Lecture 9: The Digestive System Flashcards
what is the small intestine
-muscular tube, pyloric sphincter to ileocecal valve
-site of most digestion and absorption
describe the three segements of the digestive system
- duodenum: ~25 cm, “mixing bowl” receives chyme from stomach and digestive secretions from the pancreas and liver, mostly retroperitoneal
- jejunum: ~1 m, most chemical digestion and nutrient absorption occurs here
- ileum: ~2 m, ends at the ileocecal valve
what are the modifications of the small intestine for absorption
small intestine;s length and other strutural modifications provide hugh surface area for nutrient absorption
-modifications include: circular folds, villi, and microvilli
what are the circular folds of the small intestine
-transverse folds in intestinal lining
-allow chyme to spiral through the lumen
-they are permanent features that do not disappear when small intestine fills
what are the intestinal villi
-simple columnar epithelium
-core of dense capillary bed and lymphatic capillary called luteal for absorption
-transports chylomicrons that are too large to enter blood capillaries
what is the microvilli of the small intestine
-cytoplasmic extensions of mucosal cell
-gives fuzzy appearance-> brush border
-contains membrane-bound enzymes; brush border enzymes for final CHOs and protein digestion
what are intestinal glands (intestinal crypts or crypts of lieberkuhn)
-tubular glands scattered between villi
-extend deep into lamina propria
what are the 5 main cell types in the villi and crypts
stem cells, enterocytes, goblet cells, enteroendocrine cells, and Paneth cells
what are the three types of enteroendocrine cells in the small intestine
-S cells: secretin
-CCK cells: cholecystokinin
-K cells: glucose-dependent insulinotropic peptide (GIP)
what are the duodenal submucosal glands
-produce large quantities of mucus secretion when chyme arrives from stomach
-mucus protects epithelium from acidity of chyme; contains bicarbonate ions that raise pH to neutralize gastric acid in chyme
-MALT: protects intestine against microorganisms and includes: indivual lymphoid follicles, adn Peyer’s Patches (aggregated lymphoid nodules) within lamina propria (abundant in distal part of SI, where bacterial numbers increase)
-lamina propria also contain large numbers of plasma cells that secrete IgA
what is intestinal juice
-secreted daily in response to distension or irritation of mucosa
-major stimulus for productions is hypertonic or acidic chyme
-slightly alkaline; isotonic with blood plasma, assists in buffering acids
-consists largely of water but also contains mucus (which is secreted by duodenal glands and goblet cells of mucosa
-keeps digestive enzymes and products of digestion in solution
what enzymes of the small intestine mucosa are involved in chemical digestion
-brush border enzymes: break down materials in contact wtih brush border
-enteropeptidase: brush border enzymes; activates pancreatic trypsinogen
motility of small intestine between meals
-Peristalsis increases, initiated by rise in hormone motilin
-Each wave starts distal to previous wave; referred to as migrating motility complex (MMC)
-Meal remnants, bacteria, and debris are moved toward large intestine
-Complete trip from duodenum to ileum takes ~2 hours
motility of small intestine after a meal
-Segmentation is most common motion of small intestine
-Mixes/moves contents toward ileocecal valve
-Intensity is altered by reflexes and hormones
-Parasympathetic increases motility; sympathetic decreases it
what is peristalsis
adjacent segments of the alimentary canal organs alternately contract and relax
-food is moved distally along the tract
-primarily propulsive; some mixing may occur
what is segmentation
nonadjecent segments of the alimentary canal organs contract and relax
-food is moved forward, then backward
-primarily mixes food and breaks it down mechanically; some propulsion may occur
what are the primary functions of the small intestine
- segmentations: mix chymes with digestive juices and bring food into contact with mucosa for absorption; perstalsis propels chyme through small intestine
- completes digestion of carbohydrates, proteins, lipids; begins and completes digestion of nucleic acids
- absorb about 90% of nutrients and water that pass through digestive system
what is the large intestine
-extends from end of ileum to anus
-lies inferior to stomach and liver
-frames the small intestine
What are the primary functions of the large intestine
-Reabsorption – water, nutrients (< 10%), bile salts, organic wastes,
vitamins and toxins produced by bacteria
-Compaction of intestinal contents into feces
-Storage of fecal material prior to defecation
What are the 5 sub divisions of the large intestine
Cecum, appendix, colon, rectum, anal canal
What is the cecum
sack-like first portion
-Receives and stores food from ileum
-Compaction of materials begins
What is the appendix (vermiform appendix)
-Posteromedial surface of cecum
-Lymphoid nodules
What is the appendix (vermiform appendix)
-Posteromedial surface of cecum
-Lymphoid nodules
What is the colon and the three parts
has several regions, most which are retroperitoneal (except
for transverse and sigmoid regions)
-Ascending colon: travels up right side of abdominal cavity to
level of right kidney, Ends in right colic (hepatic) flexure
-Transverse colon: travels across abdominal cavity, Ends in left colic (splenic) flexure
-Descending colon: travels down left side of abdominal cavity
-Sigmoid colon: S-shaped portion that travels through pelvis
What is the rectum
three rectal valves stop feces from being passed with gas (flatus)
What is the anal canal
last segment of large intestine that opens to body exterior at anus
What are the two sphincters of the anal canal
- Internal anal sphincter: smooth muscle
- External anal sphincter: skeletal muscle
What are the unique features of the colon
larger diameter, and thinner wall than small intestine
What is the teniae coli (in colon)
three bands of longitudinal smooth muscle in muscularis
What is the haustra (in the colon)
pocket-like sacs caused by tone of teniae coli
What are the Epiploic (Omental) Appendages (in the colon)
fat-filled pouches of visceral peritoneum
What is the microbiome of the large intestine
Microbes (bacteria, fungi) that live in and on human body
- Mainly resident in large intestine
- Normal bacteria in colon make three vitamins that supplement diet
What are the three vitamins that normal bacteria in the large intestine make
- Vitamin K (fat soluble)
- Required by liver for synthesizing four clotting factors, including prothrombin - Biotin (water soluble)
- Important in glucose metabolism - Vitamin B5 (pantothenic acid; water soluble)
- Required in manufacture of steroid hormones and some neurotransmitters
What are the organic wastes of the large intestine
• Bacteria convert bilirubin (by-product of heme) → urobilinogens/stercobilinogens
- Some urobilinogens are absorbed into bloodstream and excreted in urine
- Exposure to O2 converts urobilinogens and stercobilinogens remaining in colon to urobilins and stercobilins
• Responsible for yellow-brown coloration of feces
• Bacteria act on peptides in feces to produce:
- NH3, as NH4+ ions
- Indole and skatole – responsible for odor of feces
- H2S gas that produces “rotten egg” odor
• Bacteria feed on indigestible carbohydrates
- Produce flatus (intestinal gas
What are the main functions of the large intestine
- Haustral churning, peristalsis, and mass peristalsis drive contents of colon into rectum
- Bacteria in colon convert proteins into amino acids, break down amino acids, and produce
some B vitamins - Absorption of some water, ions, and vitamins
- Formation of feces
- Defecation
what is defecations
- Mass movements force feces toward rectum
- Distension initiates spinal defecation reflex
- Parasympathetic signals: Stimulate contraction of sigmoid colon and rectum & Relax internal anal sphincter
- Conscious control allows relaxation of external anal
sphincter - Muscles of rectum contract to expel feces: Levator ani muscle contracts → anal canal lifted
superiorly and allowing feces to leave body
what is the pancreas
-Retroperitoneal; Posterior to stomach
-Extends from duodenum toward spleen
-Produces enzymes that digest CHOs, proteins, fats, and nucleic acids
-Produces NaHCO3 which buffers stomach acid
-Empties its contents into the duodenum
what are the parts of the pancreas
-Head: broad; in a loop formed by duodenum
-Body: slender; extends towards spleen
-Tail: short and rounded
-Pancreatic duct: delivers secretions of pancreas to duodenum
explain the lobules and ducts of the pancreas
-Lobules separated by CT septum
-Ducts in lobules end in blind pockets (pancreatic acini)
describe the pancreatic islets
-Endocrine tissues of pancreas
-Scattered among pancreatic acini
-Account for about 1% of pancreatic cells
what do the endocrine cells of pancreatic islets do
Secrete insulin and glucagon into bloodstream
what are the exocrine galnds of the pancreas
-Secrete insulin and glucagon into bloodstream
Exocrine Cells
-Acinar cells and epithelial cells of duct system
-Secrete alkaline pancreatic juice (pH 8) into small intestine: Contains digestive enzymes, water, and ions
-Controlled by hormones from duodenum
list and describe the pancreatic enzymes
-Alpha-amylase:
-Lipase:
-Nucleases:
-Proteolytic enzymes:
-Proteases:
-Peptidase:
what are the proenzymes of the pancreas
-Trypsinogen: Converted to active trypsin in duodenum
-Chymotrypsinogen: Converted to active chymotrypsin by trypsin
-Procarboxypeptidas: Converted to active carboxypeptidase by trypsin
-Proelastase: Converted to active elastase by trypsin
look at slide 19 for a explanation of steps in defecation
describe the gross anatomy of the liver
-Largest visceral organ (1.5 kg)
-Functions in digestion to produce bile: fat emulsification
Divisions: Left and right lobes – anteriorly. Caudate and Quadrate lobes (part of right lobe) – posteriorly
-Falciform ligament: Separates left and right lobes, thickens posteriorly to form round ligament
-Main blood supply is the hepatic portal vein: Some venous blood from hepatic artery proper
-Hepatocytes (liver cells): Adjust circulating levels of nutrients through selective absorption and secretion
what is the histology of the liver
- Each lobe is divided by CT into lobules
- Are basic functional units of liver
- Composed of plates of hepatocytes which: Filter and process nutrient-rich blood, Production of bile, Detoxification
- Sinusoids btw plates empty into central vein
- Many stellate macrophages (Kupffer cells) in lining of
sinusoids
what do the plates of hepatocytes do
- Filter and process nutrient-rich blood
- Production of bile
- Detoxification
what is the portal triad
- Liver lobules are hexagonal in cross section
- Each of six corners has a portal triad containing:
- Interlobular vein:
- Interlobular artery:
- Interlobular bile duct:
what is the bile duct system
- Liver (hepatocytes) secretes bile: into bile canaliculi btw adjacent liver cells
- Right and left hepatic ducts which:
- Collect bile from all bile ducts of liver lobes
- Unite to form common hepatic duct (CHD)
describe the physiology of the liver
Performs several functions broadly divided into three categories: metabolic regulation, hematological regulation, and bile production
what is metabolic regulatory of the liver
- Carbohydrate metabolism
- Lipid metabolism
- Amino acid metabolism
- Waste removal
- Vitamin storage
- Mineral storage
- Drug inactivation
what is the hematological regulation of the liver
- Phagocytosis and antigen presentation
- Synthesis of plasma proteins
- Removal of circulating hormones
- Removal of antibodies
- Removal or storage of toxins
what is the production and function of bile in the liver
-Bile is produced in the liver
-Bile salts: fat emulsification in duodenum
what is the gallbladder
-Thin-walled muscular sac, posterior surface of R. lobe of liver
-Stores and concentrates bile
-Releases bile when stimulated by CCK
what are the three regions of the gallbladder
fundus, body, and neck
-Bile duct – union of cystic and common hepatic ducts
- Bile duct joins pancreatic duct before emptying into duodenal
ampulla, Opens into duodenum at duodenal papilla
describe the mechanisms of food absorption
- Absorption is the process of moving substances from lumen of gut into body
- Tight junctions ensure molecules must pass through epithelial cell rather than
btw them: Materials enter cell through apical membrane (lumen side) and exit through
basolateral membrane (blood side) - Lipid molecules can be absorbed passively through membrane, but other polar molecules are absorbed by active transport
- Most nutrients are absorbed before chyme reaches ileum
describe the digestion and absorption of carbohydrates
- Only monosaccharides can be absorbed
- Starch and polysaccharides are broken down to oligosaccharides and disaccharides
- Begins in mouth with salivary amylase
- Further broken down into lactose, maltose, and sucrose
- Final breakdown into monosaccharides (glucose, fructose, galactose)
what are the steps in digestion and absorption of carbohydrates
- pancreatic amylase breaks down starch and glycogen into oligosacchardies and disaccharides
- brush border enzymes break oligosaccharides and disaccharides into monosaccharides
- amino acids are cotransported across the apical membrane of the absorptive epithelial cell. the Na+ concentration gradient established by the Na+ K+ ATPase pump in the basolateral membrane
- amino acids exit across the basolateral membrane via facilitated diffusion and enter the capillary via intercellular clefts
slide 33
what is emulsification, digestion, and absorption of lipids
- emulsification: bile salts in the duodenum break large fat globules into smaller fat droplets, increasing the surface area available to lipse enzymes
- digestion: pancreatic lipases hydrolyze triglycerides, yielding monoglycerides and free fatty acids
- michelle formation: michelles (consisting of fatty acids, monoglycerides and bile salts) ferry their contents to epithelial cells
- diffusion: fatty acids and monoglycerides diffuse from michelles into epithelial cells
- chlyomicron formation: fatty acids and monoglycerides are recombined and packaged with other fatty substances and proteins to for chylomicrons
6.chylomicron transport: chylomicrons are extruded from the epithelial cells by exocytosis, enter lacteals, and are carried away from the intestine in lymph
describe the digestion of nucleic acids
- Nuclei of ingested cells in food contain DNA and RNA
- Pancreatic nucleases hydrolyze nucleic acid to nucleotide monomers
- Brush border enzymes, nucleosidases, and phosphatases break nucleotides down into free nitrogenous bases, pentose sugars, and phosphate ions
- Breakdown products are actively transported by special carriers in epithelium of villi
what is ankyloglossia
extremely short lingual frenulum
- Often referred to as “tongue-tied” or “fused tongue”
- Restricted tongue movement distorts speech
what is xerostomia
dry mouth, uncomfortable condition caused by too little saliva being made
what is heartburn
Caused by stomach acid regurgitating into esophagus
- Excess food/drink, extreme obesity, pregnancy, running
- Also caused by hiatal hernia: structural abnormality, part of stomach protrudes above diaphragm
what is gastritis
Inflammation caused by breach of stomach’s mucosal barrier
what are peptic or gastric ulcers and the causes
erosion of stomach wall
- Causes: bacterium Helicobacter pylori, and non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin
what are gallstones
excess cholesterol or too few bile salts
- Obstruct flow of bile from gallbladder
what is appendicitis
inflammation of appendix; usually results from a blockage by feces that traps infectious bacteria
- Most common in adolescence when entrance to appendix is at widest
what is diarrhea
watery stools, when large intestine is unable to absorb remaining water
- Cause: irritation of colon by bacteria
what is constipation
occurs when food remains in colon for extended periods of time and too much water is absorbed
- Stool becomes hard and difficult to pass; - May result from insufficient fiber or fluid in the diet
how does cystic fibrosis affect the digestive system
Genetic disease that produces thick mucus that blocks pancreatic duct
