Digestive System Part II Flashcards
Accessory Digestive Organs of the Small Intestine (duodenum, Jejunum, Ileum):
Liver:
Main Function: Produces bile (the only digestive function).
Anatomy:
Largest gland in the body (~3 lbs).
Four lobes: right (Gallbladder rests on inferior of this), left, caudate, quadrate.
Falciform ligament separates right and left lobes, attaches to diaphragm and abdominal wall.
Round ligament (ligamentum teres): Remnant of fetal umbilical vein.
Lesser omentum anchors liver to stomach.
Liver Blood Supply: Hepatic artery (O2) and hepatic portal vein (nutrient-rich blood from intestine, deoxygenated) enter at porta hepatis. Bile exits via hepatic ducts.
Microscopic Anatomy:
Liver lobules: Functional units with hepatocytes (liver cells) - filter & process nutrient rich blood
and sinusoids (leaky capillaries) - lined with macrophages (remove debris & old RBCs)
Portal triad: Contains branches of hepatic artery, hepatic portal vein, and bile duct.
Liver Functions:
Produce 900 ml of bile/day.
Process blood: Store glucose as glycogen, synthesize plasma proteins, detoxify (e.g., convert ammonia to urea).
Store vitamins: Fat-soluble vitamins.
Excrete bilirubin (from hemoglobin breakdown).
Phagocytize (Stellate macro.) old RBCs, WBCs, bacteria.
Synthesize: clotting factors (vitamin K) and lipoproteins.
Bile:
Composition: Bile salts (for fat emulsification), bilirubin (waste product from RBC breakdown) - gives feces color, cholesterol, phospholipids, electrolytes.
Enterohepatic Circulation: 95% of bile salts are recycled in the ileum and returned to the liver via hepatic portal vein.
Hepatocytes - large amounts of rough and smooth ER, Golgi apparatus, peroxisomes & mitochondria.
Recycling mechanism of bile salts:
Bile salts are secreted into the duodenum.
They aid in lipid digestion and absorption in the small intestine.
95% of bile salts are reabsorbed in the ileum.
Reabsorbed bile salts travel via the hepatic portal vein to the liver for recycling.
Only 5% of bile salts are newly synthesized.
Hepatitis: Liver inflammation from alcohol, drugs, viruses (e.g., HVA, HVB)
What fills/drains gallbladder:
Cystic duct
Bile duct is formed by union common hepatic and cystic ducts:
True
Gallbladder:
On the inferior surface of the liver.
Function: Stores and concentrates bile (by absorbing water and ions).
Location: Beneath the liver’s right lobe. expands as it fills.
Muscular Contraction: Releases bile into the duodenum through the cystic duct when stimulated by cholecystokinin (CKK)
Cystic duct: This duct connects the gallbladder to the common bile duct. It allows bile to flow into the gallbladder for storage and also lets it flow out of the gallbladder when needed for digestion.
Bile duct: The bile duct is formed by the common hepatic duct (which comes from the liver) and the cystic duct (which connects to the gallbladder). The bile duct carries bile from the liver and gallbladder to the duodenum (the first part of the small intestine) to help digest fats.
A gallstone is a solid particle that forms in the gallbladder from substances in bile, often consisting of cholesterol or bilirubin.
Bile Flow: From liver through hepatic ducts → common hepatic duct → bile duct → duodenum.
Gallbladder physiology:
Liver secretes 1L of bile/day.
Bile concentrates in the gallbladder.
CCK triggers bile release into the duodenum.
Bile salts emulsify lipids for digestion.
Pancreas:
Location: Mostly retroperitoneal, head encircled by duodenum, tail near spleen.
Endocrine Function: Secretes insulin and glucagon via pancreatic islet cells.
Exocrine Function: Produces pancreatic juice, secreted via the main pancreatic duct into the duodenum.
- Acini: Clusters of secretory cells producing proenzymes - pancreatic juice (zymogen granules).
- Duct cells: Produce bicarbonate to neutralize stomach acid.
Developmental Aspects
Embryonic development:
alimentary canal forms from endoderm.
Accessory organs (liver, pancreas, etc.) develop from endoderm.
Pancreatic Juice:
Composition: Alkaline solution (pH 8) containing bicarbonate (HCO3−) and digestive enzymes (proteases, amylase, lipases, nucleases).
Protease Activation: Proteases are secreted in inactive forms (e.g., trypsinogen) and activated in the duodenum by enterokinase (activates trypsin), which then activates other enzymes (e.g., procarboxypeptidase, chymotrypsinogen).
The pancreas secretes about 1200 to 1500 ml of pancreatic juice daily. It contains water, enzymes, and electrolytes like bicarbonate ions.
Bile and Pancreatic Secretion Into the Small Intestine:
Hepatopancreatic Ampulla: Where the bile duct and pancreatic duct meet and empty into the duodenum.
Hepatopancreatic Sphincter: Regulates the entry of bile and pancreatic juice into the duodenum.
The length of the small intestine allows for maximal nutrient absorption.
Control of bile and pancreatic Secretion:
Hormonal Regulation:
Cholecystokinin (CCK): Stimulated by protein- and fat-rich chyme, causes:
Pancreatic enzyme secretion.
Gallbladder contraction and bile release.
Secretin: Stimulated by acidic chyme, causes:
Bicarbonate-rich pancreatic juice secretion.
Weak bile secretion stimulation.
Neural Regulation:
Vagus nerve (long reflexes) can weakly stimulate bile and pancreatic secretion during cephalic and gastric phases.
Cholecystokinin (CCK) and secretin regulate bile and pancreatic secretion.
CCK is stimulated by fats and proteins in chyme and helps induce enzyme-rich pancreatic juice release and gallbladder contraction.
Secretin is stimulated by acidic chyme and promotes bicarbonate-rich pancreatic juice secretion.
Bile & Pancreatic Juice Secretion:
Chyme stimulates release of CCK (from fats/proteins) and secretin (from acidic chyme).
CCK triggers enzyme-rich pancreatic juice; secretin stimulates HCO₃⁻-rich juice.
Vagus nerve weakly stimulates secretion.
Bile secretion is mainly stimulated by bile salts and weakly by secretin.
CCK causes gallbladder contraction and hepatopancreatic sphincter relaxation, allowing bile and pancreatic juice into the duodenum.
Small Intestine:
Function: Digests and absorbs nutrients.
Length: 7-13 ft; 2.5-4 cm in diameter.
Parts: Duodenum (25 cm), Jejunum (2.5 m), Ileum (3.6 m).
Blood Supply: From superior mesenteric artery to hepatic portal vein.
Nerve Supply: Parasympathetic via vagus, sympathetic via splanchnic nerves
Small Intestine Microscopic Anatomy:
Circular folds: Slow chyme for better absorption.
Villi: Fingerlike projections with capillaries and lacteals for nutrient and lipid absorption.
Microvilli: Extensions on enterocytes with enzymes, brush border enzymes for final digestion (carbohydrates & proteins).
Absorption: Modifications increase surface area 600x (~200 m²).
Small Intestine Histology:
Cells: Enterocytes (absorption in villi), Goblet cells (mucus), Enteroendocrine cells (hormones), Paneth cells (antimicrobials), Stem cells (renewal).
Submucosa: Alkaline mucus secretion.
Mucosa: MALT protects against microbes. Peyer’s patches.
Muscularis: Circular & longitudinal muscles, serosa outer layer.
Intestinal Juice: 1-2L daily for digestion and absorption.
Digestive Processes in the Small Intestine:
Enzyme Sources: Bile, bicarbonate, and digestive enzymes from the liver and pancreas; brush border enzymes complete digestion.
Chyme Regulation: Slow entry, pH adjustment, and bile/pancreatic juice mixing, controlled by enterogastric reflex.
Motility:
After meals: Segmentation mixes and moves chyme.
Between meals: Peristalsis clears debris toward the large intestine.
Ileocecal Valve: Opens with gastroileal reflex and gastrin, closes to prevent backflow.
Large Intestine:
Structure: 1.5 m long, 7 cm diameter, shorter than SI.
Functions: Absorbs water, metabolites, stores, and eliminates feces.
Parts:
- Cecum: First part.
- Appendix: Stores bacteria.
Features:
Teniae coli: Muscle bands.
Haustra: Pouches.
Epiploic appendages: Fat-filled pouches.
Appendix: Removal alters gut bacteria and may raise cancer risk; rupture causes peritonitis.
Appendicitis: acute inflammation of appendix; usually results from a blockage by feces that
traps infectious bacteria
fLarge Intestine (Colon) Anatomy:
Regions:
3, Colon
Ascending colon: Up the right side to the right colic (hepatic) flexure.
Transverse colon: Across the abdomen to the left colic (splenic) flexure.
Descending colon: Down the left side.
Sigmoid colon: S-shaped, through the pelvis.
Rectum: Has rectal valves to prevent passing gas with feces.
Anal canal: Last segment, external to the body with two sphincters:
Internal anal sphincter (smooth muscle)
External anal sphincter (skeletal muscle)
Peritoneum: Colon is retroperitoneal (except transverse and sigmoid parts, which are intraperitoneal with mesocolons anchoring them).
Large Intestine Microscopic Anatomy:
Epithelium: Simple columnar (except anal canal, stratified squamous). no circular folds villi, digestive secretions
Mucosa: Thicker, with deep crypts and goblet cells for mucus.
Anal Canal:
Anal columns and sinuses (secrete mucus).
Pectinate line: Separates pain-sensitive (somatic) and insensitive (visceral) areas.
Hemorrhoids: Caused by dilated venous plexuses.
Hemorrhoids: Swollen veins in the anal canal.
Large Intestine: Bacterial Microbiota
Composition: 1000+ bacteria types, outnumbering human cells 10:1.
Functions:
Ferments carbs, produces fatty acids and gases.
Synthesizes B vitamins and vitamin K.
Controls pathogens by outcompeting harmful bacteria.
Health Impact: Gut bacteria affect weight, disease risk, drug response, and mood. Future health strategies may involve manipulating microbiota.
Large Intestine Digestive Processes
Residue stays for 12-24 hours.
No food breakdown, only bacterial digestion.
Reabsorbs vitamins, H2O, and electrolytes (Na+, Cl−).
Main functions: Propel feces & defecation.
Motility
Haustral Contractions: Slow, segmenting in ascending & transverse colon.
Mass Movements: Strong contractions (3-4X/day) move feces to rectum, triggered by eating.
little nutrient absorption
Defecation Process:
Mass movements push feces to rectum.
Defecation reflex: Rectal distension (stretch receptors) triggers contraction of colon/rectum and relaxation of internal sphincter (parasympatheitc). Voluntary control over external sphincter.
Expulsion: Assisted by Valsalva’s maneuver and levator ani muscle.
Diarrhea: Rapid stool transit, leading to watery stools.
Constipation: Slow transit or excessive water absorption, causing infrequent, hard stools.
Digestion & Absorption:
Digestion: Enzymes break down macromolecules into absorbable monomers via hydrolysis.
Absorption: Nutrients move from gut to body; most absorbed before the ileum. Non-lipids enter capillaries, lipids enter lacteals.
Both passively and by AT
Nutrient Processing:
Carbohydrates:
Types: mono, di, & polysaccharides.
Digested by amylase in mouth and pancreas in small intestine, and broken down to monosaccharides (glucose, galactose, fructose) by brush border enzymes. Absorbed via co-transport/secondary active transport with Na+. Fructose via facilitated diff.
Proteins:
Digested by pepsin in the stomach and proteases in the pancreas, and brush border enzymes breaking down into amino acids. AAs are co-transported across
apical membrane of absorptive
epithelial cell via secondary active
transport carriers (Na+ or H+)
▪ Amino acids exit across basolateral
membrane via facilitated diffusion.
Lipids:
Triglycerides: most abundant fats in diet
Emulsified by bile, digested by pancreatic lipases into monoglycerides and fatty acids. Absorbed via diffusion from micelles.
Fat Digestion:
Emulsification: Bile salts break fat into small droplets.
Digestion: Lipase (lingual, gastric, emulsification, pancreatic lipases) converts fats into fatty acids and monoglycerides.
Micelles: Fatty acids and monoglycerides form micelles to enter cells.
Chylomicrons: Recombined fats form chylomicrons, transported via lymph.
Carbohydrates
Three classes
▪ Mono-, di- and polysaccharides (only monosaccharides can be absorbed)
▪ the principal polysaccharide in human body is glycogen
Lactose Intolerance:
Low lactase levels prevent lactose digestion, causing diarrhea and gas.
Bloating, flatulence, and cramps result from bacterial fermentation.
Treatment: Use lactase enzyme supplements.
Nutrient Processing:
Lipids: Emulsified by bile, absorbed as fatty acids; reassembled into chylomicrons, enter lacteals, and reach the bloodstream.
Nucleic Acids: Broken down by pancreatic enzymes (pancreatic nucleases) and brush border enzymes and absorbed as nitrogenous bases, pentose sugars & Phosphate ions from nucleotides.
Vitamins:
Fat-soluble: Absorbed with micelles.
Water-soluble: Absorbed via diffusion or transporters.
B12: Requires intrinsic factor.
Electrolytes: Actively absorbed or transported (Na+, Cl−, K+, HCO3−, Fe, Ca2+ based on need).
Water: ~9L absorbed, with ~100ml excreted.
Vitamin Absorption:
Small intestine: Fat-soluble (A, D, E, K) via micelles, water-soluble (C, B) by diffusion or transport.
Large intestine: Vitamin K and B from bacteria.
Electrolyte Absorption:
Na+: With glucose/amino acids.
Fe & Ca2+: Absorbed based on need, Ca2+ regulated by vitamin D & PTH.
Water Absorption:
95% in small intestine, rest in large intestine.
Absorption Mechanism:
Nutrients pass through enterocyte membranes (apical and basolateral).
Active transport (e.g., Na+ cotransport) moves nutrients into enterocytes.
Facilitated diffusion allows nutrients to exit enterocytes and enter blood or lymph.
See all the diagrams.
Water uptake is coupled with solute uptake.
Net osmosis occurs if conc. gradient is established through AT.
Here’s a more concise version:
Pancreas & Hormones:
Pancreas: Both endocrine and exocrine functions.
Bicarbonate: Neutralizes stomach acid in the duodenum.
CCK & Secretin: Regulate bile and pancreatic enzyme release.
Digestive Structures:
Duodenum: Main site of digestion.
Jejunum: Major site for absorption.
Mesentery: Anchors intestines.
Villi & Microvilli: Increase absorption area.
Lacteals: Absorb fats into lymph.
Digestive Motility:
Segmentation: Localized mixing in the duodenum.
MMC: Clears indigestible matter; triggered by Motilin.
Gastroileal Reflex: Opens ileocecal valve for food passage.
Large Intestine:
Functions: Water absorption, compaction, and excretion.
Teniae Coli & Haustra: Help with movement.
Appendix: Stores bacteria, aids immunity.
Absorption & Secretion:
Water & Electrolytes: Absorbed in small and large intestine.
Anatomy:
Mucosa: Absorptive epithelium in small intestine.
Esophagus to Stomach: Protective stratified squamous.
Smooth vs Skeletal Muscle: Smooth for involuntary movement (peristalsis).
Colon & Rectum:
Colon’s Bacteria: Ferment food, produce gas, and vitamins.
Constipation: Caused by excessive water absorption.
Diarrhea: Due to bacterial infections.
Hemorrhoids: Swollen veins; relieved by reducing pressure.
Liver: Breaks down red blood cells, giving feces their color.
C-Diff: Bacterial infection causing severe diarrhea.
Non-Alcoholic Fatty Liver Disease (NAFLD)
Most common liver disease in North America,
Linked to obesity and insulin resistance.
Usually asymptomatic, but can lead to cirrhosis or liver cancer.
Cirrhosis - chronic liver inflammation
GI Organ Functions (Table 23.2)
1. Mouth & Accessory Organs
Ingestion, Propulsion (swallowing), Mechanical Breakdown (chewing), Digestion (starch by salivary amylase).
2. Pharynx & Esophagus
Propulsion (peristalsis to stomach).
3. Stomach
Mechanical Breakdown & Propulsion (mixing), Digestion (pepsin for proteins), Absorption (lipid-soluble substances).
4. Small Intestine & Accessory Organs
Mechanical Breakdown & Propulsion (segmentation), Digestion (pancreatic & brush border enzymes), Absorption (nutrients, water, electrolytes).
5. Large Intestine
Digestion (by bacteria), Absorption (water, electrolytes), Propulsion (mass movements), Defecation (feces elimination).
Small Intestine: Digestion & Motility
Enzymes: Bile (fat emulsification), digestive enzymes (pancreas/brush border).
Regulation: Enterogastric reflex & Enterogastrones control chyme entry.
Motility:
Segmentation: Mixing & absorption after meals.
Migrating Motor Complex: Peristalsis between meals, clears debris.
