Oxygenation

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🫁 DIFFUSION & ALVEOLAR AIR

Q: What drives diffusion of gases in the lungs?
A: Differences in partial pressures of each gas.

Q: What is the atmospheric pressure at sea level?
A: 760 mmHg

Q: Why is alveolar PO₂ ~100mmHg, not 159mmHg like inspired air?
A: Due to:

  • Water vapour (PH₂O = 47 mmHg)

  • CO₂ in alveoli (PCO₂ = 40 mmHg)

  • RQ effect (for every 40mmHg CO₂ added, ~50mmHg O₂ leaves)

Q: Alveolar Air Equation?
A: PAO₂ = FiO₂ × (Patm - PH₂O) - (PaCO₂ / RQ)
(RQ usually 0.8)

🧪 OXYGENATION MARKERS

Q: Normal A-a Gradient?
A: 5–10 mmHg

Q: P/F Ratio (PaO₂/FiO₂)

  • Normal: 400–500

  • Used for: Evaluating ARDS severity

🧱 BARRIERS TO DIFFUSION

Q: 3 barriers O₂ must cross to get from alveoli to blood?
A:

  1. Alveolar epithelium

  2. Interstitial space

  3. Capillary endothelium

Q: What law governs gas diffusion?
A: Fick’s First Law (depends on pressure gradient, area, thickness)

🩸 OXYGEN TRANSPORT

Q: 2 ways O₂ is carried in the blood?
A:

  • Dissolved (plasma)

  • Bound to hemoglobin

Q: How much O₂ dissolves in plasma per 1 mmHg PO₂?
A: 0.003 mL/dL

Q: Normal dissolved O₂ at PaO₂ of 100mmHg?
A: 0.3 mL/dL

Q: 1g of Hb carries how much O₂ when saturated?
A: 1.34 mL

🧬 HEMOGLOBIN & SATURATION

Q: What is cooperative binding?
A: Each O₂ bound increases Hb’s affinity for the next O₂

Q: Normal SaO₂ range?
A: 95%–100%

Q: What is the clinical importance of keeping PaO₂ > 60 mmHg?
A: Below this, SaO₂ drops rapidly (steep part of oxyhemoglobin curve)

Q: Does 100% SaO₂ always mean oxygenation is fine?
A: No – PaO₂ could range from 100 to 600 mmHg with same SaO₂

📈 OXYGEN CONTENT & DELIVERY

Q: Arterial O₂ content equation (CaO₂)?
A: CaO₂ = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.003)
Normal: 16–20 mL/dL

Q: Mixed venous O₂ content (CvO₂)?
A: CvO₂ = (Hb × 1.34 × SvO₂) + (PvO₂ × 0.003)
Normal: ~15 mL/dL

Q: Normal Ca-vO₂ difference?
A: 5 mL/dL
(25% of delivered O₂ is extracted at rest)

⚡️ O₂ EXTRACTION & CONSUMPTION

Q: Oxygen Extraction Ratio (O₂ER)?
A: (CaO₂ - CvO₂) / CaO₂ × 100
Normal: 25%

Q: O₂ Delivery (DO₂) formula?
A: DO₂ = CO × CaO₂ × 10
Normal DO₂: ~1000 mL/min

Q: Fick Equation (VO₂)?
A: VO₂ = CO × (CaO₂ - CvO₂) × 10

Q: What if CaO₂ drops? How is VO₂ maintained?
A: By:

  • Increasing CO

  • Increasing O₂ extraction

  • Vasodilation to increase flow

📏 BASAL METABOLIC VO₂

Q: How to estimate VO₂ at rest?
A: Body Surface Area (BSA) × 130 mL/min
e.g. BSA = 1.9 → VO₂ = 247 mL/min

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