Lecture 25: Secretion into GIT Flashcards
What are the two pathways for molecules to move across the epithelial barrier?
Transcellular pathway
▪ Across the cell membranes
▪ Through the cytoplasm
Paracellular pathway
▪ Between cells (often through ‘leaky’ tight junctions)
What is the difference between secretion and absorption in regards to the GIT?
Secretion (exocrine)
▪ Movement of solutes and water from the body to the GIT lumen.
▪ Can occur via the paracellular and/or transcellular pathway
Absorption
▪ Movement of solutes and water from the GIT lumen into the body
▪ Can occur via the paracellular and/or transcellular pathway
What is the difference between endocrine and exocrine secretions?
Endocrine secretions move into the blood (e.g. insulin, anti-diuretic hormone (ADH)
Exocrine secretions are produced by epithelia & move into the lumen
What are the three main secretions in the GIT?
Mucus
▪ Protection and lubrication
▪ Aids mechanical digestion
Electrolyte solutions
▪ Dilutes food & provides optimal pH
▪ Essential for function of digestive enzymes
Digestive enzymes
▪ Essential for chemical digestion of food
▪ Aids absorption
What are the relative proportions of saliva produced by the three salivary glands?
Sublinguinal glands = 5% (only mucus)
Submandibular glands = 70% (mixed)
Parotid glands = 25% (serous fluid w amylase)
What are the digestive enzymes produced in the salivary glands?
▪ Lingual lipase: Chemical digestion of fats
▪ Salivary amylase Chemical digestion of starch
How is salivary secretion regulated?
Regulated by the autonomic nervous system:
▪ Thought, smell, sight of food
▪ Presence of food in mouth
Autonomic Nervous system
▪ Parasympathetic
- Stimulates secretion of copious quantities of fluid
▪ Sympathetic
- Small volumes of viscous fluid
What is the secretion rate and composition of gastric juices between meals?
Slow secretion rate (15-30 mL/hour). Goblet cells secrete mucus and bicarbonate (HCO₃⁻), which protect the stomach lining from acid and abrasion.
How do mucus and bicarbonate protect the stomach lining?
Mucus shields against physical abrasion, while bicarbonate neutralizes stomach acid, protecting the stomach from its own acidity.
What additional gastric secretions occur when eating, and which cells are involved?
Secretion rate increases. Goblet cells continue to secrete mucus and bicarbonate. Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor. Chief cells secrete pepsinogen.
What are the functions of hydrochloric acid (HCl) in digestion and protection?
HCl denatures proteins, activates pepsinogen to pepsin, provides optimal pH for pepsin, and protects the body by killing microbes.
What do intrinsic factor and pepsinogen do in the stomach?
Intrinsic factor (parietal cells) aids in vitamin B12 absorption in the ileum. Chief cells secrete pepsinogen, which converts to pepsin for protein digestion.
How are hydrogen ions (H⁺) produced in the parietal cells of the stomach? (source of H+)
Carbonic anhydrase catalyzes the reaction between CO₂ and H₂O, producing hydrogen ions (H⁺) and bicarbonate (HCO₃⁻).
How are hydrogen ions (H⁺) secreted into the stomach lumen?
The H⁺-K⁺ ATPase pump in the apical membrane of parietal cells exchanges H⁺ for potassium ions (K⁺), pumping H⁺ into the lumen. K⁺ returns to the lumen through a potassium channel.
How is chloride (Cl⁻) imported into parietal cells?
An anion counter-transporter in the basolateral membrane imports Cl⁻ into the parietal cell in exchange for ejecting bicarbonate (HCO₃⁻) into the interstitial fluid.
How is chloride (Cl⁻) secreted into the stomach lumen?
Chloride (Cl⁻) diffuses across the parietal cell and exits into the lumen through a Cl⁻ channel in the apical membrane, where it combines with H⁺ to form HCl.
Describe the cephalic phase of stomach activity:
Prepare the stomach for the arrival of food
▪ Eating or being about to eat
▪ Incoming food
▪ CNS (brain) controls gastric secretion
▪ 20% of gastric secretion
Describe the gastric phase of stomach activity:
Maximise mechanical digestion and begin protein chemical digestion
▪ Food arrives at the stomach
▪ Stomach controls gastric secretion
▪ 70% of gastric secretion
Describe the Intestinal phase of stomach activity:
Slow controlled release of food to small intestine
▪ Chyme (food) is passed from the stomach to the small
intestine
▪ Duodenum (small intestine) controls gastric secretion
▪ 10% of gastric secretion
What stimuli trigger the cephalic phase of stomach activity, and how are they detected?
Stimuli:
- Seeing, smelling, tasting, or thinking about food.
Detection:
Detected by the central nervous system (CNS), which processes sensory information related to food.
How is the response coordinated during the cephalic phase?
Coordinated by:
The parasympathetic nervous system (PNS), which stimulates the enteric nervous system (ENS).
The submucosal plexus is activated, enhancing secretion in the stomach..
What are the effectors and outcomes of the cephalic phase of stomach activity?
Effectors
▪ Goblet cells
▪ Chief cells
▪ Parietal cells
▪ G cells - secrete the hormone gastrin into the
blood further stimulating Chief cells and Parietal cells
Outcome?
↑ Secretion of mucus, HCO3-, HCl acid and pepsinogen in the stomach
(basically preparing the stomach for the arrival of food yum yum)
What stimuli trigger the digestive response in the stomach (Gastric phase)?
Stretch, increased pH, and undigested food, especially proteins, act as stimuli, which are detected by:
G cells (sensing peptides) and gastric mechanoreceptors and chemoreceptors.
How is the digestive response (Gastric phase) coordinated in the stomach?
The response is coordinated by the submucosal plexus (regulating secretion) and the myenteric plexus (controlling motility).
What is the outcome of the gastric phase response triggered by G cells and neural plexuses?
The outcome is an increase in the secretion of mucus, HCO₃⁻, HCl acid, and pepsinogen, along with enhanced gastric motility through waves of peristalsis (retropulsion).
Which cells act as effectors in the stomach during the gastric phase, and what do they secrete?
Mucous cells secrete mucus and bicarbonate (HCO₃⁻)
Chief cells secrete pepsinogen.
Parietal cells secrete hydrochloric acid (HCl)
G cells secrete the hormone gastrin into the blood, which further stimulates chief cells (to release pepsinogen), parietal cells (to release HCl), and increases gastric motility by promoting peristalsis.
What stimuli trigger the intestinal phase of stomach activity, and how are they detected?
Stimuli:
- Stretch of the duodenal wall
- Decreased pH (acidic conditions)
- Presence of fatty acids and amino acids in the duodenum
Detection:
Detected by duodenal enteroendocrine cells (EECs) and mechano- & chemoreceptors in the duodenum.
How is the response coordinated during the intestinal phase?
Coordinated by:
Release of duodenal hormones: Cholecystokinin (CCK), gastric inhibitory peptide (GIP), and secretin.
Involvement of the central nervous system (CNS) leading to activation of the sympathetic nervous system (SNS) - inhibits ENS
What is inhibited during the intestinal phase, and what are the outcomes?
Myenteric plexus (reducing gastric motility):
Reduced gastric retropulsion, leads to slower gastric emptying into the small intestine - more effective digestion and absorption.
Submucosal plexus (decreasing secretory activity): Inhibits:
- Chief cells (reducing pepsinogen secretion)
- Parietal cells (decreasing HCl acid secretion)
- G cells (lowering gastrin production)
The cephalic phase of gastric secretion is stimulated by
_______________ and results in an increase in _____________ .
1. stretch in the stomach; gastric motility
2. thinking about food; secretion of gastrin
3. an increase in the pH of the stomach; secretion of HCl acid
4. peristalsis in the oesophagus; retropulsion
2
Is the pancreas an endocrine or exocrine organ?
It is BOTH
Exocrine secretions from the pancreas, are made up of TWO components made by two different types of cells: These cells are?
Pancreatic acinar cells secrete:
▪ Pancreatic digestive enzymes
Pancreatic duct cells secrete:
▪ Alkaline (HCO3-) fluid
What are the large intestine secretions?
Mucus only
What do CCK, GIP and Secretin hormones have in common?
They are all secreted from Enteroendocrine (hormone) cells in the duodenum
DONT FORGET (NOT SHOWN ON THE OTHER HORMONE ‘ROLE’ CARDS)
They also all Inhibit the stomach secretion apart from gobby cells
What is the main role of Cholecystokinin (CCK)?
(think enzymes and bile!)
Stimulates release of pancreatic juice with digestive enzymes from pancreatic acinar cells
Contraction of the wall of the gall bladder causing it to empty (releases stored bile)
Relaxation of hepatopancreatic sphincter allowing bile and pancreatic juice to enter the duodenum
What is the main role of Secretin?
(think neutralisation!)
Stimulates release of pancreatic juice with bicarbonate (HCO3-) from pancreatic duct cells
Mildly stimulates production of bile by hepatocytes (liver cells)
What is the main role of Gastric Inhibitory peptide (GIP)
Inhibits release of pepsinogen from chief cells, HCl acid from parietal cells & gastrin from G cells…
Is bile secreted always? What does bile also contain?
Hepatocytes (liver cells) continuously secrete Bile into the bile canaliculi - For chemical digestion of fat
Bile also contains excretory/waste products:
▪ Bile pigments (bilirubin) - from the breakdown of haem
▪ Excess cholesterol
Where is bile stored and concentrated?
Gall bladder
How does bile enter the duodenum?
CCK causes the initial release of bile from the gall bladder into the duodenum, this causes Contraction of the wall of the gall bladder and relaxation of the hepatopancreatic sphincter. This allows bile through into the GIT
How does bile break apart lipid droplets?
Emulsification
Is bile easy to make?
Bile is “metabolically expensive” to make
▪ 95% is reabsorbed in the ileum
▪ Transported back to the liver via the enterohepatic
circulation (REMEBER THIS)
▪ Reabsorbed by the liver so it can be secreted again
How is the liver stimulated to make more bile?
Bile returning to the liver via the enterohepatic circulation (main/strongest stimulus) - Because this signals to the hepatocytes that bile has recently been released from the gall bladder
▪ Secretin (but only mildly)
Are the enzymes produced in the pancreas active on release?
NO, Pancreatic proteolytic enzymes are secreted as inactive precursors & activated later in the duodenum so that they don’t digest proteins inside pancreatic acinar cells
What is the activation process of digestive enzymes in the doudenum?
▪ Enterokinase (= enteropeptidase) which is bound to duodenal membrane Converts trypsinogen to trypsin
(TRYPSIN SNOWBALL EFFECT)
▪ Once activated trypsin converts the other enzymes to their active form
What is the name for enzymes that break down RNA & DNA?
Nucleolytic enzymes
eg. ribonuclease, deoxyribonuclease
What is the name for enzymes that break down fat?
Lipolytic enzymes
eg. lipase (colipase = cofactor for lipase)
What is the name for enzymes that break down carbohydrates?
Amylytic enzymes
eg. : pancreatic amylase
What is the name for enzymes that break down peptides?
Proteolytic enzymes
eg. trypsinogen, chymotrypsinogen,
procarboxypeptidase
Which of the following statements is NOT correct?
1. The strongest stimulus for the liver to produce more bile is bile returning to the liver.
2. Secretin is released from duodenal enteroendocrine cells in response to acidic chyme arriving in the duodenum.
3. Cholecystokinin (CCK) relaxes the hepatopancreatic sphincter.
4. Pancreatic acinar cells secrete alkaline (HCO3-) rich fluid
4
How is the release of GIP and CCK stimulated?
Arrival of peptides/amino acids and fatty acids in the
duodenum caused by arrival of chyme from the stomach
How is the release of Secretin stimulated?
Decrease in pH in the duodenum caused by the arrival of acidic cyme from the stomach
