Chapter 15: Breathing and Exchange of Gases – Part 3

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🌬️ Chapter 15: Breathing and Exchange of Gases – Part 3

💨 Transport of CO₂ & Regulation of Respiration

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🫀 Transport of Carbon Dioxide

Carbon dioxide (CO₂) is produced continuously in tissues as a waste product of cellular respiration. It needs to be transported to the lungs for exhalation. Here's how that happens:

🧪 Modes of Transport of CO₂:

Mode of Transport	Percentage
As Bicarbonate (HCO₃⁻)	~70%
Carbaminohaemoglobin (HbCO₂)	~20-23%
Dissolved in Plasma	~7%

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1️⃣ As Bicarbonate Ions (HCO₃⁻) – Most Important

• CO₂ enters RBCs and reacts with H₂O to form carbonic acid (H₂CO₃).

• Enzyme carbonic anhydrase speeds this up.

• H₂CO₃ quickly dissociates into H⁺ and HCO₃⁻ ions.

• HCO₃⁻ diffuses into plasma for transport.

Reaction inside RBC:
CO₂ + H₂O ⟶ H₂CO₃ ⟶ H⁺ + HCO₃⁻

• At lungs, this process reverses: HCO₃⁻ converts back to CO₂ and is exhaled.

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2️⃣ As Carbaminohaemoglobin (HbCO₂)

• CO₂ combines with free amino groups of Hb (not with iron).

• This binding is reversible.

• In lungs, CO₂ is released from Hb and expelled.

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3️⃣ Dissolved CO₂ in Plasma

• A small amount (about 7%) is transported as is, dissolved in plasma.

• Plays a role in maintaining acid-base balance.

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💡 Chloride Shift (Hamburger Phenomenon)

• To balance the outflow of HCO₃⁻ from RBCs into plasma, Cl⁻ ions enter RBCs.

• This is an electroneutrality mechanism.

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🧠 Regulation of Respiration

Respiratory rhythm is not voluntary; it’s controlled by centers in the brainstem.

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🧠 Respiratory Centres

Centre	Location	Function
Medullary Rhythm Centre	Medulla oblongata	Generates basic breathing rhythm
Pneumotaxic Centre	Pons (upper brainstem)	Modulates inspiration
Chemosensitive Area	Medulla (near rhythm center)	Detects CO₂ & H⁺ levels in blood

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🔁 Role of Chemical Factors

• High CO₂ or H⁺ in blood → Stimulates chemosensitive area → Increases breathing rate.

• Low CO₂ → Breathing slows down.

O₂ concentration has less influence on normal respiration regulation compared to CO₂.

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👃 Receptors and Reflexes

• Stretch receptors in lungs send inhibitory signals via vagus nerve to prevent overinflation.

• Irritant receptors (for dust, smoke) can trigger sneezing or coughing.

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🧪 NEET PYQs – CO₂ Transport & Regulation

1. Most of the carbon dioxide is transported in blood as:

   • A) Carbaminohaemoglobin

   • B) Carbonic acid

   • C) Bicarbonate

   • D) Dissolved CO₂
     ✅ Answer: C) Bicarbonate

2. Which enzyme facilitates the conversion of CO₂ and H₂O into carbonic acid?

   • A) Carboxylase

   • B) Carbonic anhydrase

   • C) Cytochrome oxidase

   • D) Peptidase
     ✅ Answer: B) Carbonic anhydrase

3. Chloride shift is associated with:

   • A) Entry of O₂ into RBCs

   • B) Exit of CO₂ from tissues

   • C) Entry of Cl⁻ into RBCs when HCO₃⁻ leaves

   • D) None of the above
     ✅ Answer: C) Entry of Cl⁻ into RBCs when HCO₃⁻ leaves

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🧠 Summary Table

Transport Method	% of CO₂ Carried	Notes
Bicarbonate (HCO₃⁻)	~70%	Most efficient, enzyme-aided process
Carbaminohaemoglobin	~20-23%	Binds to Hb amino groups
Dissolved in Plasma	~7%	Least significant but helps pH balance

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