Secretion Flashcards
Oral mucosa and esophagus epithelium
Stratified squamous non-keratinized epithelium
(Mouth: also salivary glands // esophagus: glands, muscularis mucosae)
Stomach, intestine and colon epithelium
Columnar epithelium
(Small intestine: microvilli, villi -> crypts // colon: taeniae coli, haustras)
Transition site between simple squamous and columnar epithelium in GI tract
Cardias
Types of secretory glands
Unicelular (goblet cells)
Multicellular (exocrine or endocrine)
Unicelular glands
Part of a columnar epith
Mucous cells of stomach, globet cells from small and large intestine
Exocrine glands - location, secretes into, types
Inside submucosa / distant epith-related glands (pancreas, parotid)
Secrete the content into GI epithelium
Gastric glands (principal and parietal cells)
Duodenal glands of Brunner (mucinous cells)
Intestinal crypts of Lieberkühn (Paneth cells)
Salival glands (acinar cells)
Exocrine pancreas
Endocrine glands - location, secretes into, types
Outside epithelium
Secrete their content to the blood flow
Endocrine (alfa cells, beta cells)
Thyroid (follicular cells)
Parathyroid (principal, oxyphillic cells)
Adrenal, hypophysis
Ovaries, testicles
Process of secretion
Nutrients enter from capillary vessels into epithelial cells = glands through their basal membrane
Proteins are synthesized, folded and modified
Terminal vesicles of Golgi will form granules
When a hormone or nervous system signal arrives, the vesicles will be released through the apical side of the cells
Salivary secretions
Serosa
- by parotid and submaxillary
- contains Ptialine (amylase) and lingual lipase
Mucinous
- by sublingual and submaxillary
- contains mucous (protection and lubrication)
Salivary content
K and Bicarbonate
Na and Cl
Thiocyanate ions
Lysozyme
IgA
Salival secretions
1ª secretion
2ª secretion (modification of 1ª = rich in K + bicarbonate)
Stimuli for salivary secretion
Tongue: touch (general), taste (specific)
Info —> IX & VII CN branches —> sensory nuclei in NTS —>
- Cortex (to a region related to taste)
- Amygdala
Signal reaches hypothalamus - connects with sup and inf salivary nuclei (gastric reflex also activates them)
Efferent pathway from salivary nuclei
VII CN —> submandibular ganglion —> submandibular + sublingual glands
IX CN —> otic ganglion —> Parotid gland
Gastric / oxyntic glands secretory cells
Mucous or Principal cells → Mucus (bicarbonate)
Parietal cells → HCl = gastric acid (pH = 1-3 —> activates digestive enzymes) + Intrinsic factor (for VitB12)
Chief cells → Pepsinogen (inact) into Pepsin (act) = prot breakdown
Pyloric / Tubular glands endocrine cells
G cells: gastrin
D cells: somatostatin
Enterochromaffin-like cells (ECL): histamine
Stimulation / inhibition gastric secretions
G cells (gastrin) stimulates parietal cells (HCl)
D cells inhibit G cells (gastrin) = inhibition of parietal cells (HCl)
Mechanism of HCl secretion by parietal cells
Protons —> H+/K+ ATPase
- Needs retrodiffusion of K to lumen by passive gradient
- Needs act transport of K from interstitium (Na+/K+ pump)
Carbonic Anhydrase producing HCO3- is pumped to interstitium by cotransporter Cl-/HCO3-
Cl- will pass to the lumen through passive gradient
Cl- and H+ will combine in the lumen = HCl
Water will diffuse by osmotic gradient to the lumen
Stimuli for acid secretion
Acetylcholine —> PS nervous system (vagus nerve) which innervates parietal cells
Gastrin —> reaches parietal cells via circulation. Released upon distension, Aa in stomach, vagus stimulation,… Indirectly stimulates H+ secretion by ↑ histamine
Histamine —> reaches nearby parietal cells through paracrine diffusion
Phases of gastric secretion
Cephalic phase
- 30%
- stimuli: taste, smell, chewing, swallowing, conditional reflexes
- direct vagal stimulation or indirect
Gastric phase
- 60%
- stimuli: gastric distension, presence of peptides,…
- direct / indirect (gastrin) vagal stimulation, Aa induced Gastrin release,…
Intestinal phase
- 10%
- mediated by prot digestion products
PS-NS stimulus
Distension
PS-NS action in stomach
Increases HCL (parietal)
Increases pepsinogen (chief)
Promotes emptying
Promotes relaxation of pylorus (VIP)
PS-NS origin
Dorsal Nucleus
Vagus - Myenteric plexus
Gastrin stimulus
Distension
Proteins
Gastrin actions in the stomach
Increases HCL (parietal)
Increases pepsinogen (chief)
Increases histamin (ECL)
Promotes emptying
Promotes relaxation of pylorus (VIP)
Histamin stimulus
Gastrin
Histamin actions in the stomach
Increases pepsinogen secretion (parietal)
Secretin stimulus
Acidity, fat
Secretin actions in the stomach
Increases pepsinogen secretion (Chief)
Increases HCO3 (Brunner glands)
Somatostatin stimulus
Acidity
Somatostain actions in the stomach
Decreases HCl secretion (Parietal)
Decreases emptying
Decreases G cells, pancreatic secret, biliary secret, blood flow
Cholecystokinin (CCK) stimulus
Fat, proteins
Cholecystokinin (CKK) actions in the stomach
Increases HCl secretion (Parietal)
Decreases emptying
Increases gallbladder contraction
Increases pancreatic secretion
Secretion in duodenum is done through
Brunner glands
Brunner glands function
Secretion of alkaline mucous, in order to protect the epithelial lining
Stimuli for secretion (Brunner glands)
Touch, distension or irritants
PS nervous system stimulation (through X CN, vagus nerve)
Secretin
Inhibition of secretion (Brunner glands)
Sympathetic nervous system
Secretion along the small intestine occurs through cells located in
Lieberkühn intestinal crypts
Cell types in Lieberkühn intestinal crypts
Enterocytes
Goblet cells
Paneth cells
Stem cells
Endocrine cells: S, I, D
Enterocytes functions
Secrete water and electrolytes (Cl), enzymes for digestion
Absorb the digested products
Paneth cells function
Antimicrobial defense (defensins, lysozyme & PPL-A2)
Endocrine cells in Lieberkühn intestinal crypts
S cells → release secretin
I cells → release cholecystokinin
D cells —> release somatostatin
Mechanism of water secretion
Cl- and HCO3- actively transported
Na+ diffuses passively into lumen
Water gets dragged along with the diffusing Na+
Enzymes secreted by enterocytes
Peptidases
Sacrase, maltase, isomaltase, lactase
Intestinal lipase
Large intestine differences with small intestine
Lieberkühn crypts but NO intestinal villi
More mucus (Goblet) cells
Enterocytes don’t produce digestive enzymes and secrete little fluid
Large intestine main function
Absorption of: water, electrolytes and vitamins (B complex)
(We also find secretory processes —> to secrete Cl ions)
Absorption in large intestine is stimulates by
PS system
The function of mucus in large intestine
Protection
Feces cohesion
Exocrine pancreas cells
Ductal cells
Acinar cells
Ductal cells
Coat inter + intralobular ducts, and main pancreatic duct
Acinar cells
Form glands
Secrete their contents into the pancreatic ducts.
Stimulating pancreas hormones
Secretin —> activarte ductal cells to produce HCO3-
CCK —> activate acinar cells to produce digestive enzymes
Acinar cells main function
Digestion
Ductal cells main function
Hydration and alkalinization (protects against ulceras + sets optimal pH for pancreatic enzymes to be active)
Intracellular HCO3- (ductal secretions) comes from
Circulation (Na+/HCO3- cotransp in basolat membrane)
Intracellular production: CO2 + H2O by Carbonic Anhydrase = HCO3- & H+:
- H+ —> interstitium (Na+/H+ exchanger in basolat membr)
- HCO3- —> lumen (Cl- / HCO3- exchanger: Cl from acinar secretions & ductal cells, which secrete Cl- using CFTR)
Intracellular HCO3- (ductal secretions) function
Reacts w/ HCl to neutralize pH
HCl + NaHCO3 → NaCl + H2CO3
- H2CO3 —> H2O + CO2
- CO2 —> circulation —> respiration = stomach acid is neutralized
Stimuli for HCO3- secretion
Secretin
Ach (PS)
Pancreas
Organ which secretes most enzymes per gram of tissue
Acinus glands produce and secrete
Enzymes (proteolytic)
(accumulate on apical side and are released by exocytosis)
Acinar secretions regulators
Pro-colipase
Monitor peptide
Trypsin inhibitor
Acinar secretions stimuli
CKK (fatty acids + Aa, nervous system stimulation: CKK-releasing peptide and monitor peptide
Proteolytic enzymes
1º) Acini (trypsinogen + trypsininhibitors)
2º) Small intestine (tripsinogen —> trypsin by enterokinase)
3º) Dudenum (rest of zymogens —> active form by trypsin)
Gallbladder function
Stores bile produced by the liver and release it to the duodenum through Ampulla of Vater
Bile composition
Water
Bile salts (main one)
Phospholipids
Cholesterol
Bile pigments (bilirubin)
Na+, HCO3- and other electrolytes
Formation bile salts
Liver: colesterol —> 2 1º bile acids
Intestine: 2 1º bile acids deoxygenated = 2º bile acids
Aa, Na, K conjugated to 1º,2º bile acids = bile salts (8)
How do bile salts emulsify / break down large fat globules into small emulsified droplets?
Fragmentat/Emulsificat = larger surface area for enzymes (mainly pancreatic lipase) —> fatty acids + monoglyc.
Solubilization and Absorption: keeping them in solution + transporting them to enterocytes —> absorbe
Bile salts characteristic
Amphipatic —> assemble into micelles = emulsify / break down large fat globules into small emulsified droplets
Functions of bile
Helps digestion
Facilitates fat emulsion and solubilization —> absorption
Alkalinization of the duodenal lumen
Metabolic byproducts + excretion (bilirrubin, excess cholesterol)
Formation canalicular secretion
Composition = same as bile —> exit into canaliculi through:
- BSEP (bile salts transporter. MOST important)
- MDR3 (phospholipid transporter)
- MDR2 (xenobiotics)
- MRP2 (act secretes glutathione and conjugated bilirubin)
(Act transporters + passive diffusion of water + other substances)
Formation ductular secretion
Composition = modified (secretion of water and HCO3).
Ductal cells hydrate and alkalize the bile.
Secretion bile into ducts
Bile descends through common hepatic duct —> cystic duct —> gallbladder (not an essential structure)
Stimuli for bile secretion
CCK = gallbladder contraction, sphincter of Oddi relaxation
Ach
PS system
Phases biliary secretion
Cephalic phase
Intestinal phase
Cephalic phase (biliary secretion) - stimuli, response, result
Stimuli: food smell, touch, taste, sight
Response: Oddi sphincter relaxation (X CN)
Result: anticipation bile to chyme (duodenum)
Intestinal phase (biliary secretion) - stimuli, response, result
Stimuli: intestinal cells detect Aa + proteins
Response: I cells release CKK (duodenum)
Result: cholagoge act (gallbladder contracts + Oddi sphincter relaxation)
Stimuli: intestinal cells detect fat
Response: S cells release secretin (duodenum)
Result: choleretic act (hepatocytes increase bile, ductal cells increase HCO3-, H2O)
Enterohepatic circulation function
Bile acids and bilirrubin recycle
Enterohepatic circulation of bile salts - %, pathway
95% recycled, 5% through feces
Full field function —> portal circulation —> Na+/bile cotransporters in basolat side of enterocytes
Enterohepatic circulation of bilirrubin
RBC pahagocitosed —> heme group into indirect bilirrubin —> blood (bound to albumin)
Liver: indirect bilirubin conjugated w/ glucuronic acid = direct bilirubin —> bile
Intestine: bacteria hydrolyzes and reduce bile (w/conjugated bilirubin) = urobilinogen —>
- flow through GI tract: bacteria —> stercobilin (feces colour)
- reabsorbed into enterohepatic circulation: kidneys (urobilin —> urine colour) / liver (recycled, sent back to intestine)
