long they stay in the stomach
lipid (longest) > protein > pectin
What 2 things does the duodenum release in response to entrance of HCl, amino acids, and fatty acids entering the duodenum? What effect does this have? **
CCK (in response to fat/amino acids) and secretin (in response to acid). These inhibit secretion and emptying in the stomach. CCK also stimulates the vagus n. to stimulate pancreatic enzyme secretion
Ingesta must be hyper/hypo/isotonic to enter small intestine?
isotonic
Contents of stomach are usually hyper/hypo/isotonic?
Hypertonic. They transition to isotonic in the duodenum via secretions of pancreas, liver, and duodenum. (Can also sometimes start off at hypotonic)
factors effecting pH in small intestine
- H+ input from gastric acid
- HCO3 input from pancreatic duct cells/biliary duct cells/duodenal mucosa
- buffers from dietary protein/peptides/fatty acids/bile acids
- H+ absorption
Exocrine fxs of the pancreas
Secrete digestive enzymes, cofactors, and HCO3 into the duodenum. CCK is a major player in the fx of the pancreas
Endocrine fxs of the pancreas
secrete hormones into the portal vein
CCK stimulates pancreatic enzyme secretion by neural and hormonal pathways
:)
fx of trypsin. What is its inactive form? What is it activated by?
Activates zymogens to active enzymes and breaks down proteins. Trypsinogen = inactive form. Activated by enteropeptidase.
Where is bile stored?
gall bladder
major solid component of bile
bile acids
amylase
pancreatic enzyme that breaks down starch and glycogen into smaller sugars
lipase
pancreatic enzyme that digests long-chain triglycerides into monoglycerides and fatty acids
Which cells accomplish most of the absorption and digestion in the small intestine?
cells on the tip of the villi. Proteins intercalated into the membrane lining of these cells are responsible for digestion
intraluminal products of starch and protein digestion in the small intestine
monosaccharides, peptides, and amino acids.
consumption of meal –> intestinal blood flow
increases. Whenever there is an increase in work, there has to be increased blood flow and motility in that tissue
absorbed products of lipolysis are synthesized into triglycerides in enterocytes (intestinal absorptive cells) and exit as chylomicrons
:)
fasting –> intestinal motility
decreases
4 main functions of the colon and rectum
1) absorption of water/ions
2) bacterial fermentation of nonabsorbed nutrients
3) storage of waste and indigestible materials
4) elimination of waste and indigestible materials
What happens if there is failure o breakdown of soluble components prior to the colon?
bacteria will “go to town” on them and produce a lot of water, acid, and gas –> diarrhea
ingestion of a meal –> colonic motility. What initiates this?
increases. Initiated by hormonal signals from the stomach and intestine, and neural efferents from the CNS
