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14.5 Hepatic Processing, GI Endocrine Regulators & the Colon
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2.1 Translational Kinematics, Projectile Motion & Newton's Laws 2.2 Friction, Centripetal Force & Rotational Equilibrium in Biomechanics 2.3 Mechanical Work, Pressure-Volume Work & Forms of Energy 2.4 The Work-Energy Theorem, Power & Mechanical Advantage 2.5 Hydrostatics, Fluid Dynamics & Circulatory Hemodynamics 2.6 Periodic Motion, Wave Fundamentals, Superposition & Standing Waves 2.7 Sound Intensity, the Decibel Scale & the Doppler Effect
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5 General Chemistry: Equilibrium, Acids, Bases & Electrochemistry
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6 Organic Chemistry: Structure, Stereochemistry & Mechanisms
6.1 IUPAC Nomenclature, Functional Group Priority & Orbital Hybridisation 6.2 Resonance, Inductive Effects & Conformational Analysis 6.3 The Isomer Hierarchy, Chirality & CIP Priority Rules 6.4 Optical Activity, Diastereomers, Meso Compounds & Fischer Projections 6.5 Carbonyl Reactivity: Aldehydes, Ketones & Nucleophilic Additions 6.6 Carboxylic Acids & Derivatives: Nucleophilic Acyl Substitution
7 Bio-Organic Reactions, Laboratory Separations & Spectroscopy
7.1 Alcohol Physical Properties, Acidity, Leaving Groups & Protecting Groups 7.2 Carbon Oxidation States, Laboratory Oxidations & Biological Redox Cofactors 7.3 Extraction, Acid-Base Separation & Distillation 7.4 Analytical and Biomacromolecular Chromatography 7.5 Infrared and UV-Vis Spectroscopy 7.6 1H-NMR, 13C-NMR & Integrated Spectral Elucidation
8 Biochemistry: Amino Acids, Peptides & Enzymes
8.1 Amino Acid Stereochemistry & Side-Chain Classification 8.2 Zwitterions, Titration Curves, Isoelectric Points & the Peptide Bond 8.3 The Hierarchy of Protein Structure 8.4 Folding Thermodynamics, Haemoglobin Cooperativity & Denaturation 8.5 Catalytic Mechanisms, Enzyme Classification & Michaelis-Menten Kinetics 8.6 Enzyme Inhibition Modes & Allosteric Cooperativity
9 Biochemistry: Cellular Metabolism & Bioenergetics
9.1 Glycolysis: The Ten Steps, Regulation & Anaerobic Fermentation 9.2 Gluconeogenesis Bypass Enzymes & the Pentose Phosphate Pathway 9.3 The Pyruvate Dehydrogenase Complex & the Citric Acid (Krebs) Cycle 9.4 Mitochondrial Architecture & the Respiratory Complexes 9.5 Chemiosmotic Coupling, ATP Synthase, NADH Shuttles & Uncouplers 9.6 Fatty Acid Beta-Oxidation, Ketogenesis & Glycogen Dynamics
10 Molecular Biology: Nucleic Acids & Genetic Technologies
10.1 Nucleic Acid Chemistry, the Double Helix & Chromatin Organisation 10.2 DNA Replication: Replisome Enzymology & Fork Dynamics 10.3 DNA Damage & Cellular Repair Pathways 10.4 The Central Dogma, Strand Nomenclature & Transcription Machinery 10.5 Post-Transcriptional Processing of Eukaryotic Pre-mRNA 10.6 Regulation of Gene Expression: Operons, Enhancers & Epigenetics 10.7 The Genetic Code, Wobble & Mutation Taxonomy 10.8 tRNA Charging & the Stages of Ribosomal Translation 10.9 Translation Inhibitors & Post-Translational Processing 10.10 Restriction Enzymes, Cloning Vectors, cDNA Libraries & PCR 10.11 Blotting, Sequencing, Microarrays & CRISPR-Cas9
11 Cellular Biology & Microbiology
11.1 Eukaryotic Organelles, Membrane Transport & Cytoskeletal Dynamics 11.2 The Eukaryotic Cell Cycle, Mitosis, Checkpoints & Apoptosis 11.3 Cell Theory, Bacterial Morphology, Genetics & Sub-Viral Agents
12 Genetics, Evolution & Embryology
12.1 Mendel's Laws & the Cytology of Meiosis 12.2 Genetic Crosses, Non-Mendelian Inheritance, Linkage & Pedigree Analysis 12.3 The Hardy-Weinberg Principle, Assumptions & Clinical Calculations 12.4 Mechanisms of Evolution, Speciation & Macroevolutionary Patterns 12.5 Gametogenesis, Fertilisation, Cleavage & Implantation 12.6 Gastrulation, Germ Layer Derivatives, Stem Cell Potency & Neurulation
13 Human Physiology: Control & Transport Systems
13.1 Neuronal Anatomy, the Resting Potential & the Action Potential 13.2 Conduction Velocity, Synaptic Transmission & Postsynaptic Summation 13.3 Neurotransmitter Systems, CNS/PNS Organisation & Reflex Arcs 13.4 Hormone Classification & the Hypothalamus-Pituitary Axis 13.5 Negative Feedback Architecture, Endocrine Glands & Calcium Homeostasis 13.6 Cardiac Anatomy, Electrophysiology, the Cardiac Cycle & Cardiac Output 13.7 Vascular Hemodynamics, Starling Forces, Blood Components & Hemostasis 13.8 Respiratory Anatomy, Ventilation Mechanics & Pulmonary Surfactant 13.9 Spirometry, Dead Space, Gas Exchange & Haemoglobin Allostery 13.10 Carbon Dioxide Transport, the Chloride Shift & Acid-Base Control of Breathing
14 Human Physiology: Maintenance, Defense & Locomotion
14.1 Renal Anatomy, Glomerular Ultrafiltration & Tubular Transport 14.2 The Countercurrent Multiplier & Endocrine Regulation of Renal Function 14.3 GI Architecture, the Enteric Nervous System & Gastric Secretion 14.4 Pancreatic and Biliary Digestion & Intestinal Nutrient Absorption 14.5 Hepatic Processing, GI Endocrine Regulators & the Colon 14.6 Innate Immunity: Barriers, Complement & Cellular Effectors 14.7 Adaptive Humoral Immunity: B Cells & Immunoglobulins 14.8 Adaptive Cellular Immunity: T Cells, Thymic Selection & MHC Presentation 14.9 Muscle Types, Sarcomere Anatomy & Excitation-Contraction Coupling 14.10 Bone Architecture, Remodelling & the Integumentary System
15 CARS: Passage Analysis & Argument Mapping
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16 CARS: Question Types & Reasoning Strategies
16.1 Skill 1 Demands, Question Stems, Detail Retrieval & Paraphrase Recognition 16.2 Inferential Meaning, Distractor Taxonomy & a Worked Skill 1 Passage 16.3 Skill 2 Architecture, Question Stems, Unstated Assumptions & Evidence Strength 16.4 Rhetorical Mechanisms, Logical Fallacies & a Worked Skill 2 Passage 16.5 Skill 3 Architecture, Question Stems, Application & Analogy Mapping 16.6 Incorporating New Information, Calibrating Evidentiary Impact & a Worked Skill 3 Passage
17 Sensory Processing, Perception & Cognition
17.1 Sensation versus Perception & Sensory Receptor Classification 17.2 Psychophysical Thresholds, Weber's Law, Adaptation & Signal Detection Theory 17.3 Vision: Ocular Anatomy, Phototransduction & Central Visual Pathways 17.4 Audition & the Vestibular System 17.5 Somatosensation, Pain & Chemoreception 17.6 Models of Attention & Working Memory Architecture 17.7 Long-Term Memory Taxonomy, Encoding, Storage & Retrieval 17.8 Forgetting, Interference, Memory Distortion & Long-Term Potentiation 17.9 Problem Solving, Heuristics, Cognitive Biases & Language 17.10 Sleep Architecture, Circadian Neurobiology, Sleep Disorders & Psychoactive Drugs
18 Emotion, Stress, Motivation & Personality
18.1 Components of Emotion & Classical Theories of Emotion 18.2 The Limbic Circuit, Stress Appraisal & Selye's General Adaptation Syndrome 18.3 Intrinsic and Extrinsic Motivation, Drive-Reduction, Arousal & Incentive Theories 18.4 Cognitive and Humanistic Need Models & the Neuroendocrine Control of Hunger and Sex 18.5 The Psychoanalytic Perspective & Neo-Freudian Revisions 18.6 Humanistic, Trait & Social-Cognitive Perspectives on Personality 18.7 Biopsychosocial Etiology, Schizophrenia, Mood & Anxiety Disorders 18.8 Somatic, Dissociative, Personality & Neurodegenerative Disorders
19 Learning, Social Behavior & Identity
19.1 Non-Associative Learning & Classical Conditioning 19.2 Operant Conditioning, Reinforcement Schedules & Observational Learning 19.3 Self-Concept & Lifespan Developmental Theories 19.4 Kohlberg's Moral Development & Sociocultural Frameworks 19.5 Social Influence: Conformity, Obedience, Group Dynamics & Prosocial Behavior 19.6 Social Cognition, Attribution Theory, Prejudices, Stereotypes & Discrimination
20 Sociology, Demographics & Healthcare Disparities
20.1 Macro versus Micro Paradigms, Structural Functionalism & Conflict Theory 20.2 Symbolic Interactionism, Social Constructionism & Modern Paradigms 20.3 Major Social Institutions: Structure, Function & Dysfunction 20.4 Demographics, Population Pyramids, the Demographic Transition & Globalisation 20.5 Systems of Stratification, Socioeconomic Status & Social Mobility 20.6 Poverty, Spatial Inequality, the Socioeconomic Gradient & Health Inequities
14.5 Hepatic Processing, GI Endocrine Regulators & the Colon
Key Takeaways Explain This Topic AI
The liver executes essential metabolic buffering, synthesis of albumin and vitamin K-dependent clotting factors (II, VII, IX, X), conversion of neurotoxic ammonia to urea via the urea cycle, Phase I/II xenobiotic detoxification, and glucuronidation of hydrophobic unconjugated bilirubin into water-soluble conjugated bilirubin.
The hepatic portal vein delivers everything absorbed by the intestinal capillaries to the liver before it reaches the systemic circulation, so orally administered drugs undergo first-pass metabolism that can drastically reduce bioavailability; only lipids packaged into chylomicrons bypass this route by entering lymphatics.
Bile salts are conserved rather than consumed: roughly 95% are reabsorbed by active transport in the terminal ileum and returned to the liver through enterohepatic circulation, which is why ileal resection produces both fat malabsorption and vitamin B12 deficiency.
Last updated: September 2026
- 14.5 Hepatic Processing, GI Endocrine Regulators & the Colon
- Hepatic Architecture and Metabolic Processing
The liver receives a dual blood supply (~1.5 L/min): 75% via the Hepatic Portal Vein (nutrient-rich, partially deoxygenated blood draining the stomach, intestines, pancreas, and spleen) and 25% via the Hepatic Artery (oxygenated arterial blood). Blood from both vessels enters microscopic hepatic sinusoids , percolating past plates of hepatocytes toward the central vein , which coalesces into hepatic veins emptying into the Inferior Vena Cava (IVC). Hepatic Lobule Architecture:
Portal Triad (Portal Venule + Hepatic Arteriole + Bile Ductule) -> Hepatic Sinusoids (Lined by fenestrated
endothelium & Kupffer macrophages) -> Space of Disse -> Hepatocyte Plates -> Central Vein -> Hepatic Vein -> IVC
Primary Liver Functions
Carbohydrate Buffering : Hepatocytes express high-capacity, low-affinity GLUT2 transporters and glucokinase ( K m ≈ 10 mM K_m \approx 10\text{ mM} K m ≈ 10 mM ). In the postprandial state, insulin stimulates glycogenesis and glycolysis. During fasting, glucagon and epinephrine stimulate glycogenolysis and gluconeogenesis (converting lactate, glycerol, and glucogenic amino acids into free glucose via glucose-6-phosphatase), maintaining euglycemia (~70–100 mg/dL).
Lipid Synthesis and Ketogenesis : Synthesizes fatty acids from excess carbohydrates, assembling them into triacylglycerols packaged into Very Low-Density Lipoproteins (VLDL) . Synthesizes cholesterol and bile acids. During prolonged fasting or starvation, excessive β \beta β -oxidation of fatty acids generates acetyl-CoA that overwhelms the TCA cycle; the liver shunts acetyl-CoA into ketone bodies ( acetoacetate and β \beta β -hydroxybutyrate ), serving as vital water-soluble fuel for the brain and skeletal muscle.
Plasma Protein Synthesis : Synthesizes virtually all circulating plasma proteins, including:
Albumin : Maintains plasma colloid oncotic pressure (~25 mmHg) and transports hydrophobic hormones, bilirubin, and free fatty acids. Hypoalbuminemia leads to peripheral edema and ascites.
Coagulation Factors : Synthesizes Fibrinogen (Factor I), Prothrombin (Factor II), and Factors V, VII, VIII, IX, X, XI, XII, alongside Protein C, Protein S, and Antithrombin III. Crucially, Factors II, VII, IX, and X require Vitamin K as an essential cofactor for post-translational γ \gamma γ -glutamyl carboxylation.
The Urea Cycle (Ammonia Detoxification) : Deamination of amino acids produces free ammonia ( N H 3 / N H 4 + NH_3/NH_4^+ N H 3 / N H 4 + ), a neurotoxin that disrupts cerebral neurotransmission and TCA cycle intermediates ( α \alpha α -ketoglutarate depletion). The liver safely packages ammonia into urea for renal excretion. The rate-limiting enzyme is mitochondrial Carbamoyl Phosphate Synthetase I (CPS-1) , allosterically activated by N-acetylglutamate (NAG) . Citrulline exits the mitochondria into the cytosol, combines with aspartate, and is cleaved to release urea and regenerate ornithine.
Xenobiotic Detoxification (Biotransformation) :
Phase I Reactions (Functionalization) : Catalyzed primarily by Cytochrome P450 monooxygenases (CYP450) in the smooth endoplasmic reticulum. Introduce or unmask polar reactive handles ( − O H , − N H 2 , − C O O H -OH, -NH_2, -COOH − O H , − N H 2 , − COO H ) via oxidation, reduction, or hydrolysis.
Phase II Reactions (Conjugation) : Conjugate the Phase I intermediate with endogenous hydrophilic molecules: glucuronic acid (via UDP-glucuronosyltransferases), sulfate, glycine, or glutathione (via GST), rendering hydrophobic drugs water-soluble for biliary or urinary excretion.
Bilirubin Processing and Excretion : Senescent erythrocytes are phagocytosed by reticuloendothelial macrophages in the spleen and liver. Heme oxygenase cleaves heme into biliverdin, which is reduced to unconjugated (indirect) bilirubin . Unconjugated bilirubin is hydrophobic and circulates bound tightly to albumin. Hepatocytes take up unconjugated bilirubin and conjugate it with two glucuronic acid molecules via UDP-Glucuronosyltransferase (UGT1A1) , creating water-soluble conjugated (direct) bilirubin , which is actively secreted into bile canaliculi. In the colon, gut bacteria deconjugate and reduce bilirubin to urobilinogen ; some is oxidized to stercobilin (imparting brown color to feces), while ~10% undergoes enterohepatic recycling and renal excretion as urobilin (imparting yellow color to urine).
Gastrointestinal Endocrine and Paracrine Regulators
Hormone Site of Production Primary Stimuli Target Tissues & Molecular Actions Net Physiological Effect
Gastrin G cells (Gastric antrum, Duodenum) Luminal amino acids/peptides, distension, vagal GRP; inhibited by luminal p H < 1.5 pH < 1.5 p H < 1.5 Parietal cells ( C C K B → G q CCK_B \to G_q CC K B → G q ) and ECL cells Stimulates gastric acid ( H C l HCl H Cl ) secretion and mucosal growth
Secretin S cells (Duodenum) Luminal protons ( p H < 4.5 pH < 4.5 p H < 4.5 ), fatty acids Pancreatic duct cells ( G s → c A M P G_s \to cAMP G s → c A MP ) and biliary duct cells Stimulates pancreatic and biliary H C O 3 − HCO_3^- H C O 3 − and water secretion; neutralizes gastric acid
Cholecystokinin (CCK) I cells (Duodenum, Jejunum) Fatty acids, monoglycerides, peptides/amino acids Gallbladder smooth muscle, Sphincter of Oddi, pancreatic acinar cells ( G q G_q G q ) Stimulates gallbladder contraction, relaxes Sphincter of Oddi, stimulates pancreatic zymogens
Somatostatin D cells (Stomach, Pancreas, Intestine) Luminal acid ( H + H^+ H + ); inhibited by vagal acetylcholine G cells, ECL cells, parietal cells ( G i → ↓ c A M P G_i \to \downarrow cAMP G i →↓ c A MP ) Broad paracrine inhibitor of gastric acid, gastrin, secretin, and pancreatic exocrine secretion
Glucose-Dependent Insulinotropic Peptide (GIP) K cells (Duodenum, Jejunum) Luminal glucose, amino acids, fatty acids Pancreatic β \beta β -cells Stimulates glucose-dependent insulin secretion; blunts gastric acid secretion at high levels
The Large Intestine (Colon) and Gut Microbiome
Chyme enters the cecum via the ileocecal valve. The colon (ascending, transverse, descending, sigmoid) and rectum lack villi and plicae circulares, exhibiting deep mucosal crypts of Lieberkühn rich in mucus-secreting goblet cells. Its primary functions include:
Water and Electrolyte Reabsorption : Absorbs ~90% of the remaining 1.5–2.0 L/day of fluid presented to it, compacting fecal matter. Epithelial sodium channels (ENaC) on colonocytes reabsorb Na + \text{Na}^+ Na + under the stimulation of aldosterone , while water follows osmotically.
The Colonic Microbiome : Trillions of commensal and mutualistic bacteria (predominantly anaerobes including Bacteroides , Bifidobacterium , and Firmicutes ):
Fermentation : Ferment non-digestible dietary carbohydrates (fiber, resistant starch) into short-chain fatty acids (SCFAs) : acetate, propionate, and butyrate . Butyrate serves as the primary metabolic fuel for colonocytes.
Vitamin Synthesis : Synthesizes biologically significant amounts of Vitamin K (crucial for clotting factor activation) and water-soluble B vitamins, including Biotin (Vitamin B7) , folate, and pantothenic acid.
Colonization Resistance : Competitively excludes pathogenic microbes (e.g., Clostridioides difficile ) by consuming available nutrients, occupying mucosal niches, and producing bacteriocins.