Flashcards in Physiology Deck (77)
Layers of the GI wall?
innermost layer, layer of epithelial cells specialized for absorption and secretion, highly vascularized
- submucosa: consists of collagen, elastin, glands, and blood vessels
- circular and longitudinal smooth muscle: provides motility for GI tract
- Serosa: faces the blood, not found in the esophagus
Fxn of the mouth? Enzymes?
- mostly mechanical digestion: mastication - food is broken down into small particles so food particles can be chemically digested - form a bolus
- enzymes: lingual amylase and lingual lipase
What is involved in swallowing?
- 26 muscles and 5 cranial nerves (5, 7, 9, 10, 12)
-esophageal: begins with cricopharyngeal relaxation, involuntary
- permanent spasm ( cramping of muscle, UES will remain closed)
Composition of saliva?
- secreted by serous nad mucous cells
- 97-99.5% water, slightly acidic
- salivary amylase and lingual lipase
- metabolic wastes: urea, uric acid
- lysozyme, IgA and cyanide compound protect against microorganisms
Fxns of saliva?
- salivary glands produce 1 L/ day of saliva
- each gland delivers saliva to mouth through a duct
- initial digestion of starches and lipids by enzymes
- dilution and buffering of ingested foods
- lubrication of ingested food to aid its movement
- many more fxns
Secretion impt in GI?Produced by?
- addition of fluids, enzymes, lytes, and mucus to lumen of GI tract
- secretions produced by:
gastric mucosal cells (gastric secretion)
pancreatic exocrine cells (pancreatic secretion)
Fxn of stomach, small and large intestine?
- stomach: digestion and break down of food to smaller, absorb-able particles
- small intestine: absorption of nutrients
- large intestine: absorption of water
parts of the stomach? Specialized for?
- specialized for accumulation of food: capable of expanding, can hold 2-3 L
- gastric juice converts food into semiliquid (Chyme)
- 4 parts: cardia, fundus, body and pylorus
Fxns of stomach?
- short term reservoir
- absorption, digestion and secretion
- chemical and enzymatic digestion is initiated, particularly of proteins
- liquefication of food - into chyme
- slowly released into small intestine for further processing
- stomach uses pepsin and peptidase (enzymes) to break down proteins
- acid provides good enviro for enzymes to work in
3 phases of digestion?
- cephalic phase: cortex, amygdala, and hypothalamus (through vagus nerve)
- gastric phase: hydrochloric acid and pepsin
- intestinal phase: enterogastrone hormones secreted in duodenum and lower GI tract
Gastric secretion of enzymes?
- gastric mucosal cells secrete gastric juice: HCL and pepsinogen intiate protein digestion, intrinsic factor reqd for absorption of Vit B12. Mucus protects gastric mucosa from HCL
Cell types of gastric mucosa (secretion)?
- body of stomach contains oxyntic glands:
parietal cells: HCl and intrinsic fator
chief cells: pepsinogen
- antrum of stomach contains pyloric glands:
G cells: Gastrin into circulation,
mucous neck cells: mucus, HCO3, and pepsinogen
Secretion and fxn of gastrin?
- secreted by G cells in stomach in response to eating: stimuli include proteins, dissension of stomach, and vagal stimulation
- gastrin releasing pepride (GRP): released from vagal nerve endings onto G cells
- promotes H+ secretion by gastric parietal cells
- stimulates growth of gastric mucosa
- other fxns:
1: pepsinogen release
2: increase stomach motility
3: relax pylorix sphincter
4: contract LES
Fxn of ACh?
- released from vagus nerve
- binds to receptors on parietal cells
- produces H+ secretion by parietal cells
- atropine blocks muscarinic receptors on parietal cells
Fxn of histamine?
- released from mastlike cells in gastric mucosa
- binds to H2 receptors on parietal cells
- produces H+ secretion by parietal cells
- cimetidine blocks H2 receptors
Peptic ulcer pathogenesis?
- H pylori
- NSAIDs/ ASA
- cigarettes, ETOH
- decreased mucous secretion
- delayed gastric emptying
- decreased prostaglandin synthesis
What are segementation contractions?
- circular muscle contracts sending chyme in both directions
- intestine then relaxes allowing chyme to merge back together
What are peristaltic contractions?
- longitudinal muscle contracts propelling chyme along small intestine
- simultaneously, portion of intestine caudad to bolus relaxes
UGI bleeding most common causes?
- duodenal ulcer
- gastric erosion, itis
- M-W tear
Innervation of the GI tract?
- autonomic nervous system has extrinsic and intrinsic component
- extrinsic: sympathetic and parasympathetic innervation
- intrinsic: enteric nervous system, contained within wall of GI tract, communicates with extrinsic component
Where does parasympathetic nerve supply come from?
- nucleus ambiguus and dorsal motor nucleus of vagus nerve
- provides motor innervation to the esophageal muscular coat and secretomotor innervation of glands
Where does sympathetic nerve supply come from?
- cervical and thoracic sympathetic chain
- regulates blood vessel constriction, esophageal sphincters contractions, relaxation of muscular wall and increases in glandular and peristaltic activity
Fxn of intrinsic innervation?
can direct all fxns of GI in absence of extrinsic innervation
- controls contractile, secretory, and endocrine fxns of GI tract
- receives input from:
1. parasympathetic and sympathetic nervous systems
2. mechanoreceptors and chemoreceptors in mucosa
- sends info directly to smooth muscle, secretory, and endocrine cells
4 fxnly diff cell types in stomach glands? What are gastric pits?
- gastic mucosa has numerous openings called gastric pits
- gastric glands empty into bottom of pits
- 4 diff celly types compose glands -
Steps of cephalic phase?
1. sight and thought of food activate cerebral cortex
2. stimulation of taste and smell receptors
- activate hypothalmus and medulla oblongata - send impulse down vagus nerve
- 1. stomach distension activates stretch receptors = vagovagal reflexes - medulla an dthen vagus nerve and local reflexes
- 2. food chemicals (esp peptides and caffeine) and rising pH activate chemoreceptors - activate G cells to release gastrin into blood
- 1. presence of low pH, partially digested foods, fats, or hypertonic soln in duodenum when stomach begins to empty - leads to intestinal gastrin release to blood
(gastrin stimulates gastric emptying)
Small intestine is primary site for?
- digestion and absorption of nutrients
- bile duct and pancreatic duct empty into duodenum
Blood flow into liver?
- largest internal organ
- receives major blood supply from hepatic portal vein: brings venous blood rich in nutrients from digestive tract
- high blood flow: 1350 ml/min to liver sinusoids (1050 from portal vein and 300 from hepatic artery) = fxnl and nutritive blood circulation
- physiologically: low vascular resistance (small difference b/t pressures in portal vein and hepatic vein) - in cirrhosis: vascular resistance increases, and blood flow decreases (leads to portal HTN and ascites)
Sinusoids fxn like?
- blood capillaries
- hepatocytes are in contact with blood in sinusoids within the liver
- arranged to form fxn units called lobules
1. carb metabolism
2. lipid metabolism
3. protein metabolism
4. removal of drugs and hormones
5. excretion/secretion of bilirubin
6. synthesis of bile salts
7. storage of some compounds
9. aids in synthesis of active Vitamin D
10. urea formation from ammonium
Why is hyperglycemia common in pts with cirrhosis after eating a carb rich meal?
- combo of pathological glucose tolerance test, hyperinsulinemia and increase insulin tolerance (liver insufficiency - leads to decrease of glucose utilization - leads to hyperglycemia - leads to hyperinsulinemia - leads to downregulation fo insulin receptors and insulin resistance)
Why does hypoglycemia occur in alcohol abusers?
- alcohol suppresses citrate cycle and thereby impairs gluconeogenesis from amino acids
Process of carb breakdown?
- carbs - broken down into disaccharides and then into monosaccharides
Process of protein breakdown?
- proteins - broken down into peptides and then into AA's
Process of lipid breakdown?
- lipids get broken down into diglycerides and then into monoglycerides and fatty acids
How are carbs absorbed?
- monosaccharides (mostly glucose) are absorbed
- monomers are carrid by transporter molecules across the epithelial cells and into blood capillary present in villus - into portal vein and into liver
- oxidation of fatty acids to supply energy for other body fxn
- synthesis of large quantities of cholesterol (80% of cholesterol synthesized in liver is converted into bile salts), phospholipids, and most lipoproteins
- inactivation of steroids and their excretion from the body
What can hyperammonemia lead to and what are symptoms?
- can lead to hepatic encephalopathy = toxic effect of ammonia in the brain
- mental changes (disorientation, sleeping disorders, chaotic speech, personality changes)
- motor changes (increased in muscle reactivity, hyperreflexia, tremor)
Bile secretion is necessary for?
- digestion and absorption of liquids in small intestine
- mix of bile salts, bile pigments, and cholesterol
- bile salts emulsify lipids to prepare them for digestion
steps of bile secretion and recycling?
1. produced and secreted by liver
2. stored in gallbladder
3. ejected into small intestine when gallbladder contracts
4. after lipids absorbed, bile salts are recirculated to liver via enterohepatic circulation:
absorption of bile salts from ileum into portal circulation, and delivery back to liver
5. extraction of bile salts from portal blood by hepatocytes
How is bilirubin formed and how is it conjugated?
- from hemoglobin
- Hb is phagocytosed by tissue macrophages int he spleen
- iron released in bound to transferrin and transported in the blood
- the remainder of heme group is converted to biliverdin, which is rapidly reduced to bilirubin
- bilirubin is attached to albumin
- albumin is removed as unconjugated bili passes through the hepatocyte membrane
- bili is conjugated with glucoronic acid, sulphate
- conjugated bili has higher solubility
- hepatocytes actively transport conjugated bili into bile canaliculi
- conjugate bili then enters duodenum through sphinter of oddi
Role of small intestine in bile metabolism?
What happens to reabsorbed urobilirubin?
- bile and bile salts increase growth of intestinal bacteria
- in return, intestinal bacteria metabolize conjugated bile into urobilirubin
- this is highly soluble an dis reabsorbed back into blood
- 95% of reabsorbed urobilirubin is excreted again by liver into bile
- 5% is excreted into urine
- small part excreted into feces
When does the sphincter of oddi open?
- within 30 min of food intake, presence of aminoacids and fatty acids in duodenum activates cholecystokinin which causes gallbladder contractions and excretion of bile
- 50-1000 ml of bile is produced daily
- bile is concentrated in gallbladder
Detectable jaundice - at what total plasma bili?
- detectable when above 2 mg/dL (will see in sclera)
-excess production of bili (hemolytic anemia)
-decreased uptake of bili into hepatic cells
- disturbed intracellular protein binding or conjugation
- disturbed secretion of conjugated bili into bile canaliculi
- intrahepatic or extrahepatic bile duct obstruction
What causes obstructive jaundice?
- bile is prevented from flowing out of biliary duct
- occurs if gall stones or tumors block the duct
- conjugated bili builds up in biliary duct, pressure within biliary duct increases
- conjugated bili is returned to blood, either by rupture of bile canaliculi or via lymphatic drainage of liver
- conj. bili in blood is good dx test for obstructive jaundice
- unconjugated levels with be either normal or decreased
Fxn of gallbladder?
- stores bile (50 ml)
Other impt fxns of liver?
- formation and secretion of bile
- detoxificaiton of various substances: metabolic products of intestinal microbes, exogenous toxins (alcohol, meds, poisons), hormones
- synthesis of plasma proteins: albumin, clotting factors
coag: synth. of coag factors
- blood reservoir: filtration, storage of blood
- immunity: kupffer cells = macrophages
- vitamins: metabolism and storage of vitamins A, D, B12
- relation to blood formation: storage of vitamin B12, metabolism of Fe and its storage as ferritin, production of erythropoiein
Cirrhosis leads to what?
- leads to scar tissue which can obstruct blood and bile flow
- obstruction of hepatic venous blood flow can increase pressures within other veins leading to other circulatory diseases such as varices and ascites
How is relfux of pancreatic duct prevented?
- ductal pressure is 15-30 mm Hg vs 7-17 in CBD thus preventing reflux an damage
Main pancreatic duct?
- duct of wirsung, runs the entire length of the pancreas
- joins CBD at ampulla of Vater
Innervation of pancreas?
- sympathetic fibers from splanchnic nerves
- parasympathetic fibers from vagus
- parasympathetic fibers stim both exocrine and endocrine secretion
- sympathetic fibers have inhibitory effect
- rich afferent sensory fiber network
- ganglionectomy or celiac ganglion blockade interrupt somatic fibers (pancreatic pain)
Fxn of acinar cells of pancreas?
- Exocrine: secretes essential digestive enzymes through pancreatic duct into duodenum
Endocrine cells of pancreas?
- islets of langerhans
- 4 major cell types:
alpha cells: glucagon
beta cells: insulin
delta cells: somatostatin
F cells: pancreatic polypeptide (secretes insulin and glucagon into blood stream)
Physiology of exocrine pancreas?
- 500-800 ml pancreatic fluid secreted/day
- alkaline pH results from secreted bicarb which neutralizes gastric acid and regulate pH of the intestine
- enzymes digest carbs, proteins, and fats
What hormones stimulate and inhibit bicarb secretion?
- major stimulants: secretin, cholecystokinin, gastrin, acetylcholine
- major inhibitors: atropine, somatostatin, pancreatic polypeptide and glucagon
- secretin: released from duodenal mucoas in response to duodenal luminal pH of less than 3
Only digestive enzyme secreted by pancreas in its active form?
- fxns optimally at pH of 7
- hydrolyzes starch and glycogen to glucose, maltose, maltotriose, and dextrines
Lipase fxns optimally at what pH? fxn?
- at pH of 7-9
- emulsify and hydrolyze fat in presence of bile salts
Most impt stimulant of acinar cells?
- secretion of CCK in presence of amino acids and fatty acids in intestinal lumen
Major stimulant of ductal cells?
- aqueous secretion of HCO3
- secreted in response to H+ in intestine
Most impt adipose secretions?
How does obesity lead to hyperglycemia?
- increased adiposity leads to decreased adiponection which leads to:
- IRS-1 and decreased glucose uptake
- decrease in fatty acid oxidation and decreased FFA clearance
- increase in gluconeogenesis
Why is the 3rd portion of the duodenum the most vulnerable to traumatic rupture?
- because it is the most fixed portion of the duodenum
Fxn of GI peptides?
- includes hormones, neurocrines, and paracrines
regulate fxns of GI tract:
- contraction and relaxation of smooth muscle wall and sphincters
- secretion of enzymes for digestion
- secretion of fluids and electrolytes
- regulate secretion of other GI peptides
Fxns of the small intestine?
- mostly along length of small intestine
- Fe and Ca2+ absorption
- Na+ coupled with absorption of glucose and amino acids
- ionic iron stored in mucosal cells with ferritin
- K+ diffuses in respones to osmotic gradients
- Ca2+ absorption reg by vit D and PTH
95% absorbed in small intestine by osmosis, water uptake coupled with solute uptake
What increases the small intestine' absorptive surface area?
- intestinal lining
- villi: finger like projections of mucosa
- microvilli: tiny projections on luminal membrane of each intestinal cell
- gives apical region striated appearance called brush border
Process of carbohydrate absorption?
- monosaccharides glucose and galactose are absorbed via cotransport with Na ions, all monosaccharides enter capillary blood in villi and are transported to liver via the hepatic portal vein
- occurs in small intestine (brush border)
Process of protein absorption?
- through pepsin (stomach), pancreatic enzymes and brush border of small intestine
- amino acids are absorbed via cotransport with Na, enter capillary blood in villi and are transported to liver via hepatic portal vein
Process of fat absorption?
- emulsified by detergent action of bile salts ducted in from the liver and pancreatic lipase in small intestine
- digestion by pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then often assoc with bile salts to form micelles which ferry tehm to intestinal mucosa
- fatty acids and monoglycerides leave micelles and diffuse into epithelial cells, there they are recombined and packaged with other fatty substances and proteins to form chylomicrons
- chylomicrons are extruded from epithelial cells by exocytosis. Chylomicrons enter lacteals and are carried away from the intestine in the lymph
Fxns of the large intestine?
- reabsorb water and compact material into feces
- absorb vitamins produced by bacteria
- store fecal matter prior to defecation
Physiology of diff colon regions?
1. ascending colon: specialized for processing chyme delivered from terminal ileum
2. transverse colon: specialized for storage and dehydration of feces and is primary site for removal of water and electrolytes and storage of feces
3. descending colon: conduit b/t transverse and sigmoid colon
4. rectosigmoid region, anal canal, and pelvic floor musculature maintains fecal continence - sigmoid and rectum are reservoirs with capacity up to 500 mL
Fxn of colon?
- proximal half of colon is concerned with absorption and distal half fxns in storage
- transit of small labeled markers through large intestine occurs in 36-48 hrs
- movement of colon acccomplished by haustrations (mixing movements) and mass movements (propulsive movements)
What may trigger mass movements?
- increased delivery of ileal chyme into ascending colon following a meal (gastrocolic reflex)
- irritants: castor oil, threatening agents such as parasites and enterotoxins can initiate mass movement
- starts in middle of transverse colon and is preceded by relaxation of circular muscle and downstream disappearance of haustral contractions
Reabsorption in large intestine includes?
- vitamins: K, biotin, B5
- organic wastes: urobilongens and sterobilinogens
- bile salts
- no villi
- mucosa contains numerous tubular glands called crypts (responsible for mucus secretion)
Major fxn of the rectum?
- last portion of digestive tract
- terminates at the anal canal
- 15 cm
- storage is the major fxn
How does sensory innervation maintain continence of the bowel?
- mechanoreceptors in rectum detect distension and supply ENS
- anal canal in region of skin is innervated by somatosensory nerves that transmit signals to CNS
- this region has sensory receptors of pain, temp, and touch
- contraction of internal anal sphincter and puborectalis msucle blocks the psasage of feces and maintains continence