Lecture 25: Secretion Flashcards
What is mechanical digestion?
Breakdown of food to reduce its size and increase its surface area for chemical digestion
What is chemical digestion?
digestive enzymes extract/release the nutrients we need for absorption
What is motility?
the contraction of smooth muscle required for the mechanical digestion
How does motility aid in chemical digestion?
by moving food through the GI tract at a rate that allows chemical digestion to occur and mixes food up with enzymes to allow digestion to occur
How does motility aid in absorption?
due to mixing patterns (segmentation) in the small intestine to expose digestive products to the absorptive surfaces and move the food through at a rate that will allow chemical digestion to occur
What regulates motility?
the CNS/ENS and hormones
Are the secretions into the lumen exocrine or endocrine?
exocrine
What controls the exocrine secretions?
endocrine secretions
What are three components of exocrine secretions?
- mucous
- electrolyte solution
- digestive enzymes
What is the mucous protecting?
the thin epithelium layer so that it does not scrape off
One of the components of exocrine secretions is mucous. What is its general function?
for protection and lubrication and for aiding the mechanical digestion
One of the components of exocrine secretions is an electrolyte solution. What is its general function?
it dilutes food to allow it to be mixed with enzymes and provides the optimal pH which is essential for the chemical digestion of food
One of the components of exocrine secretions is digestive enzymes. What is their general function?
they can carry out chemical digestion to release the nutrients which is needed to absorption
Where do we secrete the electrolyte solution from?
the plasma
What is the secretion in the mouth and where is it secretion from?
saliva from the salivary glands
What are the names of the salivary glands?
parotid
submandibular
sublingual
When do we secrete saliva?
all the time but we secrete more when we are eating or about to eat
What is the composition of the saliva?
- mucous
- dilute solution of sodium bicarbonate and sodium chloride
- digestive enzymes
What is the purpose of mucous in the saliva?
for lubrication
What is the purpose of the NaHC03/HCl dilute solution?
To dilute the food to dissolve the food and also to make it the optimal pH
What are the digestive enzymes in the saliva and what are they for?
- lingual lipase for the breakdown of fats
- α amylase for the breakdown of starch
What are the functions of saliva?
it aids in talking, chewing and swallowing and is important for hygiene and for digestion of fats and starch
How is saliva important for hygeine?
the flow of saliva flushes pathogens out of the mouth and into the oesophagus to go to the stomach
there is also antibacterial/antiviral agents in saliva for oral hygiene
How is the salivary secretion regulated?
Through nervous stimulation
What causes nervous stimulation of the salivary glands?
through thought, smell, sight of food and the presence of food in the mouth stimulates secretion
The regulation of salivary secretion is done via the ________ nervous system
autonomic
Within the autonomic nervous system, is it the parasympathetic or sympathetic nervous system that stimulates salivary secretion?
both
the parasympathetic nervous system stimulates the secretion of copious quantities of fluid and then the sympathetic nervous system secretes small volumes of viscous fluid
When do we secrete into the stomach?
when eating and between meals
Between meals, we secrete ________ through the _______ ___________ in the stomach
mucous
surface epithelium
When eating, what three cells do we secrete from into the stomach?
mucous cells
parietal cells
chief cells
In the stomach, what is secreted by the mucous cells?
mucous
In the stomach, what is secreted by the parietal cells?
HCl and intrinsic factor
In the stomach, what is secreted by the chief cells?
pepsinogen
What is the function of mucous secreted into the stomach?
protection against abrasion (and likely from acid)
What is the function of intrinsic factor secreted into the stomach?
absorption of vitamin B12 in the small intestine
What is the function of pepsinogen secreted into the stomach?
pepsinogen is the precursor of pepsin (a gastric proteolytic enzyme) which starts the digestion of proteins
What is the function of acid secreted into the stomach?
- it dilutes food for dissolving
- denatures proteins so that pepsin can break it down
- activates pepsinogen into pepsin
- protection against bacteria
How do we get H+ into the stomach?
- carbonic anhydrase in the cytoplasm of parietal cells combines CO2 with H2O to make carbonic acid
- dissociation of the carbonic acid gives H+ and HCO3-
What is the name of the enzyme which forms carbonic acid from CO2 and H2O?
carbonic anhydrase
Once we have produced the H+, how do we get it from the parietal cells into the lumen of the stomach?
On the apical membrane of parietal cells, we have transport protein H+/K+ ATPase which pumps H+ ions into the lumen of the stomach in exchange for K+ ions
How do we get Cl- into the stomach?
when carbonic acid dissociates, we get H+ and HCO3-
the HCO3- are transported out of the cell on the basolateral surface and are exchanged for Cl- ions which diffuse across the cell
Once we produce Cl- ions, how to we get them from the parietal cells and into the lumen of the stomach?
it enters the lumen via the Cl- channel in the apical surface
The regulation of the gastric secretions occurs in three different phases because we need to coordinate the secretions with eating and the arrival of food. What are the three phases called?
- cephalic phase
- gastric phase
- intestinal phase
The three phases of gastric secretion are named after which region of the body controls secretion. What are the three phases called and which region of the body controls secretion in that particular phase?
- cephalic phase when the head controls secretion
- gastric phase when the stomach controls secretion
- intestinal phase when the intestine controls secretion
What is the purpose of the cephalic phase?
to secrete things into the lumen of the stomach in preparation for the arrival of food
What are the stimuli for gastric secretions in the cephalic phase?
higher centres from the thought, smell and sight of food as well and chewing action and taste
The cephalic phase of secretion is stimulated by the ________ nervous systems via the ______ nervous system
parasympathetic
enteric
How does the parasympathetic system (via the enteric nervous system) stimulate secretion into the stomach during the cephalic phase?
The sight, smell, taste or thoughts of food activate the central nervous system. Via the vagus nerve, the central nervous system activates the submucosal plexus.
The submucosal plexus activates the mucous cells, chief cells and parietal cells to secrete mucous, pepsinogen and HCl respectively.
It also activates the G cells in the antrum to secrete gastrin into the bloodstream which stimulates the chief cells and parietal cells to secrete more pepsinogen and HCl
What is the purpose of the gastric phase?
it ensures that there is sufficient secretion to handle to ingested food
What are the stimuli in the stomach that stimulate secretions during the gastric phase?
- stretch/distension of the stomach wall
- the presence of partly digested products such as peptides
- elevated pH
Ingested food stimulates gastric activity in what two ways?
by stretching the stomach and by raising the pH of its contents
Explain how distention of the stomach wall helps stimulate secretion
distension stimulates stretch receptors (local neural response) which activates the submucosal and myenteric plexuses causing mucous, chief, parietal and G cells to be simulated
the plexuses also cause the mixing waves
Explain how the elevated pH in the stomach helps stimulate secretion
elevated pH stimulates chemoreceptors (local neural response) which activates the submucosal and myenteric plexuses causing mucous, chief, parietal and G cells to be simulated
the plexuses also cause the mixing waves
Describe the hormone response in the gastric phase
The activated G cells secrete the hormone gastrin which activates the chief cells to secrete pepsinogen and the parietal cells to secrete HCl
The gastrin also stimulates the mixing waves of the stomach
As well as the local nervous reflex of the gastric phase, describe the external nervous reflex of the gastric phase
the parasympathetic nervous system takes information to the brain and then back to the ENS to stimulate secretion of mucous, pepsinogen, HCl and gastrin
What is the purpose of the intestinal phase?
to control delivery of secretions to the small intestine
What are the stimuli for gastric secretions in the intestinal phase?
- distention of the duodenum
- arrival or acid chyme, lipids and carbohydrates in the duodenum
Describe the effect of distension in the duodenum in terms of secretions
the stretch receptors in the duodenum stimulate the myenteric plexus to inhibit secretions by feedback in the enterogastric reflex
Describe the effect of the presence of lipids and carbohydrates in the duodenum in terms of secretions
CCK and GIP hormones are secreted and these are carried in the bloodstream to inhibit secretions from chief cells and parietal cells and to stop peristalsis
Describe the effect of the presence of acidic chyme in the duodenum in terms of secretions
the decreased pH stimulates the secretion of secretin which travels in the blood to the inhibit secretions from chief cells and parietal cells and to stop peristalsis
What are two components of the pancreatic secretions that are present in the small intestine?
enzymes
alkaline fluid
Where are the enzymes in the pancreatic secretions produced and what is their purpose?
from the acinar cells in the pancreas
they are used for the chemical digestion of the food
Where is the alkaline fluid in the pancreatic secretions produced and what is their purpose?
in the duct cells in the pancreas to neutralise the acid and make the optimum pH for the enzymes to work
What are four types of enzymes secreted by the pancreas?
- lipolytic
- amylytic
- proteolytic
- nucleolytic
Give two examples of lipolytic enzymes and their role
lipase and phospholipase for the digestion of fats
Give an example of amylytic enzymes and their role
pancreatic amylase for the digestion of carbohydrates
Give three examples of proteolytic enzymes and their role
trypsin, chymotrypsin and carboxypeptidase for the digestion of proteins
Give two examples of nuceolytic enzymes and their role
ribonuclease and deoxyribonuclease for the breakdown of RNA and DNA
What is the stimulation of secretion of enzymes from the pancreas into the small intestine?
the arrival of lipids and carbohydrates in the duodenum stimulating CCK secretion
What is the purpose of the pancreatic enzymes?
luminal chemical digestion of food
Proteolytic enzymes secreted into the duodenum are secreted in the _______ form and they are _______ in the _________
inactive
activated
duodenum
What is the inactive form of trypsin?
trypsinogen
What is the inactive form of chymotrypsin?
chymotrypsinogen
What is the inactive form of carboxypeptidase?
procarboxypeptidase
What activates the inactive form of the proteolytic enzymes into their active form?
an enterokinase which is bound to the duodenal membrane converts trypsinogen into trypsin and trypsin stimulates the activation of chymotrypsinogen and procarboxypeptidase into chymotrypsin and carboxypeptidase respectively
The alkaline fluid component of the pancreatic secretions into the small intestine is produced by what cells?
duct cells
What stimulates secretion of alkaline fluid from the pancreas?
secretion of secretin stimulated by the arrival of acidic chyme in the duodenum
What is the purpose of the alkaline fluid in the pancreatic secretions?
it neutralises the acidic chyme delivered from the stomach and created optimum pH for pancreatic and intestinal digestive enzymes
Where is bile secreted from?
The galbladder
Biliary secretion consists of what two things?
bile salts
HCO3- rich fluid from the ducts
What are excretory products in the biliary secretion
bile pigments and cholesterol
What is the function of bile salts?
fat digestion
What is the function of the HCO3- rich fluid?
it neutralises the acid
Bile is constantly secreted by the _________ but is stored and concentrated in the _________. It is delivered to the _______ with the arrival of ________
liver
gallbladder
duodenum
food
The initial delivery of bile from the gallbladder is under ________ control
hormonal
What hormone stimulates the secretion of bile from the gallbladder to the duodenum?
CCK produced in response to the arrival of lipids and carbohydrates in the duodenum
Relaxation of the _________ _________ allows bile into the duodenum
hepatopancreatic ampulla
How does bile stimulate its own secretion?
through the enterohepatic circulation
Describe the enterohepatic circulation (how bile stimulates its own secretion)
When the bile secreted into the small intestine reaches the ileum, it is reabsorbed back to the liver to stimulate the secretion of more bile
What three things does the small intestine secrete?
- mucous
- isosmotic fluid
- digestive enzymes
What is the purpose of the mucous secreted by the small intestine?
lubrication
What is the isosmotic fluid secreted by the small intestine made of?
a mixture of NaCl and NaHCO3
What is the purpose of the isosmotic fluid secreted by the small intestine?
it is alkaline so it helps it neutralise the acid and dilutes the food to aid digestion
How are digestive enzymes secreted by the small intestine?
we shed cells to release the enzymes
What is secreted by the large intestine and what is the purpose of this?
mucous is secreted for lubrication
How does secretion aid in mechanical digestion?
by diluting food in solution and coating the food with mucous so it doesn’t abrade the epithelial layer
How does secretion aid in absorption?
Secretion suspends the digesiton products in the fluid solution in the small intestine
Exocrine secretions are produced by ______ cells
epithelial cells
Where can a helpful summary of this lecture be found?
slides 27 and 28 of lecture 25
Is Sam a dumbass, true or false?
3/5 majority says true
