Tubular Reabsorption
Understanding the mechanisms of solute and water recovery in the nephron.
1. Basic Mechanisms
Reabsorption is the second fundamental mechanism of the kidney (after filtration). While filtration acts as a bulk dumping system, reabsorption is the process of reclaiming valuable substances.
Definition: Reabsorption
The transport of water and solutes from inside the tubule (lumen) back into the peritubular capillaries.
Key Point: There is absolutely no reabsorption in the Bowman's capsule or the initial capillary network. Reabsorption begins immediately at the start of the Proximal Convoluted Tubule.
The Renal Equation
2. Properties of Reabsorption
Highly Selective
Unlike filtration, which indiscriminately sorts by size, reabsorption discriminates by chemical nature:
- Complete Glucose & Amino Acids: 100% reabsorbed. Essential for the body.
- Partial Ions (Na+, Cl-, HCO3-): Reabsorbed based on body needs.
- None/Poor Waste (Urea, Creatinine): Little to no reabsorption; excreted efficiently.
Quantitatively Large
The volume of fluid processed is immense compared to urine output.
Example: Sodium (Na+)
Filtered ~99% Reabsorbed
Small changes in reabsorption can cause massive changes in urine volume. Reabsorbing 99% vs 99.5% of water is physiologically significant.
3. Calculating Filtered Load
To understand how much of a substance enters the nephron, we calculate the Filtered Load (Tubular Load).
Formula
Filtered Load = Px × GFR
⚠️ Important Condition
This formula only applies to substances that are Freely Filtered (not bound to plasma proteins).
Example: Calcium and Fatty Acids are partially bound to proteins, so the formula cannot be used directly without correction.
Example: Glucose Calculation
| Parameter | Value |
|---|---|
| GFR | 180 Liters/day |
| Plasma Glucose | 1g/L (100mg/dL) |
| Filtered Load | 180 grams/day |
4. Cellular Mechanisms
The Proximal Tubule Cell
The proximal tubule is the workhorse of reabsorption. Its cells are specialized for moving massive amounts of fluid and solutes.
- Brush Border: The luminal surface is covered in microvilli, increasing surface area by ~20x.
- Mitochondria: High density of mitochondria provides ATP for active transport pumps.
- Basal Channels: Infoldings of the basal membrane increase surface area for transport into capillaries.
Transport Pathways
1. Transcellular Path
Substances travel through the cell.
- Cross apical membrane (Lumen -> Cell)
- Diffuse through cytoplasm
- Cross basolateral membrane (Cell -> Interstitium)
Used by: Glucose, Amino Acids, Na+
2. Paracellular Path
Substances travel between cells.
Moves through "tight junctions" and intercellular spaces. In the proximal tubule, tight junctions are "leaky," allowing significant water and ion flow.
Used by: Water, Cl-, K+
Mechanism of Sodium Reabsorption
Note: Sodium reabsorption is the engine. The Na+/K+ ATPase on the basal side keeps intracellular Na+ low (-70mV), creating a gradient that pulls Na+ in from the lumen.