Flashcards in GI Secretory Functions, Digestion, Absorption Deck (53)
What stimulates gland secretion?
-food contact and local epithelial stimulation
-autonomic stimulation (parasympathetic)
-higher brain centers
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
-presence of alpha-amylase, lingual lipase, and kallikrein
What is the difference between low and high saliva flow rates?
-lowest Na, Cl, and CO3-
-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.
-second messenger is cAMP
What are some factors the increase saliva production?
-food in mouth (para)
What are some factors that decrease saliva production?
-inhibition of parasympathetic system:
What are the secretory cells in gastric glands?
-mucus neck cells
What are the different gland types?
-crypts of Lieberkuhn
-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?
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?
-CN X innervates parietal cells
-stimulates H+ secretion directly
-utilizes ACh and muscarinic receptor
-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?
-somatostatin binds to receptors on the parietal cell that are coupled to adenyl cyclase via a GI protein
-antagonistic to stimulatory action of histamine
-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
+secreted by the glandular cells
+necessary to prevent the action of trypsin on the pancreatic tissues themselves
What is the digestive enzyme for carbs?
What are the digestive enzymes for fat?
-pancreatic lipase, cholesterol esterase, phospholipase
What are some characteristics of pancreatic secretions?
-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