Lecture 25 Flashcards
Secretion in the GIT
What is the epithelial membrane called in the GIT lumen side?
Apical membrane
What is the epithelial membrane lining the blood and lymph lumen?
Basal membrane
What is the epithelial membrane between the gap junction?
Lateral membrane
What are the two pathways of diffusion from the GIT lumen to the blood and lymph?
Transcellular pathway and paracellular pathway
What is the transcellular pathway?
Across the cell membrane and through the cytoplasm
What is the paracellular pathway?
Between the cells through the tight junction
What is secretion?
The movement of solutes and water from the body to the GIT lumen. Can occur via the paracellular and/or transcellular pathway
What is absorption?
The movement of solutes and water from the GIT lumen into the body. Can occur via the paracellular and/or transcellular pathway
What is endocrine secretion?
Hormones into the blood
What is exocrine secretion?
Secretions produced by the epithelia
What are the components of exocrine secretions in the GIT?
Mucus, electrolyte solutions and digestive enzymes
What is the function of mucus?
To protect and lubricate and also aid in mechanical digestion
What is the function of electrolyte solutions?
Dilutes food and provides optimal pH as well as being essential for digestive enzymes
What is the function of digestive enzymes?
Essential for chemical digestion of food and also aids absorption
Why is RE-absorption important for secretion?
Because we secrete 8L/day of fluid from 3L of plasma therefore need to recycle it
How many salivary glands do we have?
3
How much fluid do the salivary glands secrete per day?
1.5L
How much fluid does basal (resting) secretion secrete?
0.3mL/min
How much fluid does stimulated secretion secrete?
1.5mL/min
What is the composition of saliva?
Musus, dilute solutions of NaHCO3 and NaCl (electrolytes) and digestive enzymes
What s the function of electrolytes in saliva?
To dilute food for tasting, swallowing and also talking. Provides the optimal pH for digestive enzymes and for hygiene
What are the digestive enzymes of the salivary glands?
Lingual lipase and salivary amylase
What does lingual lipase do?
Chemical digestion of fats
What does salivary amylase do?
Chemical digestion of starch
What is secretion of the salivary glands regulated by?
The autonomic nervous system
What are the two components of the autonomic nervous system?
Parasympathetic and sympathetic
What does the parasympathetic nervous system do for the salivary glands?
Stimulates secretion of copious quantities of fluid
What does the sympathetic nervous system do for the salivary glands?
Stimulate secretion of less and more viscous fluid
How much fluid does the stomach secrete per day?
2-3L
What is the volume of stomach secretion between meals?
Slow rate of 15-30mL/hr
What cells are active between meals in the stomach?
Goblet cells on the surface of the stomach
What do the goblet cells secrete?
Mucus and bicarbonate
What does bicarbonate do?
Protects the proteins in the lining of the stomach from the acid, preventing denaturing
What cells are active while eating in the stomach?
Goblet cells, parietal cells and chief cells
What do the parietal cells secrete?
HCl acid and intrinsic factor
What is the purpose of HCl in the stomach?
To denature proteins and convert pepsinogen to pepsin. Also protects from microbes
What is the purpose of intrinsic factor in the stomach?
To bind to vitamin B12 in the stomach and then help its absorb in the ilium (SI)
What do the chief cells secrete, and why?
Pepsinogen, a precursor for pepsin that starts chemical digestion of proteins
What is the source of hydrogen ions for HCl in the stomach?
Carbonic anhydrase and production of H+ and HCO3- from H2O and CO2
What is the process of secretion of stomach acid (H+) in the stomach?
H+ - K+ ATPase in the apical membrane of parietal cells pumps the H+ ions into the GIT lumen in exchange of K+ (into cell lumen) which returns to the GIT lumen through a passive channel
What is the source of chloride in the stomach?
Anion counter transporter in the basolateral membrane of parietal cells imports Cl- ions into the cell in exchange for ejecting HCO3- into interstitial fluid
What is the process of the secretion of Cl- in the stomach?
Cl- diffuses across the cell into the lumen via Cl- channel in apical membrane
What are the three phases of gastric secretion and what proportion of gastric secretions do they provide?
Cephalic 20%, gastric 70%, intestinal 10%
What is the function of the cephalic phase?
To prepare the stomach for the arrival of food
What is the stimulus of the cephalic phase?
Seeing, smelling, tasting or thinking about food + chewing
What is the stimulus of the cephalic phase detected by?
CNS
What is the response from the CNS in the cephalic phase?
PSN stimulates ENS (submucosal plexus) via the vagus nerve
What are the effectors of the cephalic phase?
Goblet cells, chief cells, parietal cells, G cells
What are G cells?
A type of enteroendocrine cell
What do the G cells do?
Secrete gastrin into blood which stimulates chief and parietal cells (for the second time)
What is gastrin?
A hormone secreted by G cells (an EEC) into the blood
What do goblet cells secrete?
Mucus and bicarbonate
What do chief cells secrete?
Pepsinogen
What do parietal cells secrete?
HCL acid
What is the outcome of the cephalic phase?
Increased secretion of mucus, HCO3-, HCl acid and pepsinogen in the stomach
What is the function of the gastric phase?
To maximise mechanical digestion and begin protein chemical digestion
What is the stimulus of the gastric phase?
The arrival of food through stretch of the stomach wall, increased pH and detection of undigested food (esp. protein)
What is the stimulus of the gastric phase detected by?
G cells (from peptides), gastric mechano- & chemoreceptors
What is the response from the G cells in the gastric phase?
Secrete gastrin to stimulate chief and parietal cells
What is the response from the receptors in the gastric phase?
Stimulus of the myenteric and submucosal plexus
What are the effectors of the submucosal plexus?
Goblet cells, chief cells, parietal cells, G cells
What are the effectors of the myenteric plexus?
Smooth muscle of the stomach to increase motility (retropulsion)
What is the outcome of the submucosal plexus stimulus in the stomach?
Increased secretion of mucus, HCO3-, HCl acid, pepsinogen and gastrin in the stomach
What is the outcome of the myenteric plexus stimulus in the stomach?
Increases motility; retropulsion, waves of peristalsis
What is the function of the intestinal phase?
To allow slow, controlled release of food to the small intestine
What is the stimulus of the intestinal phase?
Stretch of the duodenal wall, decreased pH from acid, arrival of fatty and amino acids in the duodenum
What is the stimulus of the intestinal phase detected by?
The duodenal enteroendocrine cells (EECs) and mechano- & chemoreceptors
What is the response from the EECs in the intestinal phase?
Secretion of duodenal hormones; CCK, GIP (fatty and amino acids) and secretin (pH)
What is the response from the receptors in the intestinal phase?
To stimulate the CNS (SNS) to inhibit the ENS - submucosal and myenteric plexus
What does inhibition of the submucosal plexus cause?
Decreased secretion of pepsinogen and HCl acid
What does inhibition of the myenteric plexus cause?
Inhibition of G cells to decrease gastrin and motility
What is the outcome of the intestinal phase?
Decreased secretion of HCl acid, pepsinogen and gastrin, decreased motility; retropulsion
What is the pancreas?
An endocrine and exocrine organ
How much fluid does the pancreas secrete per day?
1-1.5L
What are the two components of the pancreas?
The pancreatic acinar cells and pancreatic duct cells
What do the pancreatic acinar cells secrete?
Pancreatic digestive enzymes
What do the pancreatic duct cells secrete?
Alkaline fluid (HCO3-)
What stimulates the release of pancreatic juice with digestive enzymes from pancreatic acinar cells?
Cholecystokinin (CCK) from the duodenum EECs - CCK stimulated by the arrival of fatty acids and amino acids in the duodenum
How are digestive enzymes classified?
Based on what type of molecule they digest
What do nucleolytic enzymes digest?
Ribonuclease and deoxyribonuclease
What do lipolytic enzymes digest?
Fat; lipase
What do amylytic enzymes digest?
Carbohydrates; pancreatic amylase enzyme
What do proteolytic enzymes digest?
Peptides; trypsinogen, chymotrypsinogen, procarboxypeptidase
What are pancreatic proteolytic enzymes secreted as?
Inactive precursors that are activated in the duodenum
Why are pancreatic proteolytic enzymes secreted as precursors?
So that they don’t digest proteins inside pancreatic acinar cells
What is the active form of Trypsinogen?
trypsin
What is the active form of chymotrypsinogen?
Chymotrypsin
What is the active form of procarboxypeptidase?
Carboxypeptidase
What converts trypsinogen to trypsin?
Enterokinase (enteropeptidase) which is bound to the duodenal membrane
What converts chymotrypsinogen and procarboxypeptidase to their active forms?
Trypsin
What stimulates the release of pancreatic juice with bicarbonate from pancreatic duct cells?
Secretin from the EECs - stimulated by the arrival of acidic chyme from the stomach in the duodenum
What does the alkaline fluid in the pancreatic duct cells do?
Neutralises acid in chyme from the stomach and provide the optimum pH for pancreatic enzymes
What volume of fluid does the liver secrete per day?
0.5L
What are the liver cells?
Hepatocytes
What do the hepatocytes make and secrete, and why?
Bile into the bile canaliculi for chemical digestion of fat
What do the bile duct cells secrete, and why?
Alkaline fluid which neutralises the acid in the chyme from the stomach
What does bile contain?
Excretory/waste products with bile pigments (bilirubin) from the breakdown of haem and also excess cholesterol
Where is bile sotred?
In the gallbladder where it is also concentrated
What stimulates the release of bile from the gallbladder into the duodenum?
CCK causes contraction of the wall of the gallbladder and relaxation of the hepatopancreatic sphincter
What stimulates the release of CCK?
Fatty acids in the duodenum
What happens to the bile after it is secreted through the duodenum?
About 95% is reabsorbed into the ilium and transported back to the liver via the enterohepatic circulation system where it can be reabsorbed and secreted again
How is the liver stimulated to produce more bile?
Through the return of bile to the liver via the enterohepatic circulation system because this signals to the hepatocytes that bile has recently been released from the gallbladder. Also stimulated through secretin (mildly)
What volume of fluid does the small intestine produce per day?
1.5L
Where does the secretion in the SI come from?
The glands (crypts)
What does the SI secrete?
Mucus and isosmotic fluid
What is isosmotic fluid?
An alkaline mixture of NaCl and NaHCO3 which helps neutralise acid and dilute food to aid digestion
What does the large intestine secrete?
Mucus
