GI Physiology Flashcards
What cells make up a gastric gland?
Surface mucous cells
Mucous neck cells
Parietal cells
Chief cells
What do mucous neck cells secrete?
Mucus
What do chief cells secrete?
Pepsinogen
What do parietal cells secrete?
HCl and intrinsic factor
What is the mechanism for HCl secretion from parietal cells?
CO2 + H2O -> H2CO3
H2CO3 -> H + HCO3
H pumped out across apical membrane
HCO3 pumped out across basolateral membrane in exchange for Cl ion
CL leaves cell via apical membrane
H and Cl are in lumen of stomach with H2O
What transporter is essential for HCl secretion?
H+/K+ ATPase
What 4 factors act on H+/K+ ATPase to control gastric acid secretion?
Gastrin
Histamine
Prostaglandins
Acetylcholine
What is the function of Gastrin?
Binding to receptor causes increase in intracellular calcium.
Calcium acts on protein Kinase C which increases activity of ATPase
What is the function of Histamine?
Coupled to adenylate cyclase to convert ATP → cAMP.
cAMP acts on Protein kinase A to increase activity of ATPase
What is the function of ACh?
Binding to receptor causes increase in intracellular calcium.
Calcium acts on protein Kinase C which increases activity of ATPase
What is the function of prostaglandins?
Coupled up to adenylate cyclase to inhibit production of cAMP and switch the mechanism off.
What is the neurocrine control of gastric acid secretion?
Vagus nerve/ local reflexes
What is the endocrine control of gastric acid secretion?
Gastrin
What is the paracrine control of gastric acid secretion?
Histamine
What cells release histamine?
Enterochromaffin like cell (ECL)
What is the stimulation for histamine release?
Increased Gastrin/ ACh levels
What is the stimulation for ACh and gastrin release?
Increased vagal activity
What is involved in the cephalic phase for stimulating gastric acid production?
Sight/smell/taste of food
What is involved in the gastric phase for stimulating gastric acid production?
Distention of stomach
Peptides in lumen
What is involved in the cephalic phase for inhibiting gastric acid production?
Stopping eating
What is involved in the gastric phase for inhibiting gastric acid production?
Low pH due to HCl causes gastrin levels to lower
What is involved in the intestinal phase for inhibiting gastric acid production?
Acid in duodenum -> enterogastric reflex and secretin release
Fat/CHO in duodenum -> HIP release
The result is reduced activity of G cells and parietal cells
What are enterogastrones?
Hormones released from gland cells in duodenal mucosa
- Secretin
- Cholecystokinin (CKK)
- GIP
What is the stimulation of enterogastrones?
- Acid in duodenum
- Hypertonic solutions
- Fatty acids or monoglycerides in duodenum
All signs that the stomach is emptying its contents
What is the action of enterogastrones?
- Inhibit gastric acid secretion
- Reduce gastric emptying
What is intrinsic factor?
Glycoprotein released by stomach parietal cells to absorb vitamin B12
What is gastric motility?
The process by which food travels through the digestive tract via a series of muscular contractions called peristalsis
What produces gastric peristaltic waves?
Peristaltic rhythm (3/min) generated by pacemaker cells within longitudinal muscle layer
What is the impact of gastrin on motility?
Increased contraction
What is the impact of stomach wall distention on motility?
Reflexes cause increased contraction
What is the impact of Fat/acid/amino acid/hypertonicity in duodenum on motility?
Reduced contraction
What is the result of acid in the duodenum?
Vagal and ENS reflexes
Secretin release from S cells
The result is HCO3 secretion to neutralise the acid
What cells is HCO3 secreted from in the duodenum?
Brunner’s glands in the submucosa
What are long and short reflexes?
ENS- short
Vagal- long
What is the endocrine portion of the pancreas?
Pancreatic Islets
Produce insulin, glucagon and somatostatin
What is the exocrine portion of the pancreas?
Acini -> Lobules -> intralobular ducts -> Main pancreatic duct -> common bile duct -> hepatopancreatic ampulla
What do acinar cells secrete?
Digestive enzymes
What to duct cells secrete?
Bicarbonate
What is the action of enterokinase?
Converts trypsinogen to trypsin
What is the action of trypsin?
Converts all other zymogens to active forms
What is secretin released in response to?
Acid in duodenum
What is Cholecystokinin (CCK) released in response to?
Fat/amino acids in duodneum
What is the mechanism for glucose transport?
SGLT1 undergoes conformational change when sodium and glucose bind
Secondary active transport
Creates osmotic gradient
What is the mechanism for fructose transport?
Facilitated diffusion through GLUT-5 and GLUT-2
No sodium involvement
No water uptake associated
What is the mechanism for sodium transport?
SAAT transports sodium and amino acid across apical membrane
Sodium independent transporter in basolateral membrane
Creates osmotic gradient
What is PepT1?
Transports dipeptides and tripeptides via hydrogen ion dependant process
What are the fat soluble vitamins?
A, D, E, K
What are the water soluble vitamins?
B, C, folic acid
What doe sDMT1 transport?
Iron into the duodenal enterocytes
What do iron ions bind to?
Ferritin
What do iron in blood bind to?
Transferrin
What do iron in blood bind to?
Transferrin
