🫁 Alveolar Gas Exchange

Define PO₂, O₂ content, and %Hb saturation.

• PO₂ (tension): partial pressure of dissolved O₂ in blood.

• O₂ content: total O₂ carried (dissolved + bound to Hb).

• %Hb saturation (SaO₂): fraction of Hb binding sites occupied by O₂.

Oxyhemoglobin Dissociation Curve

Shape and significance?

• Sigmoidal due to cooperative binding of Hb.

• Loading zone: high PO₂ (lungs) → Hb saturates rapidly.

• Unloading zone: low PO₂ (tissues) → Hb releases O₂ efficiently.

Oxyhemoglobin Dissociation Curve

Normal values?

• PO₂ 100 mmHg → SaO₂ ≈ 100%

• PO₂ 40 mmHg → SaO₂ ≈ 75% (mixed venous)

• PO₂ 25 mmHg → SaO₂ ≈ 50%

• PO₂ 15 mmHg → SaO₂ ≈ 25%

How much O₂ is dissolved in plasma?

~0.3 vol% (≈15 ml/min at rest). Insufficient for metabolism → Hb required.

Hb O₂ capacity?

1.35 ml O₂/g Hb × 15 g Hb/dl ≈ 20.3 ml O₂/dl blood.

Hb structure?

4 subunits (2α, 2β), each with Fe²⁺ heme. Cooperative, reversible O₂ binding.

Mb vs Hb?

• Mb: monomer, 1 O₂ site, in muscle, stores O₂.

• Hb: tetramer, 4 O₂ sites, in RBCs, transports O₂.

Factors shifting curve right (↓ affinity, ↑ P50)?

↑ H⁺ (↓ pH), ↑ CO₂, ↑ temperature, ↑ 2,3-BPG.

Factors shifting curve left (↑ affinity, ↓ P50)?

↓ H⁺, ↓ CO₂, ↓ temperature, ↓ 2,3-BPG.

Bohr Effect Mechanism?

↓ pH (↑ H⁺) → Hb affinity ↓ → O₂ released at tissues. At lungs: O₂ binding releases H⁺ → facilitates CO₂ unloading.

How does CO₂ affect Hb affinity?

↑ PCO₂ → right shift (↓ affinity). Mechanism: carbamino-Hb formation + carbonic anhydrase reaction → more H⁺ → Bohr effect. At lungs: ↓ PCO₂ → Hb affinity ↑ → O₂ loading.

Role of 2,3-BPG?

Negative allosteric effector. Binds β-subunit pocket in T-state Hb → stabilizes deoxyHb → ↓ affinity → enhances O₂ unloading. ↑ in chronic hypoxia → more O₂ released to tissues.

Effect of anemia?

↓ Hb content → ↓ O₂ capacity. PaO₂ & SaO₂ normal, but O₂ content ↓.

Effect of CO poisoning?

CO binds Hb with 250× affinity vs O₂ → Hb-CO.

• PaO₂ normal, SaO₂ falsely normal.

• Curve shifts left → remaining sites hold O₂ tightly → ↓ tissue release.

• Symptoms: hypoxia without cyanosis, bright red blood, headache, nausea.

• Fatal when 70–80% Hb bound to CO.

What regulates Hb synthesis?

Local tissue hypoxia sensed by kidney → EPO release → bone marrow stimulation.

Forms of CO₂ in blood?

• Dissolved: ~7%

• Carbamino-Hb: ~23%

• Bicarbonate (HCO₃⁻): ~70%

Enzyme essential for CO₂ transport?

Carbonic anhydrase (in RBCs).

What is the Haldane effect?

O₂ binding to Hb at lungs → Hb becomes stronger acid → releases CO₂. Mechanisms:

1. Acidic Hb ↓ affinity for CO₂.

2. H⁺ released binds HCO₃⁻ → CO₂ regenerated → exhaled.

What is the chloride shift?

At tissues: HCO₃⁻ diffuses out of RBC → Cl⁻ moves in to maintain electroneutrality → osmotic water entry. At lungs: reversed → HCO₃⁻ reenters RBC, Cl⁻ exits, CO₂ regenerated.

How fast do O₂ and CO₂ equilibrate in capillaries?

Both equilibrate within ~0.25 sec (1/3 capillary length).

• O₂: slower gradient, but Hb binding accelerates.

• CO₂: smaller gradient, but 20× higher solubility → diffuses rapidly.

Define hyperventilation vs hypoventilation.

• Hyperventilation: ↑ VA, constant VCO₂ → ↓ PACO₂ → hypocapnia → alkalemia.

• Hypoventilation: ↓ VA, constant VCO₂ → ↑ PACO₂ → hypercapnia → acidemia.