Renal System
Each nephron acts like a mini chemistry lab — filtering the blood, adjusting its composition, and returning it clean. Let’s break down the three essential processes that together create urine:
⚙️ 1️⃣ Glomerular Filtration — The First Step
Definition: It’s the process by which fluid and solutes are filtered from blood into Bowman’s capsule.
Filtration Barrier (3 layers):
- Endothelial cells of glomerular capillaries (fenestrated — holes for passage)
- Basement membrane (negatively charged — repels proteins)
- Podocytes (slit diaphragms — size-selective filter)
💡 Filters ~180 L/day of plasma, but reabsorbs 99% back!
📊 Glomerular Filtration Rate (GFR)
GFR = Volume of fluid filtered per unit time (~125 mL/min or 180 L/day).
💡 Mnemonic: “Starling forces” — hydrostatic vs. oncotic pressures drive filtration.
🧠 Clinical note: GFR measured by creatinine clearance; low GFR = kidney failure.
💧 2️⃣ Tubular Reabsorption — The Recycling Center
Of the 180 L filtered, 99% is reabsorbed back into blood — only 1–2 L becomes urine.
| Segment | Reabsorbed Substances | % Reabsorbed |
|---|---|---|
| PCT | Na⁺, water, glucose, amino acids, HCO₃⁻ | 65–70% |
| Loop of Henle | Na⁺, Cl⁻ (ascending), water (descending) | 25% |
| DCT | Na⁺, Ca²⁺, fine-tuning | 5–10% |
| Collecting Duct | Water (ADH), Na⁺ (aldosterone) | Variable |
💡 Glucose reabsorbed via SGLT transporters in PCT; threshold ~180 mg/dL → glycosuria in diabetes.
🚮 3️⃣ Tubular Secretion — The Waste Disposal
Adds substances from peritubular capillaries into tubules for excretion.
- Main site: PCT and DCT
- Secreted: K⁺, H⁺, organic acids (e.g., creatinine, drugs like penicillin)
🧠 Regulated by aldosterone (↑ K⁺ secretion) and pH (↑ H⁺ secretion in acidosis).