GI Secretory Functions, Digestion, Absorption Flashcards
What stimulates gland secretion?
- food contact and local epithelial stimulation
- autonomic stimulation (parasympathetic)
- higher brain centers
- hormonal stimulation
What are the functions of mucus?
- adheres to food and other particles
- spreads thin films over surfaces
- coats wall of gut, preventing actual contact with food
- causes fecal particles to adhere to one another
- resistance to digestion by GI enzymes
- has atmospheric properties making it useful for buffering small amounts of acids and bases
What are the functions of saliva?
- initial starch digestion and initial triglyceride digestion
- lubrication of food and protection of mouth and esophagus
What is the composition of saliva?
- high K and bicarbonate conc
- low Na and Cl
- hypotonicity
- presence of alpha-amylase, lingual lipase, and kallikrein
What is the difference between low and high saliva flow rates?
Low:
- lowest osmolarity
- lowest Na, Cl, and CO3-
- highest K
High:
-composition closest to plasma
What are the salivary gland secretions?
- parotid gland secretions are almost entirely serous
- submandibular and sublingual secretions are mixed
What occurs during the first stage of salivary secretion?
- occurs in acini
- secretion contains ptyalin (alpha-amylase)
- composition is isotonic with ionic conc similar to plasma
What happens during stage two of salivary secretion?
- occurs in salivary ducts
- active reabsoption of Na
- active secretion of K
- active/passive secretion of bicarbonate ions
- passive reabsoption of Cl due to -70mv in ducts
What controls salivary gland secretion?
- cranial nerves VII and IX
- production if controlled mostly by parasympathetic system, but also by sympathetic system, both can result in saliva production
Describe how parasympathetic stimulation of salivary gland secretion functions.
- muscarinic cholinergic receptors on both acinis and ducts
- second messenger is inositol 1,4,5-triphosphate (IP3) and increased Ca+2
Describe how sympathetic stimulation of salivary gland secretion functions.
- B-adrenergic receptors
- second messenger is cAMP
What are some factors the increase saliva production?
- food in mouth (para)
- smells
- conditioned reflexes
- nausea
What are some factors that decrease saliva production?
-inhibition of parasympathetic system: \+sleep \+dehydration \+fear \+anticholinergic drugs
What are the secretory cells in gastric glands?
- mucus neck cells
- chief/peptic cells
- parietal/oxyntic cells
What are the different gland types?
- unicellular mucous
- crypts of Lieberkuhn
- tubular glands
- complex glands -> salivary glands, pancreas, liver
What do chief cells do?
- secrete pepsinogen (pH range of activity: 1.8-3.5)
- secrete intrinsic factor
What stimulates the release of pepsinogen?
- ACh from vagus nerves or gastric enteric nervous plexus
- response to acid in stomach
What do parietal cells secrete?
-secrete HCl
What is the mechanism of secretion for HCl?
- dissociation of water inside the cell into H+ and OH-
- OH- + CO2 -> HCO3 +H+
-HCO3 is exchanged for Cl- ions
+HCO3 increases blood pH and will even tail be secreted by pancreas to neutralize H+ in duodenum
- Cl- ions are secreted through chloride channels into the canaliculi
- H+ are pumped out of the cell in exchange for K+
- K+ leaks outside the cell but is transported back in via H-K ATPase pump
- Na+ is reabsorbed into the cell due to Na-K basolateral pump
What are the two pathways that parasympathetic stimulation increases H+ secretion?
DIRECT:
- CN X innervates parietal cells
- stimulates H+ secretion directly
- utilizes ACh and muscarinic receptor
INDIRECT:
- CN X innervates G cells
- stimulates gastrin secretion
- gastrin stimulates secretion of H+
- neurotransmitter is GRP (gastrin releasing peptide)
How is gastrin secreted?
- secreted by gastrin G) cells in pyloric glands
- two forms: G-34 and G-17 (more abundant)
- released in response to presence of protein in pylorus
- causes enterochromaffin-like cells to release histamine -> stimulates H+ secretion
- second messenger on parietal cells is IP3/Ca+2
How is histamine stimulated?
- stimulates secretion of H+ by activating H2 receptors on parietal cell membrane
- H2 receptor is coupled to adenyl cyclase via Gs protein
- second messenger is cAMP
What are the two pathways that somatostatin inhibits gastric H2 secretions?
DIRECT:
- somatostatin binds to receptors on the parietal cell that are coupled to adenyl cyclase via a GI protein
- antagonistic to stimulatory action of histamine
INDIRECT:
-inhibits the release of histamine and gastrin
What do prostaglandins do?
-inhibits gastric H2 secretions by activating G1 protein, inhibiting adenyl cyclase
What are the three phases of gastric secretions?
- cephalic phase (via vagus, parasympathetic so excite pepsin and acid production)
- gastric phase (1. Local nervous secretory reflexes 2. Vagal reflexes 3> gastrin-histamine stimulation)
- intestinal phase (1. Nervous mechanisms 2. Hormonal mechanisms)
What are the digestive enzymes for proteins?
-trypsin, chymotrypsin, and carboxypolypeptidase
-trypsin inhibitor
+secreted by the glandular cells
+necessary to prevent the action of trypsin on the pancreatic tissues themselves
What is the digestive enzyme for carbs?
-pancreatic amylase
What are the digestive enzymes for fat?
-pancreatic lipase, cholesterol esterase, phospholipase
What are some characteristics of pancreatic secretions?
- high volume
- isotonic
- same Na and K ion concs as plasma
- higher bicarbonate conc than plasma
- Lowe Cl- conc than plasma
- low flow rates: isotonic fluid composed mostly of Na and Cl
- high flow rates: isotonic fluid composed mostly of Na and bicarbonate
How do the acinar cells help form pancreatic secretions?
-produce small volume of pancreatic secretion composed mainly of Na and Cl
How do ductal cells help in the formation of pancreatic secretions?
- secrete bicarbonate ion and reabsorb CL via a Cl-HCO3 exchange mechanism
- ducts are permeable to water, so water moves into ducts to make secretion isotonic
What does ACh do in the regulation of the pancreatic secretions?
- from parasympathetic nerves and enteric nervous system
- released in response to H+, small peptides, aas, and FA in duodenum
- stimulates enzyme secretion by acinar Cole sand potentiates the effect of secretin
What does CCK do for pancreatic secretions?
- release is stimulated by presence of food in upper intestine, especially small peptides, aas, and FAs
- secreted by duodenal and upper jejunal mucosal cells
- results in dramatic increase in secretion on pancreatic enzymes
- potentiates effect of secretin on ductal cells to stimulate bicarbonate secretion
- second messengers are IP3 and increased intracellular Ca
How does secretin regulate pancreatic secretions?
- release is stimulated by presence of acidic foods in upper intestine
- secreted by duodenal and upper jejunal S mucosal cells
- stimulates release of large amounts of HNaCO3 by ductal cells
- second messenger is cAMP
What factors stimulate the release of bile from the gallbladder?
-presence of fatty food in duodenum
In the gallbladder, ow is the bile concentrated?
-by active transport of Na followed by secondary absorption of Cl-, water, and other diffusible constituents
What are the functions of bile salts?
- emulsification
- complex with lipids to form micelles for absorption across intestinal mucosa
How are bile acids formed?
- colic acid and chenodeoxycholic acid are synthesized from cholesterol by hepatocytes
- bacteria in the intestine convert the primary bile acids into secondary bile salts (deoxycholic and lithocholic acid)
How is bile formed?
- bile acids are conjugated with Gly and taurine
- electrolytes and water are added to bile
- bile is concentrated in the gallbladder as a result of isomotic absorption of solutes and water
What are the different cell types in the crypts of Lieberkuhn?
- goblet cells
- enterocytes (absorptive cells)
- paneth cells (secrete antimicrobial proteins to limit bacteria-enterocyte contact)
- enteroendocrine cells (secrete peptide hormones controlling several GI system functions)
What is the difference between hydrolysis and condensation reactions?
- condensation reactions are used to remove H+ and OH- ions from building blocks in order to allow the binding monomers into polymers
- hydrolysis is the reverse, incorporating water molecules in such a way that polymers are broken down into monomers
Describe the absorption of carbs.
- only monosaccharides are absorbed
- absorption of glucose and galactose in SI
- absorption of fructose is entirely by facilitated diffusion in SI
- absorption of lactose requires lactase in brush border
What are the causes of gall stones?
- Too much absorption of water from bile
- Too much absorption of bile acids from bile
- Too much cholesterol in bile
- Inflammation of epithelium
Describe the absorption of glucose and galactose.
-active transport of Na followed by secondary active transport involving a sodium co-transport mechanism (SGLT 1) in luminal membrane
Describe the absorption of fructose.
-cannot be absorbed against a concentration gradient
Describe the absorption of lactose.
- lactase breaks lactose down into glucose and galactose
- absence of lactase results in lactose intolerance, causing lactose to remain in lumen and cause osmotic diarrhea
What are proteins absorbed as?
-amino acids or peptides
How are aas and peptides absorbed into the SI?
- protein digestive products are absorbed as aas, dipeptides, and tripeptides
- sodium-aa cotransporter mechanism in luminal membrane
- facilitated transport from enterocyte into blood
- four separate transports: neutral, acidic, basic, imino aa
- hydrogen ion-dipeptide/tripeptide so-transporters on luminal membrane, cytoplasmic peptidases then convert them to aas
What does CCK do in terms of lipid digestion?
-bile acids emulsify lipids in SI
-pancreatic lipases hydrolyze lipids to FAs, monoglycerides, cholesterol, and lysolecithin
+pancreatic lipase, cholesterol ester hydrolase, phospholipase A2
- these products of enzymatic digestion are hydrophobic and are solubilized in micelles by bile acids
- micelles bring the products of digestion into contact with cell membrane of enterocyte and contents diffuse into cells
- intracellularly, products of lipid digestion are converted into triglycerides, cholesterol ester, and phospholipids
- these products are combined with apoproteins to form chylomicrons
- chylomicrons are exocytosis and transferred to lymph vessels
How is NaCl absorbed?
- passive diffusion of Na through Na channels
- Na-glucose or Na-aa cotransporters
- Na-H ion exchange
- passive absorption of Cl
- Na-Cl cotransport
- Cl-HCO3 ion exchange
Absorption and secretion of K
- K can be absorbed passively via paracellular route
- K secretion in colon is stimulated by aldosterone
Absorption of water
- secondary to solute absorption
- isomotic in SI and gallbladder
- permeability to water is much lowere in colon than in SI -> feces may be hypertonic
Secretion of electrolytes
-secretory mechanisms are located in the crypts (absorption is in the villi)
