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BS1060 > Block 9 - GI physiology and homeostatic integration > Flashcards

Block 9 - GI physiology and homeostatic integration Flashcards

1
Q

What are the 4 functions of the stomach?

A

1) Digestion of protein
. pepsinogen and acid secretion, churning
. chief cells

2) Secretion of acid, regulation of acid secretion and protection of mucosa from acid
. pepsinogen and acid secretion
. parital cells

3) Defence from microbes
. acids, pepsins, microbes
. kills bacteria - first defence, first hollow organ to recieve chyme

4) Motility
. churning
. propulsive movements along GI tract
. mechanically mix to expose particles to digestive enzymes - churning movements

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2
Q

What is the GI tract?

A
  • A hollow tube that is open at both ends (mouth and anus)
  • Considered external
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3
Q

What is the structure of the GI tract inside to out?

A
  • The hole inside is called the lumen (where chyme travels)
  • Next layer is the mucosa (where villi and glands are)
  • Lamina propria then Muscularis mucosae (both parts of the mucosa
  • Submucosa
  • Muscularis externa - muscle layer. Inner cirular and outer longitudal
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4
Q

What forms the mucosa?

A

Inside to out:
- Lumen. The space.
- Epithelial, touch the chyme. The ones that move the food/ have a function.
- Lamina propria. Layer of connective tissue.
- Muscularis mucosae. Muscle layer.

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5
Q

What forms the submucosa?

A
  • One layer, formed of loose collagenous connective tissue.
  • Contains larger blood vessels, lymphatics and nerves.
  • Contains glands.
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6
Q

What forms the muscularis externa?

A
  • longitudinal smooth muscle
  • circular smooth muscle
  • enteric nerves between muscle layers
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7
Q

What is the enteric nervous system (ENS) and what does it control?

A

ENS is a divison of the autonomic nervous system located in the GI tract - primarily in the mysenteric and submucosal plexus.

  • Controls motility, secretion and absorbtion

plexus = braid

  • Contains over 100 million neurons (more than the spinal chord and rest of the entire ANS)
  • A reflex circuit that can function without any other input
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8
Q

What forms the serosa?

A

Connective tissue that envelops the GI tube, outermost layer.

Covered with a layer of squamous epithelial cells.

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9
Q

What is the structure of the stomach?

A
  • First of the hollow organs
  • The body (main/ upper area)
  • Antrum (lower area)

Same as GI tube but mucosa has gastric pits.

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10
Q

What is the structure of the gastric pits and the role of each cell type?

A

Top to bottom:
- Superficial epithelial cell; HCO3- secretion
- Mucous neck cell; mucous secretion and protection from luminal acid.
- Stem/ regenerative cell; repair
- Parietal (oxyntic) cell; HCl secretion
- Chief cell; pepsinogen secretion
- Endocrine cell; hormone secretion
. G cells; gastrin
. D cells; somatostatin
. Entero-chromaffin-like cells; histamine

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11
Q

How does gastric acid secretion work?

A

Parietal cells change shape when stimulated to secrete acid (gastric acid).
- tubulovesicular structure rearranges into a canalicular structure (inc surface area x100)

Involved:
. Cl- channels - apical
. Proton pump - apical
. Na+/ H+ exchanger - basolateral
. Sodium pump - basolateral
. K+ channel - basolateral
. HCO3-/ Cl- exchanger - basolateral
. Carbonic anhydrase - basolateral

Mechanism for action:
1) Carbonic anhydrase - converts carbon dioxide and water into hydrogen carbinate and protons
2) Protons are extruded through proton pump on the apical membrane in exchange for potassium
3) Biproduct of carbonic anhydrase is hydrogen carbonate, exchanges for Cl- at the HCO3-/Cl- exchanger on the basolateral membrane
4) Chloride channel on apical membrane passively diffuses Cl- across into the lumen and makes acid with H+
5) Sodium pump creates the sodium gradient - keeps cell alive.
6) Excess H+ removed through H+/Na+ exchanger on basolateral membrane to avoid cell getting too acidic (pH control of cytoplasm)
7) Excess K+ recycled through K+ channel on basolateral membrane.

pH can get as low as 1 (between 5 and 1)

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12
Q

Where are the apical and basolateral membranes found?

A

Apical - side with lumen
Basolateral - side with interstitial space

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13
Q

What are irreversible inhibitors of the proton pump used for?

E.g. Esomprazol, omeprazole, losec, prilosec, nexium

A

Biggest selling drug in pharmaceuticals.

Used for heartburn and ulcers.

Irreversible doesn’t matter bc the cells die quickly and get replaced. Blocks the proton pump.

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14
Q

What are the pathways for acid secretion?

A

Direct pathway:
Acetylcholin, gastrin and histamine stimulate the parietal cell triggering the secretion of H+ into the lumen.

Indirect pathway:
Acetylcholine and gastrin stimulate the ECL (entero-chromaffin-like) cells to secrete histamine. Histamine acts of the paretial cell.

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15
Q

How is acid secretion regulated? STIMULATION

A

STIMULATION - body
1) The vagus nerve stimulates the parietal cells directly through the release of ACh and increases acid secretion

2) Vagal timulation of ECL increases histamine release

3) Histamine stimulates acid secretion

4) Vagal timulation of G cells triggers gastrin release

5) Gastrin directly stimulates acid secretion from the parietal cell

6) Gastrin indirectly stimulates acid secretion by increasing hisatmine release from ECL cells

INHIBITON - antrim
7) Luminal acid stimulates D cells to release somatostatin

8) Somatostatin inhibits acid secretion directly at parietal cell

9) Somatostatin also inhibits gastrin release at G cell

10) Somatostatin also inhibit histamine release at ECL cell

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16
Q

How does the mucous barrier work?

A

Mucus gel (from mucous neck cells) and hydrogen carbonate secretion.

Hydrogen carbonate is used by surface epithelial cells which is secreted into the mucous gel - this is alkaline.

Diffusion barrier against hydrogen ions and pepsins - creates a muscus gel neutralisation zone (pepsins inactivated).

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17
Q

What is the structure of the small intestine?

A

. Duodenum
. Jejunum
. Ileum

1/3 digestion
2/3 absorbtion
abt 17ft long

Mucosa have villi and folds of kerckring

18
Q

What are the folds of kerckring?

A

Circular folds in the small intestine to increase surface area. Can be seen with the naked eye.

19
Q

What do the villi of the small intestine contain?

A

Base of villi called crypts of Leiberkühn (the pits)

  • Lymph vessles
  • Capillaries
  • Arterioles and venules
  • Nervous supply
  • Epithelial cells
20
Q

What is the cellular structure of the small intestine?

A

Top to bottom:
- Absorbative cell; transporting epithelia with microvilli-brush boarder
- Goblet cell; secretory cells that produce mucin
- Enteric endocrine cell; hormone production
- Stem/ progenitor cell; differentiation and migration to replace sloughed cells
- Paneth cell; specialised for secretion of antibacterial enzymes such as lysosyme and defensins

Villi:
Absorbative and goblet cells.

Crypt:
Absorbative, enteric endocrine, stem, paneth cells.

21
Q

What is the process of digestion in the small intestine?

A

Digestive juices from pancreas are secreted into duodenum.

  • Lipase breaks down fat
  • Amylase breaks down carbohydrates
  • Protease and trypsin break down protein

Oligosaccharides for carbohydrate digestion are expressed on the brush boarder (e.g. lactose brocken down by lactase into galactose)

Amino-peptidases for protein digestion are on the apical membrane of the brush border. Breaks down protein into amino acid constituents.

22
Q

What is the process of glucose absorbtion in the small intestine?

A

-Sodium-glucose linked transporter - apical membrane
- Glucose uses secondary active transport by linking to sodium and travels down it’s gradient. (secondary)
- Sodium/ potassium pump on basolateral membrane (primary)

23
Q

What is the process of amino acid absorbtion in the small intestine?

A

-Sodium-amino acid symporter - apical membrane
- Amino acid travels through with sodium (secondary)
- Sodium/ potassium pump on basolateral membrane (primary)

24
Q

What is the process of H2O absorbtion in the small intestine?

A
  • Water passively flows down the osmotic gradient (e=generated by secretion and absorbtion)
  • Osmotic substances include glucose, amino acids, Na+, Cl-
25
Q

What is the process of sodium absorbtion in the small intestine?

A

Na+/H+ exchanger (also used for fat absorbtion)
^ secondary active transport

  • primary active transport is the sodium potassium pump (sodium ion pump for keeping cell alive)
26
Q

What is the process of fat absorbtion in the small intestine?

A
  • Lipases secreted from the pancras digest complex dietary fats into simple lipids in the small intestine
  • Bile salts act as detergents and form small micelles.
  • Micelles are small enough to diffuse amongst microvilli.
  • Na+/ H+ exchanger creates a microclimate at the brush boarder and micelles release fatty acids in acid environment.
  • Free fatty acids are absorbed into absorbative cells through fatty acid transport protein.
27
Q

What happens to fatty acids post absorbtion?

A

2-monoglycerides, lysophospholipids and cholesterol are re-synthesised in the smooth ER into triglycerides, phospholipids and cholesterol ester.

  • Intracellular micelles called chylomicrons are formed with a lipoprotein coat
  • Chylomicrons leave the basolateral membrane by exocytosis and diffuse into the villi lacteals where they leave the intestine through the lymphatic system
28
Q

What is the small intestine defence system?

A

Non-immune defence:
- secretion of mucous (Goblet cells)
- secretion of defensins (Paneth cells)
- tight junctions between cells

Immune defence:
- gut associated lymphoid tissue (immune cells in the villi and Peyer’s patches)

29
Q

What are Peyer’s patches?

A

Distinctive structures with epithelia lining containing B and T lymphocytes surrounding a lymphoid nodule.

  • Found in the lamina propria of the mucosa.
30
Q

Explain the motility of the small instestine

A

1) Migrating motor complex
2) Peristalsis
3) Segmental movements

  • Contraction of the lonitudinal and circular layers of muscilaris externa in GI motility
    . controlled by the ENS
  • Smooth muscle contraction (found in muscularis externa and muscularis musosae)
    . cross-bridge cycling
    . requires calmodulin to detect calcium and myosin light chain kinase for phosphorylation of myosin for contraction.
31
Q

What is the migrating motor complex?

A
  • Between meals, sweeps remnants into the intestine
  • Rhythmic contractions of the stomach and small intestine in the fasting state.
32
Q

What is peristalsis?

A

Circular muscles contract whilst longitudinal muscles relax just behind bolus of food, pushing food into receiving segment. Receiving segment circular msucles relax whilst longitudinal ones contract.

  • Forward movements
  • Progressive waves that move food bolus towards the next area of the GI tract
33
Q

What are segmental movements?

A
  • Responsible for mixing and churning.
  • 1-5cm segments of the intestine alternately contract and relax
  • Churning occurs when circular muscles contract in segments
  • Occurs randomly along intestine
34
Q

What are slow waves in digestion?

A

Oscillations in the membrane potential of GI smooth muscle cells.

Several slow waves per minute (around 6)

When slow waves exceed the electrical threshold (around -40mV) action potentials will fire.
. when action potentials fire the force of smooth muscle contraction is higher.

35
Q

What is homeostasis?

A

The tendancy towards a relatively stable equillibrium between interdependent elements, especially as maintained by physiological processes

36
Q

What is redundancy in homeostasis?

A

The more vital a parameter is, the more systems there are to regulate it.

If one system fails then there are others that maintain homeostasis.

37
Q

What is hypovolamemic shock?

A

Severe blood loss
Hypo = low, vol= volume, aemia = blood
Also called haemorrhagic shock or circulatory shock.
30% or more blood loss = ~1.5 L

38
Q

What is the average blood volume in humans?

A

~5 Litres

39
Q

List 5 compensatory mechanisms against blood loss

A

1) Reabsorbtion of tissue fliuds into the plasma
2) Baroreceptor reflexes: high and low pressure
3) The sympathetic urge
4) Release of other vasoconstrictor substances
5) Renal conservation of salt and water

40
Q

Explain the process of transapillary refill

A

The movement of fliud from the interstitum to the vasulature (reabsorbtion of tissue fluids into the plasma)
- major defence against blood loss
- AKA autoinfusion
- Only lasts a few hours due to limited supply of water in interstital place

Driven by:
. Hydrostatic pressure - difference between capillary hydrostatic pressure and extravascular pressure (interstitial fluid)
. Oncotic pressure - difference across the capillary wall. Albumin drives this process

41
Q

How do baroreceptors work?

A

High bp:
High stretch = inc firing rate (afferent fibres) = signals a decrease in vasoconstrictor (brainstem) = vasodilation

Low bp:
Low stretch = decreased firing rate (afferent fibres) = signals an increase in vasoconstrictor (brainstem) = vasoconstriction

42
Q

How is respiration controlled through the brainstem?

A

Neurons regulate the rate and depth of breathign in respinse to the physiological needs of the body for O2 and CO2 exchange and for blood acid-base balance.

Stretch receptors in the lungs

Peripheral chemoreceptors
Central chemoreceptors

BS1060 (10 decks)

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