GI Track Exam 6 Flashcards
Digestive System: Continuous Tube
From mouth to anus
-mouth, pharynx, esophagus, stomach, small intestine, large intestine
Digestive system: Accessory Digestive Organs
Teeth, tongue, salivary glands, liver, gallbladder, pancreas
-Breakdown of food (teeth, tongue)
-Secretions into GI tract (salivary glands, liver, gallbladder, pancreas)
—Enzymes and mucous
Mucosa (inside)
Epithelium
Lamina propria is connective tissue
Musclaris mucosa -> thin layer of smooth muscle (small movement mucosal surface)
Submucosa
Connective tissue containing blood and lymph vessels
Network of neurons (submucosal plexus)
Muscularis External
-Circular (thick; narrowing the lumen) and longitudinal muscles (shorten the tube) in most of the tract
-Myenteric plexus (neurons) between muscle layer
Serosa (outside ab cavity)
Connective tissue -> connects serosa to ab wall
Enteric Nervous System (ENS): myenteric and submucosal plexus
Myenteric (Auerbach’s) Plexus:
Control motility
Submucosal (Meissner’s) Plexus:
-Innervates epithelial cells and smooth muscle of muscularis
-It senses the environment in the lumen -> control local secretion, absorption and local blood flow
Innervation of interneurons and sensory neurons
Interneurons:
-Integrate info from sensory neurons to motor neurons in plexus
Sensory Neurons:
-receives info from sensory receptors in mucosa and muscles
What is the overall function of the digestive system
Process ingested foods into molecular forms that can be absorbed and circulation can distribute them to cells
What are the 5 major processes of the digestive system
Secretion (cells in GI tract and accessory organs)
-Release of water, acid, buffers, and enzymes into lumen
-Food mix with the secretions
Digestion:
-Mechanical digestion by churning food (muscle contraction)
-Chemical digestion (enzymes) -> hydrolysis of food-producing smaller molecules
Motility:
-Muscular movements (wavelike) along part of the GI tract
Absorption:
-Molecular forms pass into blood or lymph
Elimination:
-Elimination of waste by defecation
Processes of the GI tract
-Absorption products of digestion, water, minerals, vitamins into blood
-Secretion of ions, enzymes, and bile salts into the GI tract
-Larynx and esophagus are not important in digestion, but muscles in the wall are involved in swallowing
-Small intestine is where most digestion, absorption, and secretion occurs
What does the mouth, esophagus, stomach, small intestine, colon, anus do?
Mouth: bolus formation starts digestion
Esophagus: moves bolus to stomach
Stomach: chyme formation, digestion and storage
Small intestine/Colon: absorption of water and alcohol (limited amount)
Anus: Waste elimination
Mechanical Digestion
Food grounded by teeth into smaller particles
Mixed with saliva and formed into a bolus (soft, rounded mass)
Chemical Digestion
Enzymes
-Salivary amylase
-Salivary or Lingual lipase
Enzymes in Saliva: salivary amylase, salivary or lingual lipase, lysozyme
Salivary amylase:
Activated by food (breakdown carbohydrates)
Salivary or Lingual Lipase:
Activity continues into stomach (breakdown of lipids)
Lysozyme:
Enzyme that kills bacteria -> protection of oral cavity (breakdown of proteins)
The stomach functions
-Dissolves and partially digests macromolecules
-Mixing chamber -> mixing saliva, good, gastric juice
-Bolus is converted into CHYME
-Storage -> reservoir for food before releasing it into the small intestine
-Regulated the rate at which it empties chyme into small intestine
What does secretion of mucus, gastric acid and intrinsic factors do
-Mucus and HCO3- -> protective glands
-Gastric acid (HCl) -> kills bacteria, denatures proteins
-Intrinsic factor -> Vit B12 absorption
What does the secretion of pepsinogen, gastric lipase, and gastrin do
-Pepsinogen originates pepsin that digests proteins
-Gastric Lipase -> triglycerides digesition
-Secretion of gastrin (hormone)
—Causes release of gastric acid
—Stimulates pancreas (enzymes), liver (bile), intestine (movement)
Acid Secretion by Parietal Cells: H+ and Cl- transport
Secrete H+ and Cl- separately into the lumen of the stomach and then HCL is formed in the gastric lumen
H+ comes from carbonic acid and are transported into the lumen of the stomach by H+/K+ ATPase
Cl- moves into cells in exchange for HCO3- using the Cl-/HCO3- antiporter
Regulation of gastric acid secretion in parietal cells
Acetylcholine -> released by parasympathetic neurons
Gastrin -> secreted from G cells
Histamine -> released by ECL cells
Direct Regulation of Gastric Acid
-Direct regulation involves the binding of ACh, gastrin and histamine to receptors on the surface of parietal cells
-Increase cAMP (histamine) and Ca2+ (Ach and gastrin) activate PKA
-PKA phosphorylates and activates H+/K+ ATPase -> production of gastric acid
Indirect regulation of gastric acid secretion: gastrin and ACh
Gastrin:
-gastrin activates CCK receptors on ECL cells
ACh:
-ACH activates M1 muscarinic receptors on ECL cells
-ACh release from neurons of the enteric system
What cells release histamine
ECL cells
Histamine activates H2 receptors on _________ cells -> production of _______ acid
parietal, gastric
What is the main regulatory pathway in indirect gastric acid secretion
Gastrin - histamine pathway
______________-like cells are located in the gastric gland of the gastric mucosa -> close to ________ cells
enterochromaffin, parietal
Chemical Digestion in Stomach: Gastric Acid
Secreted by parietal cells secrete -> H+/K+ ATPase
-gastric acid partially denature proteins
-gastric acid activates pepsinogen and gastric lipase
-pepsinogen (inactive molecule), secreted by chief cells, is activated to pepsin by HCl
Chemical Digestion in Stomach: what does pepsin do, how are triglycerides broken down
Pepsin breaks down peptide bonds
Gastric Lipase together with lingual lipase break down triglycerides in the stomach
Chemical Digestion in Stomach: how are stomach cells protected
They are protected from acid by alkaline mucus produced by surface and neck mucous cells
Mechanical Digestion in Stomach: Mixing waves
Mixing -> several minutes after food enter stomach mixing waves pass over stomach every 15-25 seconds
Mechanical Digestion in Stomach: movements, peristaltic waves
-Propulsion and retropulsion movements
-Peristaltic waves produced by pacemaker cells in the longitudinal smooth muscle -> signal is then passed to circular muscle
Mechanical Digestion in Stomach: Movements resulting in chyme
-Salivary amylase is inactivated and lingual and gastric lipases are activated in the stomach
-Lower esophageal sphincter prevents stomach contents from entering the esophagus
Mechanical Digestion in Stomach: Gastric Emptying
Chyme passes through the pyloric sphincter
3 ml at a time
Small Intestine: absorption and secretion
Absorption of most nutrients
Finishing the digestion of proteins, carbs, and lipids for absorption
Secretion from accessory organs (pancreas, liver, gallbladder) to help with digestion
What are the three regions of the small intestine
Duodenum (short)
Jejunum
Ileum (longest)
What two parts of the small intestine cause chemical digestion and absorption of most nutrients
Duodenum and Ileum
Anatomy of the small intestine
Circular Folds:
Folds of mucosa and submucosa in intestinal wall -> cause the chyme to spiral
Villi:
fingerlike projections of the mucosa that cover the folds
Microvilli:
-Small projections of the apical membrane of absorptive cells (enterocytes) on the surface of each villus
-Known collectively as brush border
Special features of Villi center
Capillary Network (arteriole and venule)
Lateral (lymphatic capillary)
-Nutrients absorbed by the lateral reaches the general circulation from the lymphatic system into large veins
Epithelial Cells: Absorptive, globet, enteroendocrine, paneth cells
Absorptive Cells (enterocytes):
Digestion and absorption
Globet:
Secrete mucous for lubrication and protection
Enteroendocrine cells secrete:
Secretin (HCO3-)
Cholecystokinin (pancreatic and gastric secretion and gallbladder contraction)
Paneth Cells (at the bottom):
Secret Lysozyme
Involved in phagocytosis
Brush-Boarder Enzymes: digestive enzymes, microvilli
-Digestive enzymes are synthesized by absorptive cells or enterocytes and inserted in the plasma membrane of microvilli
-Microvilli form a fuzzy line called the brush boarder that extends into the lumen of the small intestine
Brush-Boarder Enzymes: enzymatic digestion and specific enzymes
-Some enzymatic digestion occurs at surface of enterocytes rather than in the lumen of the small intestine
Enzymes:
carb digesting, protein digesting (peptidases) & nucleotide digesting
Mechanical digestion and absorption in duodenum of small intestine
Mechanical Digestion:
Peristalsis and segmentation movements (rapidly in duodenum or small intestine)
Absorption:
Digested food forms pass through the absorptive epithelial cells lining the mucosa into blood and/or lymphatic vessels
Peristalsis and Segmentation in mechanical digestion in small intestine
Peristalsis: pushing chyme forward by contracting cells behind (longitudinal cells)
Segmentation: circular muscles contract breaking chyme into smaller pieces until chyme mixed with digestive juices
Accessory Organs in small intestine functions
Pancreas -> exocrine cells (acini) secrete pancreatic juice containing enzymes
Liver -> secretes bile
Gallbladder -> stores secreted bile between meals
Acini in pancreas
-99% of clusters
-Glandular epithelial or acinar cell clusters
-Exocrine portion of the organ secretes pancreatic juice with digestive enzymes
What is pancreatic secretions stimulated by
secretin and CCK
Pancreatic Secretions: Sodium Bicarbonate
Changes chyme to less acidic -> stop pepsin activity
Important for activity of enzymes in small intestine
Pancreatic Secretions: Digestive Enzymes
Pancreatic Enzymes -> starch metabolism
Trypsin, chemotrypsin, carboxypeptidase, elastase -> metabolism of proteins
Pancreatic lipase -> metabolism of triglycerides
Deoxyribonuclease & Ribonuclease -> nucleic acid metabolism
Pancreatic Enzymes
Inactive enzymes:
Trypsin, chemotrypsin, carboxypeptidase, elastase
Trypsinogen is activated to trypsin by enterokinase enzyme in small intestine and then activates the other peptidases
What is the liver functional unit
Hepatic Lobule -> contains cells or hepatocytes and portal triads with branches of the bile duct, hepatic and portal veins and hepatic artery
What do the Kupffer cells do in sinusoids in the liver
They are macrophages that destroy old WBCs and RBCs, bacteria and foreign matter
Hepatic artery brings _________ blood with ________ to the liver
oxygenated, nutrients
______ veins bring absorbed substances to the liver from the small intestine. _____ veins drain into the ________
portal, portal, sinusoids
Substances absorbed from the small intestine end up in the ________ ________
hepatic sinusoids -> can reach vena cava via the central vein or they are taken up by the hepatocytes to be processed
Eliminated substances and bile are secreted by the __________ into the bile _________, which form the common hepatic bile duct
hepatocytes, canaliculi
What is enterohepatic circulation
Recycling pathway from the liver to the intestine and back to the liver
What do the bile salts do in enterohepatic circulation
-Are secreted into the small intestine to digest a fatty meal, absorbed by the transporter in the ileum
-Absorbed and taken to the liver via the hepatic portal vein to the liver
-Then secreted back into the bile
-Small amounts do not recycle -> eliminated in the feces and liver synthesizes more to replace them
Function of the gallbladder: bile
-Stores and concentrates the bile produced by the liver
-Gallbladder muscle contracts when food enters releasing bile via the cystic duct into the common bile duct and duodenum
-Bile emulsifies fats
Gallstones are ____________ that crystallized in the gallbladder
cholesterol
Digestion is completed in the small intestine by components in …
pancreatic juice, bile, and intestinal juice
What do carbohydrates intake
Starch, disaccharide glucose, and lactose
What enzymes metabolize simple monosaccharides
Pancreatic Amylase and Brush Boarder Enzymes
How is fructose, glucose and galactose absorbed in carb digestion and absorption
Fructose -> absorbed into the cell by facilitated diffusion via glucose transporter
Glucose and Galactose -> absorbed by active transport coupled to sodium
Monosaccharides are absorbed across the basolateral membrane by facilitated diffusion
Protein Digestion
Starts in stomach by pepsin and continues in small intestine
What pancreatic juices enzymes need to be activated for protein digestion
Trypsin, chymotrypsin, and elastase -> cleave peptide bond between specific AA and its neighbor
Carboxypeptidase -> Splits off the AA at the carboxyl end of a peptide (also in brush border)
Brush-boarder peptidases in protein digestion
They complete protein digestion
Aminopeptidase cleaves AA at the amino end of a peptide
Dipeptidase splits dipeptides into single AA
Carboxypeptidase
Protein Absorption
Amino Acids enter absorptive cells via Na+ dependent active transporter
Small peptides use secondary active transport with H+
- peptides are then hydrolyzed to single AA in the cell
Animo Acids move out of the cell (basolateral) via facilitated diffusion via different specific transporter
-They are transported to the liver and then to circulation
Emulsification of Lipids (small and large droplets)
-Lipids are insoluble in water and aggregate into large lipid droplets
-Pancreatic lipase is a water-soluble enzyme and works only at the surface of droplets
-Large droplets are divided into many very small droplets -> increase lipase action and rate of digestion
-Needed for emulsification
-Mechanical disruption of large droplets -> GI tract motility
-Emulsifying agent (prevents reaggregation of smaller droplets) -> phospholipids and bile salts in bile and phospholipids in food
Formation of Micelles
-Formation of micelles increases the rate of absorption of water-insoluble products of the lipase reaction
-Micelles are similar to emulsion droplets but much smaller
-Micelles consist of bile salts, fatty acids, monoglycerides and phospholipids in a cluster
-Non-polar portion forms the micelle core, where there is also vitamins and cholesterol
-Micelles are in equilibrium with fat digestion product (free in solution) -> individual molecules release from the micelles are absorbed
Lipids and Bile Salts Absorption
-Fatty acids and monoglycerides enter the enterocytes
-In the cell, fatty acids and monoglycerides are used to resynthesize triglycerides in smooth ER
-Triglycerides combine with phospholipids, cholesterol and lipid-soluble vitamins and then coated with proteins forming lipoprotein known as chylomicron
-Chylomicron are released from cells
-Bile salts are reabsorbed and go back to the liver
Chylomicrons Absorption
-Chylomicrons are extruded from the cells
-They enter lacteals of the villi and are transported away from the intestine
-Chylomicron-rich lymph then drains into the lymphatic system, which rapidly flows into the blood
Electrolytes and Nutrients Absorption
Sodium Ions: diffusion, actively transported out by Na/K ATPase
Bicarbonate, Cl, I, N: passively follow sodium or actively transported
Ca: Active transported stimulated by calcitriol (active Vit D)
Fe, K, Mg, P: Active transport
Vitamins:
-Fat-soluble vitamins (A,D,E,K), ingested with dietary lipids in micelles, simple diffusion
-Water soluble (B,C), simple diffusion, vitamin B12 combines with intrinsic factor (active transport in ileum)
Water: Via osmosis following concentration gradients
Functions of Large Intestine
Large intestine (colon) is much wider and shorter than the small intestine
Lacks digestive enzymes and secretions contain mucus and fluid with HCO3 and K
Function is to absorb water, salts, and vitamins and remove solid waste -> time of food in here is important
Final stages of digestion
Chyme is prepared for elimination by the action of bacteria present in the cells
-Ferment carbs (not digested in small intestine)
—Release hydrogen, CO2, methane gases -> causes flatulence (excessive gas)
Break amino acids into indole, skatole, hydrogen sulfide
Decompose bilirubin into stercobilin
Produces vitamins (B and K) -> absorption
Absorption and Feces Formation
Chyme remained in the large intestine and becomes solid or semi-solid due to water reabsorption -> feces
What does feces consist of
Water, inorganic salts, sloughed epithelial cells, bacteria, products of bacterial decomposition, unabsorbed digestion material, indigestible parts of food
Absorption Pathways
-Fats enter the lacteal where lymph vessels empty in a large vein near the heart -> chylomicrons deliver content to body cells
-All other absorbed nutrients move to intestinal capillaries then to veins
-Venous drainage from small and large intestine, pancreas and portion of stomach goes to the liver through the portal vein
-Liver can process these materials if needed
-In the liver the blood goes into capillaries and then leaves for the heart
Problems with lipid digestion
Lipids in food present as large globules -> it is difficult for lipase to act on them
Lipids are hydrophobic, and poorly soluble in the aqueous environment of the GI tract
Lipase (water soluble) only works at the surface of fat globules -> low rate and efficiency
Emulsification process -> solution
During emulsification, the large globules break down into smaller globules -> bile is important
Smaller globules are held in suspension
Lipase can act on larger surface area -> increase the efficiency of lipid digestion
