Kidney Tubule Functions: Making Urine & Balancing Your Body

⏱️ Glomerular Filtration Rate (GFR)

Your kidneys filter about 125 ml of fluid every minute (that’s 180 liters per day!). This is called GFR. If GFR drops, special cells (juxtaglomerular apparatus/JGA) release renin to boost blood flow and restore normal filtration. 💧

🔁 Reabsorption vs. Secretion

Reabsorption = Taking good stuff back into blood
Secretion = Adding waste to the filtrate

Of the 180L filtered daily, only 1.5L becomes urine! 😲 That means 99% of the filtrate gets reabsorbed by kidney tubules.

🧩 Parts of the Nephron & Their Jobs

1. Proximal Convoluted Tubule (PCT)

  • 🧠 Lined with “brush border” cells (maximizes surface area!)
  • ✅ Reabsorbs: ALL essential nutrients (glucose, amino acids), 70-80% electrolytes (Na+), and water.
  • ⚖️ Maintains pH by: Secreting H+ and ammonia into filtrate + Reabsorbing HCO3–.

2. Henle’s Loop

  • ⬇️ Descending Limb: Lets water out → Filtrate concentrates.
  • ⬆️ Ascending Limb: Lets electrolytes (NaCl) out → Filtrate dilutes. Water can’t leave here!
  • 📈 Creates high saltiness (osmolarity) in the medulla (super important for concentrated urine!).

3. Distal Convoluted Tubule (DCT)

  • ⚠️ Conditional reabsorption of Na+ and water (depends on body’s needs).
  • ⚖️ Maintains balance by:
    • Reabsorbing HCO3–
    • Secreting H+, K+, and NH3

4. Collecting Duct

  • 💧 Reabsorbs LOTS of water → makes urine concentrated.
  • ♻️ Lets a little urea leak into medulla → helps keep it “salty”.
  • ⚖️ Secretes H+/K+ to maintain blood pH and ion balance.

🌟 NEET Super-Important Concepts 🌟

  1. JGA & Renin: Activated by low GFR → fixes filtration rate.
  2. PCT = Major Reabsorber! ~100% nutrients, 70-80% water/electrolytes.
  3. Henle’s Loop: Creates medullary gradient (descending: water OUT; ascending: electrolytes OUT).
  4. DCT: Adjusts Na+/water reabsorption & secretes ions (H+/K+) for pH/K+ balance.
  5. Collecting Duct: Final water conservation + urea recycling for osmolarity.

🔬 How Urine Gets Concentrated

Henle’s loop and nearby blood vessels (vasa recta) work together! Filtrate flows in opposite directions in the loop’s two limbs (countercurrent flow), building a super-strong salt gradient in the kidney medulla. This lets the collecting duct pull out water → hyper-concentrated urine! 🎯

Fun fact: This is why desert animals have long loops! 🌵