Chapter 16: Excretory Products and Their Elimination – Part 2

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🚽 Chapter 16: Excretory Products and Their Elimination – Part 2

💧 Urine Formation & Water Balance in the Body

🧬 Overview of Urine Formation

Urine formation in humans involves three key steps:

  1. Glomerular Filtration

  2. Tubular Reabsorption

  3. Tubular Secretion

All these processes occur within the nephron.

1️⃣ Glomerular Filtration (Ultrafiltration)

  • Occurs in Bowman’s capsule

  • Blood enters via afferent arteriole into the glomerulus

  • High pressure filters small molecules:

    • Water, glucose, urea, amino acids, salts

  • Large molecules (RBCs, proteins) do not filter

  • Filtrate is called: Glomerular filtrate

🧠 GFR (Glomerular Filtration Rate) = ~125 mL/min = 180 L/day

2️⃣ Tubular Reabsorption

  • Reabsorption of essential substances from filtrate back into blood

  • Occurs mainly in PCT (Proximal Convoluted Tubule)

  • What gets reabsorbed?

    • Glucose, amino acids, water, Na⁺, Cl⁻, HCO₃⁻

Segment Reabsorbed Substances
PCT 70% of water, glucose, ions
Loop of Henle Water (descending), Na⁺/Cl⁻ (ascending)
DCT & Collecting Duct Fine-tuning via hormones like ADH & Aldosterone

3️⃣ Tubular Secretion

  • Active transport of substances from blood to filtrate

  • Removes:

    • H⁺, K⁺, drugs, creatinine

  • Occurs in DCT and collecting duct

  • Helps maintain acid-base and ionic balance

💦 Osmoregulation

The process of maintaining water and salt balance in the body.

🧠 Role of Hormones

Hormone Gland Function
ADH (Vasopressin) Posterior Pituitary Increases water reabsorption (makes urine concentrated)
Aldosterone Adrenal Cortex Reabsorbs Na⁺ (indirect water retention)
ANF (Atrial Natriuretic Factor) Heart Promotes Na⁺ & water excretion; antagonistic to Aldosterone

🌀 Counter Current Mechanism

Occurs in Loop of Henle and vasa recta (blood vessels).

How it works:

  • Loop of Henle:

    • Descending limb: Permeable to water

    • Ascending limb: Impermeable to water, reabsorbs Na⁺ & Cl⁻

  • This creates a concentration gradient in medulla

  • Vasa recta:

    • Blood flows in opposite direction to filtrate

    • Maintains the osmotic gradient

🔁 This helps kidneys conserve water and produce hypertonic urine

NEET PYQs – Urine Formation & Osmoregulation

  1. Filtration in Bowman’s capsule is due to:
    A) Secretion
    B) Active transport
    C) Osmotic pressure
    D) Blood pressure
    ✅ Answer: D) Blood pressure

  2. Which hormone increases water reabsorption in kidneys?
    A) Insulin
    B) Oxytocin
    C) ADH
    D) Aldosterone
    ✅ Answer: C) ADH

  3. Counter current mechanism helps in:
    A) Reabsorption of urea
    B) Reabsorption of glucose
    C) Concentration of urine
    D) Filtration of blood
    ✅ Answer: C) Concentration of urine

  4. Which of the following is not reabsorbed in PCT?
    A) Glucose
    B) Urea
    C) Amino acids
    D) Creatinine
    ✅ Answer: D) Creatinine

🔁 Recap of Part 2

  • Urine formation = filtration + reabsorption + secretion

  • PCT does bulk reabsorption

  • Loop of Henle sets up counter current system

  • Hormones like ADH & Aldosterone regulate water/salt levels

  • ANF works against Aldosterone to lower BP

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