Study Notes: Oxygen Content, Saturation, and Hemoglobin Physiology

This section is into understanding oxygen physiology, because disorders of oxygen content and saturation often accompany hematologic conditions.

1. Oxygen Pathway in the Body

1. Inhalation:
   Air (21% oxygen) enters the lungs.

2. Gas Exchange:
   Oxygen diffuses from alveoli → arterial blood → hemoglobin (Hb).

3. Tissue Delivery:
   Hemoglobin releases oxygen → tissues → mitochondria for oxidative phosphorylation (ATP production).

4. Carbon Dioxide (CO₂) Removal:

   • CO₂ produced during metabolism returns to blood.

   • Travels as carbaminohemoglobin to lungs.

   • Exhaled during respiration.

2. Oxygen Terms & Symbols

Term	Symbol	Meaning
Fraction of Inspired Oxygen	FiO₂	% of oxygen in inhaled air (≈ 21% or 0.21)
Alveolar Oxygen Pressure	PAO₂	Partial pressure of O₂ in the alveoli
Arterial Oxygen Pressure	PaO₂	Partial pressure of O₂ in arterial blood (≈ 95–100 mmHg)
Oxygen Saturation	SaO₂	% of hemoglobin bound to oxygen (≈ 97%)

Key Point:
For oxygen to move from alveoli to blood, a pressure gradient must exist (PAO₂ > PaO₂).

3. Hemoglobin and Oxygen Saturation

• SaO₂ (Hemoglobin Oxygen Saturation):
  % of hemoglobin molecules carrying oxygen.
  “The oxygen that’s on the hemoglobin.”

• Normal: ~97%
  Problem threshold: <93% (detected with pulse oximeter)

• Measurement:

  • ABG (Arterial Blood Gas): Measures PaO₂ and SaO₂ directly.

  • Venous samples do not reflect true oxygenation levels.

4. Oxygen Content (CaO₂)

Definition:
The total amount of oxygen in the blood (both bound and dissolved).

Components:

1. Hemoglobin concentration (amount of Hb available to carry O₂)

2. Oxygen bound to hemoglobin (SaO₂)

3. Oxygen dissolved in plasma (PaO₂)

Formula (conceptual):

O₂ Content = (Hb × 1.34 × SaO₂) + (0.003 × PaO₂)

5. Oxygen Content in Anemia

• Anemia: ↓ Red cell mass → ↓ Hemoglobin concentration

• Effects:

  • PaO₂: Normal (oxygen in plasma unaffected)

  • SaO₂: Normal (each Hb still fully saturated)

  • O₂ Content: Decreased (fewer Hb molecules overall)

Example:
If Hb is low, O₂ content drops even though pulse oximeter shows 97%.
This explains why an anemic patient can appear “normal” on pulse oximetry but still have poor oxygen delivery.

6. Oxygen–Hemoglobin Dissociation Curve

• Y-axis: SaO₂ (% saturation)

• X-axis: PaO₂ (mmHg)

At the lungs:
Oxygen binds to Hb → ↑ PaO₂, ↑ SaO₂

At the tissues:
Oxygen leaves Hb → ↓ PaO₂, ↓ SaO₂

The curve is S-shaped due to cooperative binding between oxygen molecules.

7. Curve Shifts

Right Shift

→ Hb releases oxygen more easily (to tissues)

Cause	Explanation
↑ H⁺ (↓ pH, acidosis)	Bohr effect
↑ Temperature	Fever, exercise
↑ 2,3-BPG	Chronic hypoxia, altitude
↑ CO₂	Hypercapnia

Mnemonic: “Right = Release” (right hand gives oxygen to tissues)

Left Shift

→ Hb holds onto oxygen (less release to tissues)

Cause	Explanation
↓ H⁺ (↑ pH, alkalosis)	Opposite of Bohr effect
↓ Temperature	Hypothermia
↓ 2,3-BPG	Fetal hemoglobin, stored blood
Methemoglobin, HbF	Stronger oxygen affinity

Mnemonic: “Left = Left behind” (oxygen stays on Hb; tissue left behind)

8. Clinical Case (Methemoglobinemia)

Case Summary:

• 31-year-old man treated with lidocaine for sore throat

• Returns with dusky skin, blue lips, headache, fatigue

• Low SaO₂ but PaO₂ normal or high

Diagnosis:
Methemoglobinemia — Iron in Hb oxidized to Fe³⁺ form, which cannot bind oxygen properly.
Blood appears dark brown; pulse oximetry shows low saturation despite normal PaO₂.

9. Key Takeaways

• FiO₂: Oxygen entering the body (21%)

• PaO₂: Oxygen dissolved in plasma

• SaO₂: Oxygen bound to hemoglobin

• O₂ Content: Total oxygen carried in the blood

• Anemia: Normal saturation, but low O₂ content

• Right shift: Easier O₂ delivery

• Left shift: Harder O₂ delivery

Summary:

Concept	Increased	Decreased	Result
pH (↑, alkalosis)	Left shift		↓ O₂ delivery
pH (↓, acidosis)		Right shift	↑ O₂ delivery
Temperature ↑	Right shift		↑ O₂ release
2,3-BPG ↑	Right shift		↑ O₂ release
HbF, MetHb	Left shift		↓ O₂ release
Anemia	↓ O₂ content	Normal SaO₂	↓ O₂ delivery