✅ Chapter 17: Breathing and Exchange of Gases – Part 2

🔍 Exchange of Gases and Transport of Respiratory Gases

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📚 Gas Exchange at the Alveoli:

The actual exchange of oxygen (O₂) and carbon dioxide (CO₂) between the lungs and blood occurs at the alveolar surface.

Alveoli are tiny, balloon-like structures that provide:

• Large surface area

• Thin walls

• Rich capillary network

✅ Key point:

Alveoli and capillaries together form the respiratory membrane, which is extremely thin (~0.5 micrometers).

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⚙️ Mechanism of Gaseous Exchange:

Passive Diffusion is the main mechanism based on differences in partial pressure.

Gas	Alveolar Air (mm Hg)	Deoxygenated Blood (mm Hg)
Oxygen (O₂)	104	40
Carbon dioxide (CO₂)	40	45

✅ Direction of diffusion:

• O₂ diffuses from alveoli → blood (higher to lower partial pressure).

• CO₂ diffuses from blood → alveoli.

Thus, gases move down their pressure gradients without requiring energy.

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🌍 Partial Pressure:

Partial pressure (P) of a gas is the pressure contributed by that gas in a mixture of gases.

• Atmospheric air composition:

  • O₂: ~21%

  • CO₂: ~0.04%

  • N₂: ~78%

✅ Total atmospheric pressure = 760 mm Hg

Thus,

• PO₂ = 159 mm Hg (21% of 760)

• PCO₂ = 0.3 mm Hg

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🚗 Transport of Gases:

Blood transports gases between lungs and tissues.

1. Oxygen Transport

2. Carbon Dioxide Transport

Let's dive into each!

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🧬 Oxygen Transport:

Oxygen is transported in two ways:

1. Bound to Hemoglobin (~97%)

2. Dissolved in plasma (~3%)

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🔵 Hemoglobin and Oxygen:

Hemoglobin (Hb) is a red pigment present in RBCs. Each molecule can bind four O₂ molecules.

✅ Reaction:

$$\text{Hb} + 4\text{O}_2 \rightarrow \text{HbO}_8 \quad (\text{Oxyhemoglobin})$$

✅ Key point:

Binding of O₂ to Hb is reversible.

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📈 Oxygen-Hemoglobin Dissociation Curve:

• The graph between % saturation of Hb with O₂ and PO₂.

• It is sigmoid (S-shaped).

✅ Important features:

• In lungs (high PO₂), Hb binds O₂ strongly.

• In tissues (low PO₂), Hb releases O₂ easily.

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🧠 Factors Affecting O₂ Binding:

• Increased CO₂

• Increased H⁺ concentration (low pH)

• Increased temperature

All these cause Bohr’s effect — promote oxygen release at tissues.

✅ Bohr's Effect:

High CO₂ levels lower the affinity of Hb for O₂.

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🧬 Carbon Dioxide Transport:

Carbon dioxide is transported in three forms:

1. Dissolved in plasma (~7%)

2. As carbaminohemoglobin (~20–25%)

3. As bicarbonate ions (HCO₃⁻) (~70%)

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🔵 Carbon Dioxide Transport in Detail:

✅ 1. Dissolved form:
Some CO₂ dissolves directly in blood plasma.

✅ 2. Carbaminohemoglobin (HbCO₂):
CO₂ binds with amino groups of hemoglobin.

✅ 3. Bicarbonate Formation (Major Method):

Inside RBCs:

$$\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{Carbonic Anhydrase}} \text{H}_2\text{CO}_3 \quad (carbonic\ acid)$$ $$\text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^-$$

Bicarbonate ions diffuse out into plasma and are carried to lungs.

✅ Key enzyme:

Carbonic anhydrase – accelerates the reaction by 5000 times!

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📤 Release of CO₂ at the Lungs:

At alveoli:

• Low CO₂ pressure

• Bicarbonate reforms into CO₂ and H₂O

• CO₂ diffuses into alveoli to be exhaled

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📈 Haldane Effect:

At lungs, the release of CO₂ is promoted by the binding of O₂ to hemoglobin.

✅ Haldane’s effect:

Oxygenation of blood in the lungs displaces CO₂ from hemoglobin, increasing CO₂ removal.

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🧠 Important Terms:

Term	Meaning
Oxyhemoglobin	Hb + O₂
Carbaminohemoglobin	Hb + CO₂
Carbonic anhydrase	Enzyme catalyzing CO₂ hydration

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⚡ Quick Recap:

Gas	Main Transport Form
O₂	Oxyhemoglobin
CO₂	Bicarbonate ions

✅ Note:

Oxygen binds loosely to Hb at tissues (helps release easily), but binds strongly at lungs (for efficient loading).

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🧠 Fun Fact:

• Hemoglobin gives blood its red color only when bound with oxygen.

• Without oxygen, deoxygenated blood is darker!

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🌟 Related Blogs to Explore:

👉 biologyatease1.blogspot.com
👉 neetpyqall.blogspot.com

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