lecture 25 - secretion Flashcards
What is secretion?
Movement of solutes and water from the body to the lumen, across some kind of membrane or barrier
What is absorption?
Movement of solutes and water from the lumen into the body across some kind of membrane
What are exocrine secretions produced by?
Epithelia, can then move into the lumen
What are the 3 key components of GI exocrine secretions?
Mucus, electrolyte solution/serous fluid, digestive enzymes
What is the function of electrolyte solution/serous fluid in the GI tract?
Dilutes food, provides optimal pH and aids in chemical digestion
What are the functions of digestive enzymes?
Chemical digestion and aid in absorption
What part of the GI tract has the lowest pH?
The stomach (~1)
What part of the GI tract shows the greatest change in pH?
The duodenum and upper jejunum as stomach acid is neutralised
Is the pancreas slightly acidic or slightly basic?
Basic - ion produces bicarbonate
What is osmolarity?
The total ion/solute concentration
What part of the GI system has the lowest osmolarity?
Saliva - 100mOsm
Why is reabsorption of electrolyte solution from the stomach important?
Approx. 8L a day is secreted, while we only have about 3L of plasma
What is the approximate volume of electrolyte solution secretied from the GI tract daily?
8L
What is the basal/resting secretion rate of saliva?
0.3ml/min
What is the stimulated secretion rate of saliva?
1.5mL/min
What are the 3 key components of saliva?
Mucus, dilute solution of NaHCO3/NaCl, digestive enzymes
What is the function of the mucus found in saliva?
lubrication
What is the function of the dilute solution of NaHCO3 (sodium bicarbonate) and NaCl (sodium chloride) in saliva?
Dilution of food and providing an optimal pH for digestive enzymes
What are the 2 types of digestive enzyme secreted in saliva?
Lingual lipase, alpha-amylase
Where are the sublingual glands located?
Under the tongue
Where are the submandibular glands located?
Under the jaw
Where are the parotid glands located?
Just below/in front of the ears
What salivary gland secretes the majority of the saliva?
Submandibular glands
How does salvia aid in chewing and swallowing?
dissolves and lubricates food
What is the condition resulting in a dry mouth from reduced or absent saliva?
Xerostomia
What does lingual lipase digest, and where?
Fats, in the mouth as secreted in saliva
What does alpha-amylase digest, and where is it found?
Starch, released from salivary glands and the pancreas
What is nervous system regulation of salivary secretion triggered by?
Thought, smell sight of food or prescience of food in the mouth
What is the result of parasympathetic activation of salivary glands?
Secretion of copious quantities of fluid
What is the result of sympathetic activation of salivary glands?
secretion of small volumes of viscous fluid
What is the volume of gastric fluid secreted each day?
2-3L
What do the surface epithelial cells off the stomach secrete?
Bicarbonate, and goblet cells secrete mucus
What is the function of intrinsic factor, which is secreted by parietal cells?
binds to vitamin b12, stabilising it to prevent it being digested so it can be absorbed in the small intestine
What is needed to convert pepsinogen to pepsin?
HCl
Where are enteroendocrine cells found in gastric glands?
At the very bottom of gastric glands
What is the upper part of a gastric gland called?
a gastric pit
What is the function of the mucus secreted n the stomach?
Protects against abrasion. Contains surface bicarbonate which buffers the acid
What is pepsin?
Gastric proteolytic enzyme - begins digestion of proteins in the stomach
How does stomach acid have a protective role?
Natures bacteria and viruses to prevent infection
How does pH impact protein?
low pH denatures protein structure
How are H+ ions produced in parietal cells to form acid (HCl)?
H2CO3 (carbonic acid) is produced by the reaction of CO2 and H2O using the enzyme anhydrase. This then dissociates forming bicarbonate and hydrogen ions. The hydrogen ions are then pumped into the stomach making it acidic.
How is H+ pumped into the lumen from parietal cells?
Via H+-K+ ATPase
How does H+-K+ ATPase work to pump H+ ions into the stomach from parietal cells?
Sits in the apical membrane and pumps H+ ions into the lumen in exchange for K+
What is the source of chloride (Cl-) in the secretion of HCl from parietal cells - how is Cl- brought into these cells?
An anion counter transporter in the serosal membrane (adjacent to the capillaries) of parietal cells ejects bicarbonate into the interstitial fluid while importing Cl-
What type of transporter is used to import Cl- into parietal cells?
Anion counter transporter
What type of transporter is used to pump H+ from parietal cells into the lumen of the stomach?
H+-K+ ATPase
How do Cl- ions enter the stomach lumen from the parietal cells?
Cl- diffuse across the apical membrane via a Cl- channel
What are the 3 phases of gastric secretion regulation?
Cephalic, gastric and intestinal phase
What controls secretion in the cephalic phase?
The head - higher CNS centres and the parasympathetic nervous system
What nervous system pathways are involved in the cephalic phase of gastric secretion?
Higher CNS centres detect the thought, smell and sight of food. The vagus nerve is stimulated. The Parasympathetic nervous system via the ENS stimulates gastric secretory cells and the secretion of the hormone gastrin
Where is gastrin released?
From G cells into the blood
What stimulates G cells to secrete gastrin into the blood?
The Parasympathetic nervous system via the ENS, then
What cells does gastrin stimulate in the cephalic phase?
Targets secretion from parietal cells and chief cells
What percentage of secretion associated with a meal occurs in the cephalic phase?
20%
What percentage of secretion associated with a meal occurs in the gastric phase?
70%
What percentage of secretion associated with a meal occurs in the intestinal phase?
10%
What stimuli in the stomach triggers the gastric phase of gastric secretion?
Stretch/distension of stomach wall. products of digestion in lumen, elevated pH
What do the enteric and parasympathetic reflexes of the gastric phase of secretion trigger?
Gastric gland secretion, motility and secretion of gastrin
What does gastrin trigger in the gastric phase?
Targets secretion from chief and parietal cells as well as generating motility - e.g. mixing patterns
What is the function of the intestinal phase of gastric secretion?
Controls delivery to the small intestine and slows gastric secretion as stomach empties
What stimuli triggers the intestinal phase of secretion?
Distension of the duodenum and detection of the entry of acidic chyme containing lipids and carbohydrates
What hormones are released in the duodenum in the intestinal phase of secretion?
GIP, CCK, secretin
What is the function of hormonal secretion of GIP, CCK and secretion during the intestinal phase of gastric secretion?
Slows down peristalsis and inhibits chief and parietal cell secretion.
What is the function and process of nervous feedback in the intestinal phase of gastric secretion?
Nerves stimulated by duodenal stretch and chemoreceptors trigger the enterogastric reflex, which targets the myenteric plexus to inhibit secretion and motility from the stomach
What reflex inhibits gastric motility and secretion in response to delivery of chyme to the duodenum?
Enterogastric reflex
What are the 2 components of pancreatic secretions?
Enzymes and alkaline fluid
Where are enzymes secreted from in the pancreas?
Acinar cells
What is the function of acinar secretion from the pancreas?
Produces enzymes for the chemical digestion of food in the small intestines
Where is the alkaline fluid of the pancreas produced?
In duct cells
What is the fucntion of the alkaline fluid produced in the pancreas?
Neutralises acidic chyme to provide an optimum pH for enzymes
What are the 4 classes of enzyme secreted by the pancreas for the digestion of all classes of food?
lipolytic, amylytic, proteolytic, nucleolytic
What are the lipolytic enzymes secreted by the pancreas?
Lipase, phospholipase
What is the amylytic enzyme secreted by the pancreas?
Pancreatic amylase
What are the 3 proteolytic enzymes secreted by the pancreas?
Trypsin, chymotrypsin, carboxypeptidase
What are the nucleolytic enzymes secreted by the pancreas?
Ribonuclease, deoxyribonuclease
What stimulates the secretion of acianr cells in the pancreas?
The hormone CCK - the secretion of which is stimulated by the arrival of lipids and carbohydrates in the duodenum
In what form are proteolytic enzymes secreted?
as inactive precursors/zymogens
Where are the inactive precursors of the pancreatic proteolytic enzymes activated?
In the duodenum
What is the inactive precursor of trypsin?
Trypsinogen
What is the inactive precursor of chymotrypsin?
Chymotrypsinogen
What is the inactive precursor of carboxypeptidase?
procarboxypeptidase
What enzyme activates trypsinogen to trypsin in the small intestine?
Enterokinase/enteropeptidase
What is the process of activation of inactive precursor of proteolytic enzymes in the small intestine?
Enterokinase/enteropetidase converts trypsinogen to trypsin, trypsin activates chymotrypsinogen to chymotrypsin as well as procarboxypeptidase to carboxypeptidase
Where is enterokinase/enteropepetidase found and made?
Made bound to epithelial cells in the duodenum and then remains bound to the duodenal membrane where it can activate trypsinogen
What is the secretion of alkaline fluid from the pancreas stimulated by?
The secretion of the hormone secretin, which is stimulated by the arrival of acidic chyme in the duodenum.
What are the 4 components of biliary secretions?
Bile salts, HCO3- (bicarbonate) rich fluid, bile pigments (waste products), cholesterol
Where is HCO3– produced in the liver?
Duct cells
What is the function of bile salts?
Fat digestion
What is the function of bile pigment release in biliary secretions?
Excretion of waste
When is bile delivered to the duodenum?
Delivered from gall bladder upon the arrival of food
What control mechanism is used for the initial delivery of bile to the duodenum?
Hormonal - using mostly CCK and also secretin
How does CCK stimulate biliary secretion?
CCK produced in response to products of digestion of digestion. It then acts to contract the gallbladder and relax the hepatopancreatic ampulla so that bile can enter the duodenum
What does CCK act on to release bile?
Contracts the gallbladder, relaxes the heptopancreatic ampulla
What is the role of secretin in biliary secretion?
Mild stimulation of bile by liver
How does bile ‘stimulate its own secretion’?
Via the enterohepatic circulation - Bile salts are reabsorbed from the duodenum back to the liver, as the are metabolically expensive to produce. The transport back stimulates further secretion from the liver.
What is the enterohepatic circulation?
A circulation that involves the cycling of bile salts from the liver to the gall bladder then duodenum, where it is reabsorbed into the hepatic portal vein and travels back to the liver to be reabsorbed.
What proportion of bile salts are reabsorbed?
95%
Why is bile reabsorbed via the enterohepatic circulation?
As it is metabolically expensive to produce
Where are secretions made in the small intestine, excluding accessory organs?
crypts/glands
What is isosmotic fluid secreted in the small intestine?
A slightly alkaline fluid of NaCl and NaHCO3 that dilutes food to aid in digestion and neutralises stomach acid
