Secretion in the GI Tract Flashcards
what are the two pathways that things can travel from the lumen into the blood and lymph?
- the lumen of the intestine is continuous with the outside world
- the intestinal epithelium is a barrier
Two pathways:
Transcellular:
- across the cell membranes
- through the cytoplasm
Paracellular:
- between cells (via the tight junctions)
what are the specific names for the direction of movement for water and solutions across an epithelial barrier?
Secretion (exocrine)
- movement of solutes and water from the body to the GIT lumen
- can occur via that 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
what is secreted in the GIT?
endocrine vs. exocrine secretion:
- endocrine: move into the blood (eg. insulin, ADH)
- exocrine: produced by epithelia and move into the lumen
Components and functions of exocrine secretions in the GIT:
Mucus:
- protection and lubrication
- aids mechanical digestion
Electrolyte solutions:
- dilutes food and provides optimal pH
- essential for function of digestive enzymes
Digestive enzymes:
- essential for chemical digestion of food
- aids absorption
things we need to know about that table of electrolytes and fluid in the GIT
- secreting a lot of fluid so it is important that we reabsorb a lot of it
- saliva, very dilute
- stomach is very acidic (whereas everywhere else is just a bit alkaline)
- the stomach values are for gastric juice, which will end up in the stomach lumen but it doesn’t have any bicarbonate in it (??)
describe the volume, composition and functions of salivary secretion
- 3 pairs of salivary glands produce and secrete 1.5L fluid per day (resting is less than stimulated secretion)
Composition:
Mucous: lubrication
Dilute solution of NaHCO3/NaCl:
- dilution of food for tasting, swallowing, talking
- optimal pH for digestive enzymes
- hygiene, irrigation of mouth
Digestive enzymes:
- lingual lipase: chemical digestion of fats
- salivary amylase: chemical digestion of starch
describe the regulation of salivary secretion
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
describe the volume and composition of gastric secretions between meals and when eating
stomach secretes 2-3L of fluid per day
Between meals:
Slow rate of secretion
Goblet cells on the surface: mucus and bicarbonate:
- mucus: protection from abrasion
- with bicarbonate: protection from stomach acid
When eating:
Extra secretion added to basal/resting rate
Goblet cells on the surface: mucus and bicarbonate (HCO3-)
Parietal cells:
- HCl acid: denatures (unfolds proteins) and converts pepsinogen to pepsin (active form of enzyme) and creates optimum pH for pepsin to work, as well as protection from microbes
- intrinsic factor: absorbtion of vitamin B12 in the ilium (small intestine)
Chief cells:
- pepsinogen: proteolytic enzyme starts chemical digestion of proteins
describe the process of secretion of acid by parietal cells
- source of hydrogen ions
- carbonic anhydrase
- production of H+ and HCO3- from CO2 and H2O - secretion of acid (H+)
- H+/K+ ATPase in the apical membrane of parietal cells: pumps H+ ions into lumen in exchange of potassium ions (K+)
- K+ returns to lumen through channel - Source of chloride
- anion counter transporter in the basolateral membrane of parietal cells: imports Cl- ions into cell in exchange for ejecting HCO3- into interstitial fluid - secretion of chloride (Cl-)
- Cl- diffuses across the cell
- diffuses into the lumen bia Cl- channel in apical membrane
what are the three stages that regulate gastric secretion?
- Preparation: cephalic
- eating or being about to eat
- incoming food
- CNS controls gastric secretion
- 20% of gastric secretion - Race: gastric
- food arrives at the stomach
- stomach controls gastric secretion
- 70% of gastric secretion - Handover: intestinal
- chyme (food) is passed from the stomach to the small intestine
- duodenum (small intestine) controls gastric secretion
- 10% of gastric secretion
describe the preparation: cephalic phase of gastric secretion
function: prepare the stomach for the arrival of food
detected by: the CNS
response coordinated by: parasympathetic nervous system
- stimulates ENS which stimulates the submucosal plexus for secretion
effectors:
- goblet cells
- chief cells
- parietal cells
- G cells: secrete the hormone gastrin into the blood, stimulating: chief cells and parietal cells (so double activation of those two cell types)
outcome: increased secretion of mucous, HCO3-, HCl acid and pepsinogen in the stomach
describe the race: food arrives phase of gastric secretion
function: maximise mechanical digestion and begin protein chemical digestion
stimulus: stretch, increased pH, undigested good (esp protein)
detected by: G cells (peptides), gastric mechano- and chemoreceptors
response coordinated by: submucosal and myenteric plexus’
effectors:
- mucous cells, chief cells, parietal cells, G cells
- G cells secrete gastrin into the blood stimulating: chief cells, parietal cells and gastric motility
outcome: increased secretion of mucous, HCO3-, HCl acid and pepsinogen in the stomach as well as increased motility (retropulsion and waves of peristalsis)
This is an example of a short reflex pathway
describe the handover: intestinal phase of gastric secretion
function: slow controlled release of food to small intestine
stimulus: stretch of the duodenal wall, decreased pH (acid), fatty acids and amino acids in the duodenum
detected by: duodenal enteroendocrine cells (EECs), mechano- and chemoreceptors
response coordinated by:
- duodenal hormones: cholescytokinin (CCK), gastric inhibitory peptide (GIP) and secretin
- sympathetic nervous system (SNS) which inhibits ENS and therefore inhibits myenteric and submucosal plexus’
inhibition of:
- chief cells, parietal cells, G cells
- myenteric plexus and submucosal plexus
outcome: decreased secretion of HCl acid and pepsinogen as well as decreased motility (retropulsion)
long reflex pathway
describe the volume and composition of pancreatic secretions
endocrine and exocrine organ
- secretes 1-1.5L of fluid per day
- exocrine secretions are made up of TWO components made by two different types of cells
- panreactic acinar cells secrete: pancreatic digestive enzymes
- pancreatic duct cells secrete: alkaline (HCO3-) fluid
describe pancreatic digestive enzyme secretion and regulation
the pancreas is the most important source of digestive enzymes
The arrival of fatty acids and amino acids in the duodenum activates receptors on duodenal enteroendocrine cells which release cholecystokinin (CCK):
- CCK stimulates release of pancreatic juice with digestive enzymes from pancreatic acinar cells
Digestive enzymes are classified based on what type of molecule the chemically digest:
- nucleolytic (break down DNA and RNA); ribonuclease, deoxyribonuclease
- lipolytic (break down fat): lipase (colipase: cofactor for lipase)
- amylitic (break down carbohydrates): pancreatic amylase
- proteolytic (break down peptides): trypsinogen, chymotrypsinogen, procarboxypeptidase
describe the activation of pancreatic proteolytic enzymes
pancreatic proteolytic enzymes are secreted as inactive precursors and activated in duodenum, so that they don’t digest proteins inside pancreatic acinar cells
- trypsinogen -> trypsin
- chymotrypsinogen -> chymotrypsin
- procarboxypeptidase -> carboxypeptidase
Activation in the duodenum:
- enterokinase (=enteropeptidase) which is bound to duodenal membrane (converts trypsinogen to trypsin)
- once activated trypsin converts the other enzymes to their active form
