A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.

Proximal Tubule Reabsorption and Secretion

College Depth 239 in the knowledge graph ☐ I know this ☆ Set as goal

142topics build on this

1,286prerequisites beneath it

Glomerular Filtration and Filtration Rate Regulation Active Transport→→Kidney Tubular Processing and Urine Formation Loop of Henle and Osmotic Gradient Generation

Core Idea

The proximal tubule performs selective reabsorption of glucose, amino acids, sodium, and other filtered nutrients via active and passive transport coupled to the Na+-gradient, while also secreting drugs, organic acids, and excess H+ from peritubular blood into the tubular lumen. These two processes together determine which substances are retained and which are excreted.

Explainer

From glomerular filtration, you know that the kidney produces about 180 liters of filtrate per day — essentially plasma minus proteins. That filtrate contains everything the body needs: glucose, amino acids, electrolytes, bicarbonate, water. If the nephron simply excreted all of it, you would lose your entire plasma volume in minutes and all your blood glucose in under an hour. The proximal tubule prevents this catastrophe by reclaiming roughly 65% of the filtered water, sodium, and solutes before the fluid even reaches the loop of Henle.

The engine driving nearly all proximal tubule reabsorption is the Na⁺/K⁺-ATPase pump on the basolateral membrane (the side facing the blood). This pump continuously moves sodium out of the tubular cell and into the interstitial fluid, keeping intracellular sodium concentration low. This creates a steep sodium gradient across the apical membrane (the side facing the tubular lumen), and that gradient is harnessed by a family of cotransporters and exchangers on the apical surface. Sodium-glucose cotransporters (SGLT2 and SGLT1) carry glucose into the cell by riding sodium's downhill gradient. Sodium-amino acid cotransporters do the same for amino acids. A sodium-hydrogen exchanger (NHE3) swaps sodium inward for hydrogen ions outward, which drives bicarbonate reabsorption (the secreted H⁺ combines with filtered bicarbonate in the lumen to form CO₂ and water, which diffuse into the cell and are reconverted to bicarbonate). In each case, the Na⁺/K⁺-ATPase is the ultimate energy source — ATP hydrolysis creates the sodium gradient, and that gradient powers everything else.

Water follows the solutes. As sodium, glucose, and other solutes are reabsorbed, the osmolarity inside the tubular lumen drops slightly relative to the interstitium. Water then moves out of the lumen by osmosis through aquaporin-1 channels in both the apical and basolateral membranes. This water reabsorption is so efficient and so tightly coupled to solute reabsorption that the fluid leaving the proximal tubule is still approximately isosmotic with plasma — the same concentration, just much less of it. The proximal tubule also reabsorbs most filtered phosphate, citrate, lactate, and small peptides, making it the nephron's primary recovery site.

Running in the opposite direction is tubular secretion: the proximal tubule actively pumps certain substances from the peritubular blood into the tubular lumen. Organic anion transporters (OATs) and organic cation transporters (OCTs) on the basolateral membrane take up drugs (like penicillin), toxins, and metabolic waste products (like urate and creatinine) from the blood, and apical transporters then dump them into the lumen. This is physiologically important because some substances are protein-bound in plasma and therefore cannot be filtered at the glomerulus — secretion provides a second route for their elimination. The proximal tubule also secretes hydrogen ions (via NHE3 and H⁺-ATPase), contributing to acid-base balance. Together, reabsorption and secretion in the proximal tubule accomplish the bulk of the nephron's work: recovering what the body needs while adding extra waste to the fluid destined to become urine.

Practice Questions 5 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → One-to-One Correspondence → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Making 10 as an Addition Strategy → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Iterated Integrals and Fubini's Theorem → Double Integrals in Cartesian Coordinates → Double Integrals in Polar Coordinates → Double Integrals in Polar Coordinates → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → The Measurement Problem → Interpretations of Quantum Mechanics → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Acid-Base Chemistry → Weak Acid Ionization → Weak Base Ionization → Acid and Base Strength: Ka, Kb, and Ionization → Leaving Groups and Nucleofugality → SN2 Substitution Reactions → SN1 Substitution Reactions → E1 Elimination Reactions → Alcohols and Ethers: Structure, Properties, and Nomenclature → Reactions of Alcohols → Aldehydes and Ketones: Structure and Reactivity → Oxidation Reactions in Organic Chemistry → Oxidation of Alcohols to Aldehydes and Ketones → Aldehyde and Ketone Structure and Nomenclature → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → IUPAC Nomenclature of Carbonyls and Carboxylic Acids → IUPAC Nomenclature of Alkenes → Electrophilic Addition to Alkenes → Aromaticity and Benzene → Electrophilic Aromatic Substitution (EAS) → Nucleophilic Aromatic Substitution (SNAr) → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Peptide Bonds and Polypeptide Formation → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Enzyme Structure and Function → Transcription: DNA to RNA → RNA Types and Structure → RNA Structure and Intramolecular Base Pairing → RNA Processing and Splicing → Translation: RNA to Protein → Ribosomes: Protein Synthesis Machines → Translation: Initiation and Elongation → Post-Translational Modifications → Proteasomal Degradation and Ubiquitin-Mediated Marking → Cell Cycle Regulation and Checkpoints → Cell Cycle Checkpoints: Ensuring Genome Integrity → Cell Cycle Checkpoints and Cancer Prevention → Mitotic Spindle Checkpoint and Chromosome Segregation → Kinetochore Structure and Function → Mitochondria: Structure and Function → Cellular Respiration Overview → Glycolysis → Pyruvate Oxidation → The Krebs Cycle (Citric Acid Cycle) → Electron Transport Chain → ATP Synthesis and Oxidative Phosphorylation → ATP Hydrolysis and Cellular Free Energy → The Na+/K+-ATPase: Maintaining Ion Gradients → Resting Membrane Potential → Ligand-Gated Ion Channels → Voltage-Gated Sodium Channels → Action Potential Phases → Cardiac Electrophysiology and Action Potentials → Cardiac Pacemaker Activity and the Sinoatrial Node → Atrioventricular Node Conduction and Physiological Delay → Heart Rate Control and Autonomic Modulation → Cardiac Output and Stroke Volume Regulation → Blood Pressure Regulation → Renal Physiology and Fluid Balance → Glomerular Filtration and Filtration Rate Regulation → Proximal Tubule Reabsorption and Secretion

Longest path: 240 steps · 1286 total prerequisite topics

Prerequisites (2)

Glomerular Filtration and Filtration Rate Regulationhard Active Transportsoft

Leads To (2)

Kidney Tubular Processing and Urine Formationhard Loop of Henle and Osmotic Gradient Generationhard