What chemical and mechanical digestion occur across the GIT?
- Oral cavity
> Teeth/Salivary glands - Stomach
- Small intestine
> Smooth muscle, Pancreas, Gal bladder, Brush boarder - Large intestine
= produces Water, Amino acids, Monosaccharides, Fatty Acids.
Describe what is absorbed across the GIT.
- Oral cavity
- Stomach
- Small intestine
- Large intestine
- These components are then absorbed across the mucosa of the gastrointestinal tract, and then into the blood
How is gastrointestinal secretion and motility carefully regulated?
- Endocrine + neural signalling
> Neural – Fast onset, specific responses, energetically expensive
Endocrine – Slow onset, coordinated responses, cheaper energetically
= both, when need a fast onset of response,
but need to maintain this for a longer period
What are the three phases of GI secretion and where is the stimulus sensed?
- Cephalic stimulated by external environment and mouth
- Gastric stimulated by food in the stomach
- Intestinal stimulated by food in the intestines
1- How is GI secretion initiated in Cephalic stage?
2- What type of control is this known as?
3- What are the 4 function of Cephalic phase?
1- sensory stimuli related to the anticipation of food > sight, smell, taste & thoughts of food
(Conditioned reflex)
2- Feedforward control = GI secretion triggered before food arrives in GIT.
3-
1- lubrication with saliva
2- Ensuring acid is present in stomach to kill bacteria
3- Bicarbonate is present to neutralise any acidic chyme escaping stomach
4- To have active enzymes in place when food arrives
How is the cephalic phase mediated? (Neural/Hormonal regulation)
How does the cephalic phase prepare the GIT to receive food?
Sensory stimuli → Vagus nerve →
1- ACh → muscarinic cholinergic receptor → parietal cells secrete acid
- Gastrin release from G-cells in antrum →
1- gastrin receptors (G) on parietal cells → acid secretion
2- enterochromaffin-like (ECL) cells → histamine (H) release → parietal cells secrete acid
3- inhibit the release of somatostatin from D-cells
(Normally: D-cells → somatostatin (SST) → parietal cells → inhibit of acid secretion)
1- When does gastric phase of GI secretion begin?
2- What are the 3 functions of the gastric phase?
3- What is the function of stretch receptors + Chemoreceptors in gastric phase?
1- Food enters the stomach
2-
- enhance secretions started in cephalic phase
- acidify chyme
- initiate protein digestion
3-
> Stretch receptors - matches degree of secretion to predicted quantity of food ingested
> Chemoreceptors - matches degree of secretion to expected nutritional quality of food ingested
How is the gastric phase mediated? (Neural + Hormonal)
Describe flow of processes in the gastric phase when:
1- The stomach is distended by food
2- Proteins are digested
1- Distention: stretch-sensitive neurons → PNS mediated vagovagal reflex (quantity ≈ secretion)
2- Amino acids & peptones:
- G-cells in stomach secrete gastrin → stimulates acid & acinar cell secretion
(quality ≈acinar cell secretion)→ stimulate chief cells to secrete pepsinogen
What happens when luminal PH < 2?
- Motility and secretions are temporarily suspended.
1- When does the intestinal phase of GIT secretion begin?
2- What are the 2 functions of the intestinal phase?
1- Chyme starts entering small intestine from stomach
2-
- control rate of chyme entry into duodenum
- maintain optimal conditions for enzymatic digestion of food
How is the intestinal phase mediated? (Neuronal + Hormonal)
1- In the intestinal phase Acidic chyme enters the duodenum what does this stimulate?
1- S-cells to release secretin, if the luminal pH falls below 4.5. > Stimulates duct secretion primarily (maintains optimal conditions for enzymatic digestion of food)
2- Secretin also stimulates HCO3- secretion from burners gland, slows H+ secretion by stomach parietal cells as well as constriction of pyloric sphincter
In the intestinal phase, what do fatty acids and peptones stimulate?
- Vagovagal reflex which further maintains parasympathetic stimulation of acinar and duct cells
(quality ≈ pancreatic secretion)
> Vagus nerve (CN X) stimulation for release of PANCREATIC ENZYMES
In the intestinal phase G cells in the duodenum secrete?
- Gastrin in response to amino acids and peptones in the stomach. = Stimulates acinar cell secretion.
(acinar cell secretion ≈ quality)
In the intestinal phase Fatty acids and to a lesser extent peptones stimulate?
- Duodenal I-cells to release cholecystokinin (CCK).(pancreatic secretion ≈ quality)
- CCK also stimulates the relaxation of the hepatopancreatic sphincter as well as contraction of the gall bladder.
> Bile salts assists lipid digestion by emulsifying lipid droplets thus increasing surface area for digestion.
In addition to its ability to directly stimulate pancreatic secretion, CCK stimulates pyloric sphincter constriction
Why is this helpful?
- Reduces rate of chyme release from the stomach
> Digestion & absorption takes time
The rate of passage through the gastrointestinal tract can be controlled by:
1- Contraction of sphincters
2- Changing the rate of Peristalsis
> CCK elicits contraction of the pyloric sphincter, slowing the rate of release of chyme into the duodenum.
Less chyme released = slower flow through intestinal tract, giving more time for digestion & absorption.
What does CCK stimulate in the hypothalamus?
- Satiety centres
Describe how GIT secretion is dysregulated in the following conditions:
- Inflammatory bowel disease (IBD)
- Gastroesophageal reflux disease (GERD)
- Exocrine pancreatic insufficiency
- Peptic ulcers
Pancreatic fluid is a major supply of digestive enzymes for the duodenum.
1- Describe the path of pancreatic juices into the duodenum?
2- What PH is pancreatic juice?
1- Main pancreatic duct + Common bile duct → Major duodenal ampulla/ Hepatopancreatic ampulla → Hepatopancreatic sphincter → Duodenum
2- Bicarbonate-rich fluid to neutralize acidic chyme
Describe the two major secretory cell types of the pancreas and their principal function.
1- Pancreatic Acinar cells - Enzyme secretion (proteases, pancreatic lipase, pancreatic amylase)
> Isotonic fluid to carry enzymes
(≈25% of total fluid)
2- Pancreatic Duct cells - HCO3- secretion
> Hypotonic fluid to carry enzymes
(≈75% of total fluid)
What is a zymogen?
- Pancreatic enzymes are packaged within zymogen granules (Z), which can be secreted upon stimulation of the acinar cell
- The enzymes within the zymogen granules are inactive precursors of the mature enzyme (zymogens) to prevent autodigestion of the cell
Describe the 2 hormones that control exocrine pancreas secretions.
