Intro to GI Physiology Lecture Sep 18 Flashcards
WHat are the roles of the GI system?
Provide a connection with the external environment for ingesting nutrients and eliminating wastes
Regulate the secretory and motility processes involved in normal gut function
** Assure that ingested nutrients are digested to absorbable forms**
Provide an absorptive surface for nutrients (etc.) uptake
Protect against ingested pathogens
What is the general order of the GI system?
Oral cavity
esophagus
stomach
small intestine
large intestine
rectum
(plus the accessory organs of salivary glands, gall blader, and pancreas)
WHat is the GI tract’s role in fluid regulation?
A large amount of fluid is secreted into the GI tract in order to aid in digestion (from the salivary glands, bile, gastric secretions, pancreatic secretions, and intestinal secretions). In total about 9 liters.
IF all that fluid was lost in our feces, we would dehydrate rapidly, so most of the volume of fluid entering the GI tract will be reabosrbed and only about 100 ml will leave through feces.
The fluid is reabsorbed primarily through the small intestine, but also in the large intestine.
All in all, the volume of fluid entering the GI tract must equal the volume leaving the GI tract
WHat are the four basic processes of the digestive system?
digestion (enzymes breaking it down)
secretion (fluids and enzymes going into the gut, blood or endothelial cells)
absorption (bringing in nutrients from the gut into the blood)
motility (keeping the food moving through the bowel)
What are the controllable variables of the digestive system? What are not controllable?
Secretion and motility are the controlled variables.
Absoprtion and digestion are not controllable - they’ll happen either way.
What are some characteristics of stomach anatomy?
It’s characterized by several layers. THe first is the mucosal layer which has right folds and long pits that extend down into the gastric pits.
Beneath the mucosa is the submucosa which contains veins, arteries, and lymph vessels
Below that is the circular muscle layer
Next is the myenteric plexux with nerves
After that is the Longitudinal muscle layer
Which cells of the gastric mucosa are responsible for the acidic enviornment of the stomach?
What is the function of this acidity?
The parietal cells
They secrete gastric acid in response to acetylcholine, gastrin, and histamine.
THe acid activates pepsin and kills bacteria
What do the chief cells secrete into the stomach?
Pepsinogen and gastric lipase for the digestion of proteisn and fats
What cells secrete gastrin into the stomach?
G cells
This stimulates gastric acid secretion
Describe in more detail how parietal cells secrete hydrochloric acid into the lumen of the stomach.
hydrogen ions need to be actively transported into the lumen of the stomach down the electrochemical gradient.
This is done using an ATP-coupled antiporter that pumps H+ in to the lumen and pumps K+ into the parietal cell
THe H+ comes from the dissociation of water. The OH- combines with CO2 trhgouht eh carbonic anhydrase reaction to make bicarbonate which exits the parietal cell into the intersitial fluid
As bicarbonate is pumped intot he interstitial fluid, Cl- is pumped into the parietal cell, after which is can diffuse through facilitated diffusion into the lumen of the stoamch
The H+ and Cl- then combine in the lumen to form HCL stomach acid.
Describe the intestinal anatomy?
THe outer mucosa has big projections called villi that extend up and then dip down into the crypts. This provides an incredibly large surface area for things to be absorbed.
There are cells along the mucosa that are specialized for absoprtion, secretion, immune function, and hormone production.
Below that is the submucosa with arteries, veings and nerves (in the submucosal plexus)
THen there’s the circulat muscle
THen the myenteric plexus with nerves
THen the longitudinal muscle
What are the long and short reflexes of the digestive system?
Short reflexes respond to local stimulus (such as stretch receptors that utilize the myenteric plexus to contract behind the bolus and relax in front of the bolus to promote motility OR chemoreceptors that promote activity of secretory cells)
Long reflexes are messages that involve the cephalic brain as well - this can either be from external stimuli such as the signt and smell of food OR it can be info sent to the brain first from the sensory resepcotrs within the enteric nervous system and then the cephalic brain sends down another message to the neurons of the myenteric plexus via sympathetic and parasympathetic neurons)
Explain the reflexes that involve GI peptides.
- A stimulus occurs (cephalic phase of digestion or distension or acid or presence of food)
- A receptor/integrating center detects the stimulus (secretory cells of the stomach and small intesting)
- GI peptides are released down the efferent pathway
- the GI peptides reach the effectors: GI smooth muscle, exocrine cells of stomach, pancresae, intestin, endocrine cells, and nervous system
- This leads to response: change in motility, release of bile and pancreatic secrestion, enzymes/aids/bicarbonate are synthesized and released, hunger and satiety
What are the four steps of the gastric phase?
- Food or cephalic relfexes initiate gastric secretions
- Gastrin stimulates acid secretion by direct action on parietal cells or indirectly through histamine (from the ECL cells) which also increases the parietal cell secretion of H+
- Acid stimulates short reflex secretions of pepsinogen from the chief cells, and the acid cleaves the pepsinogen into pepsin which can digest peptides
- As a negative feedback when the bolus reaches the entrance to the small intestine, somatostatin is released by the D cells and inhibits parietal cell release of H+, ECL release of histamine, and G cell release of histamine (it basically shuts the gastric phase off)
Describe what happens during the intestinal phase of digestion.
- Once chyme enters the small intestine, receptors at the entrance will response to the stretch and to the contents (carbs, fats, acids, etc.) and activate secretin release from cells in the duodenum
- Secretin slows down gastric motility, acid secretion, and enzyme secretion. So secretin does something similar to somatostatin–it inhibits the gastric phase.
- Secretin also acts on the pancrease to produce bicarbonate to buffer the acid
- The cells that repond to the fat and protein in the chyme will promote the secretion of CCK.
- CCK acts on the pancrease to produce enzymes that will help to digest those fats and proteins, and to increase satiety.
- Carbs in the intestine will promote insulin secretion which will promote uptake of glucose into tissues
