Physiology Flashcards
What is the function of gastric acid?
- Activates pepsin
- Denatures proteins
- Helps kill bacteria and other microorganisms
- Inactivates salivary amylase
What mechanisms control gastric acid secretion?
- Neurocrine - vagus nerve/local reflexes
- Endocrine - gastrin
- Paracrine - histamine
Where is gastrin secreted from?
G cells
Where are G cells found?
Deep within gastric glands
What stimulates the release of gastrin?
- Amino acids
- Peptides
- Distention of stomach
- Neural reflexes
What cells produce histamine?
Enterochromaffin-like cells (ECL)
What cells secrete gastric acid?
Parietal cells
Where are parietal cells found?
Body of the stomach
Draw a diagram of how gastric acid is produced by parietal cells
How do histamine, gastrin, and acetylcholine cause gastric acid secretion?
All cause intracellular reactions which activate protein kinases, leading to acid production
What are the different phases of gastric acid production?
- Cephalic phase
- Gastric phase
- Intestinal phase
How does the sight/smell/taste of food stimulate gastric acid production (cephalic phase)?
Stimulates the vagus nerve, which in turn acts on parietal cells through:
- Direct - ACh
- Indirect - G cells
Gastrin/ACH also acts on ECL cells, which secrete secrete histamine, which acts on parietal cells
What does gastric acid stimulate the release of?
Pepsinogen - converted to pepsin by acid itself
How does the gastric phase lead to Gastric acid production?
- Distention of stomach -> vagal/enteric refelxes -> Parietal cells
- Peptides in lumen -> Gastrin secretion -> Parietal cells
- Gastrin/ACh released -> ECL -> Parietal cells
How is the gastric phase of gastric acid secretion inhibited?
Decreased pH -> decreased gastrin production
What is the net result of the gastric phase?
Digestion of protein, formation of chyme, and controlled entry of chyme into the small intestine
What occurs during the intestinal phase of gastric acid production?
Chyme enters duodenum -> Acid sensed by enterogastric reflex, which causes:
- Secretin release
- Decreased Gastrin production and action on parietal cells
Fat in the duodenum -> GIP release, which decreases:
- Gastrin secretion
- HCl secretion
When is gastric inhibitory peptide released?
In response to carbohydrates
When is secretin released?
In the presence of acidic chyme
When is cholecystokinin released?
In the presence of fats
What is the funciton of CCK?
Slows gastric motility - to promote fat digestion as it takes longer than protein/carb
What is the function of Secretin?
- Inhibits gastric motility
- Stimulate Bicarb release from pancreas -> neutralise acid
What cell is pepsinogen secreted from?
Chief cells
Where is intrinsic factor secreted from?
Parietal cells
Where is gastric mucus secreted from?
Mucus neck cells
What is the role of gastric mucus?
- Protects mucosal surface from mechanical injury
- Protects against acid corrrosion -> buffered by HCO3
What cells do waves of depolarisation causing gastric motility originate in?
Cells of Cajal - pacemaker cells
What direction does gastric muscle contraction push food in?
Body -> fundus
Where does mixing primarily occur?
Antrum
How is gastric acid neutralised?
Bicarbonate production by pancreas
What cells secrete bicarbonate to neutralise gastric acid?
Brunner’s gland duct cells
What is the function of pancreatic duct cells?
- Protect the duodenum from the acidic content of chyme
- Provide an alkaline condition for the intestinal enzymes to be active
- Lubricate the intestinal walls
What controls Bicarbonate secretion from duct cells in the pancreas?
- Vagal/ENS reflex
- Secretin from S cells
What stimulates the release of pancreatic enzymes?
- Distention of small intestine
- Presence of food in small intestine
- Neural signals
- CCK
What cells produce pancreatic enzymes?
ACinar cells
How is trysinogen converted to trypsin?
Secreted by pancreas, then is converted by membrane bound enterokinase to trypsin. Then can activate other inactive enzymes secreated by the pancreas
What are proteases?
Enzymes which cleave peptide bonds
What are nucleases?
Enzymes which hydrolyse DNA/RNA
What are elastases?
ENzymes which digest collagen
What are phospholipases?
Enzymes which convert Phospholipids to fatty acids
What are lipases?
Enzymes which convert triglycerides to fatty acids and glycerol
What is alpha-amylase?
Enzyme which converts starch to maltose and glucose
What stimulates symogen secretion?
CCK
What are the functions of crypt cells?
Secretes Cl + Water
By what process does the majority of intestinal absorption occur?
Sodium coupled active transport
What are the main functions of the duodenum?
- Gastric acid neurtalisation
- Digestion
- Iron absorption
Where does the majority of nutrient absorption take place?
Jejunum - 95%
What is the main site of water absorption in the gut?
Ileum
How much fluid does the small intestine secrete per day?
1500 mL
How is water secreted into the lumen of the gut?
Epithelial cells lining the crypts of lieberkuhn - screted passively as a consequence of Cl secretion into lumen
What channel is responsible for Cl release from gut lumen cells into the lumen?
CFTR
What is segmentation?
This is segmental contraction and relaxation of the intestine. COntraction moves chyme up and down into adjacent areas of relaxation. Relaxed areas then contract and push cyme back
What is the purpose of segmentation?
Provides thorough mixing of contents with digestive enzymes, and brings chyme in contact with absorptive surface
When does segmentation most commonly occur?
During meals
What is peristalsis?
Coordinated contraction of the gut which moves contents in single direction. This is driven by the migrating motility complex. It ends at the terminal Ileum
What causes cessation of the migrating motility complex?
Food entering the stomach
What is the purpose of the migrating motility complex?
- Move undigested material into large intestine
- Limit bacterial colonisation of small intestine
- Motilin (hormone) involved in initiation of MMC
What plexus controls peristalsis?
Myenteric plexus
What is the gastroileal reflex?
- Gastric emptying -> increased segmentation activity in ileum
- Opening of ileocaecal valve (sphincter)
- Entry of chyme into large intestine
- Distension of colon
- Reflex contraction of ileocaecal sphincter (prevents backflux into small intestine)
What is the composition of saliva?
- Water: 99% of secreted fluid. Softens, moistens, dilutes particles. Solvent
- Mucins: Major protein component. Mucins + water = mucus.
- a-Amylase: Catalyses breakdown of polysaccharide into disaccharides
- Electrolytes: Tonicity/pH (e.g. Na+, K+ etc.)
- Lysozyme: Bacteriocidal - cleaves polysaccharide component of bacterial cell wall
