Lecture 25: Secretion into GIT Flashcards by Angus Henry

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)

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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

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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

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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

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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)

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What are the digestive enzymes produced in the salivary glands?

▪ Lingual lipase: Chemical digestion of fats

▪ Salivary amylase Chemical digestion of starch

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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

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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.

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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.

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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.

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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.

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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.

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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⁻).

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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.

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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.

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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.

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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

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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

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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

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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.

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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

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

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