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Oxygen and Carbon Dioxide Transport Physiology

1. Overview

Gas transport depends on two mechanisms working together: diffusion (driven by partial pressure gradients) and blood flow (cardiac output). Hemoglobin increases O₂-carrying capacity ~70-fold and CO₂-handling reactions increase CO₂ transport ~17-fold compared to simple dissolution alone.

"The presence of hemoglobin increases the O₂-carrying capacity of the blood 70-fold, and the reactions of CO₂ increase the blood CO₂ content 17-fold." — Ganong's Review of Medical Physiology, 26th Ed.

2. Oxygen Transport

2.1 Forms of O₂ in Blood

O₂ is carried in two forms:

Form	% of Total O₂	Key Points
Dissolved O₂	~2%	Obeys Henry's Law: concentration = PO₂ × 0.003 mL/100 mL/mmHg; only dissolved O₂ creates partial pressure and drives diffusion
Bound to Hemoglobin (HbO₂)	~98%	Reversible binding; far greater capacity

Why dissolved O₂ alone is insufficient: At a normal PaO₂ of 100 mmHg, dissolved O₂ = only 0.3 mL/100 mL blood. At rest, O₂ demand is ~250 mL/min; dissolved O₂ alone delivers only ~15 mL/min (cardiac output 5 L/min × 0.3 mL/100 mL). — Costanzo Physiology, 7th Ed.

2.2 Hemoglobin Structure and O₂ Binding

Hemoglobin (Hb) is a globular protein with 4 subunits; each contains a heme moiety (iron-binding porphyrin ring with Fe²⁺) and a polypeptide chain (α or β)
Adult Hb (HbA) = α₂β₂
Each subunit binds one O₂ molecule → total: 4 O₂ per Hb molecule
The Fe²⁺ remains ferrous (Fe²⁺); the reaction is oxygenation, not oxidation

Cooperative binding (T→R transition):

Deoxygenated Hb = tense (T) configuration → low O₂ affinity
When first O₂ binds, bonds break → relaxed (R) configuration → exposes more binding sites → 500-fold increase in affinity
This cooperativity produces the characteristic sigmoid-shaped O₂-Hb dissociation curve

Sequential binding reactions:

Hb₄ + O₂ ⇌ Hb₄O₂
Hb₄O₂ + O₂ ⇌ Hb₄O₄
Hb₄O₄ + O₂ ⇌ Hb₄O₆
Hb₄O₆ + O₂ ⇌ Hb₄O₈

— Ganong's Review of Medical Physiology, 26th Ed.

2.3 Oxygen–Hemoglobin Dissociation Curve

O₂-Hemoglobin Dissociation Curve showing left and right shifts with physiological factors

Key values:

P₅₀ = PO₂ at which Hb is 50% saturated = ~26 mmHg (normal)
Normal arterial PO₂ = 100 mmHg → SaO₂ ~97–98%
Normal venous PO₂ = 40 mmHg → SvO₂ ~75%

Factors shifting the curve:

Shift	Direction	Factors	Effect
Left (↑ affinity, ↓ P₅₀)	←	↑ pH, ↓ PCO₂, ↓ temperature, ↓ 2,3-DPG	Less O₂ delivered to tissues
Right (↓ affinity, ↑ P₅₀)	→	↓ pH, ↑ PCO₂, ↑ temperature, ↑ 2,3-DPG	More O₂ delivered to tissues

Bohr Effect: CO₂ and H⁺ (↓ pH) cause a right shift — promotes O₂ unloading in metabolically active tissues.

2,3-Diphosphoglycerate (2,3-DPG):

Generated in RBCs via glycolysis
Binds to deoxyhemoglobin, stabilizing the T-state → right shift
Elevated in: anemia, chronic hypoxia, high altitude → facilitates O₂ delivery
Decreased in: stored blood (old bank blood loses 2,3-DPG) → left shift, impaired O₂ delivery

Fetal Hemoglobin (HbF):

γ chains replace β chains; γ chains bind 2,3-DPG poorly → left shift relative to adult HbA
Higher O₂ affinity facilitates O₂ transfer from maternal to fetal circulation

2.4 Oxygen Delivery (DO₂)

DO₂ = Cardiac Output (CO) × Arterial O₂ Content (CaO₂)

CaO₂ = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.003)

Where 1.34 mL O₂/g Hb = O₂-carrying capacity of each gram of hemoglobin.

DO₂ depends on:

Amount of O₂ entering the lungs
Adequacy of pulmonary gas exchange
Blood flow (cardiac output)
O₂-carrying capacity of blood (Hb concentration + affinity)

2.5 Oxygen Delivery in the Pulmonary Capillary

Alveolar PO₂ ≈ 104 mmHg; Mixed venous PO₂ ≈ 40 mmHg
Initial diffusion gradient = 104 − 40 = 64 mmHg
Blood equilibrates with alveolar gas within the first third of the pulmonary capillary
During exercise, diffusing capacity increases ~3-fold (more capillary surface area, better V/Q matching)

2.6 Myoglobin

Found in skeletal muscle; resembles Hb but binds 1 O₂ per molecule (no cooperative binding)
Dissociation curve = hyperbola (not sigmoid), shifted left relative to Hb → higher O₂ affinity
Stores O₂ in muscle; releases it only at very low PO₂ (e.g., during intense exercise when blood flow is compressed)

3. Carbon Dioxide Transport

3.1 Forms of CO₂ in Blood

CO₂ is carried in three forms:

Form	% of Total CO₂	Notes
Dissolved CO₂	~5%	Solubility = 0.07 mL/100 mL/mmHg (20× more soluble than O₂)
Carbaminohemoglobin	~3%	CO₂ bound to terminal amino groups on Hb (different site from O₂)
Bicarbonate (HCO₃⁻)	>90%	The dominant form; produced inside RBCs

"By far, HCO₃⁻ is quantitatively the most important of these forms." — Costanzo Physiology, 7th Ed.

3.2 Bicarbonate Formation – The Key Reaction

CO₂ transport diagram showing carbonic anhydrase reaction and chloride shift inside RBC

Step-by-step in systemic capillaries (tissue level):

CO₂ produced by aerobic tissue metabolism; diffuses down its partial pressure gradient into the RBC
Inside the RBC, carbonic anhydrase catalyzes: CO₂ + H₂O ⇌ H₂CO₃ (extremely fast)
H₂CO₃ dissociates: H₂CO₃ ⇌ H⁺ + HCO₃⁻
H⁺ is buffered by deoxyhemoglobin inside the RBC (forms Hb-H); deoxyhemoglobin is a better H⁺ buffer than oxyhemoglobin — convenient because Hb has already given up its O₂ to the tissues
HCO₃⁻ is exchanged for Cl⁻ across the RBC membrane via the band 3 anion exchanger protein → this is the Chloride Shift (Hamburger shift). HCO₃⁻ then travels in plasma to the lungs

In the lungs — all reactions reverse:

HCO₃⁻ re-enters RBCs (Cl⁻ exits)
H⁺ released from Hb, recombines with HCO₃⁻ → H₂CO₃ → CO₂ + H₂O
CO₂ is expired

— Costanzo Physiology, 7th Ed.

3.3 The Haldane Effect

Definition: Deoxygenation of hemoglobin increases its affinity for CO₂ (as carbaminohemoglobin) and also increases its capacity to buffer H⁺.

In tissues: Hb releases O₂ → becomes deoxyhemoglobin → binds more CO₂ and buffers more H⁺ → facilitates CO₂ loading
In lungs: Hb binds O₂ → affinity for CO₂ falls → CO₂ is released → expired

The Haldane and Bohr effects are complementary and mutually reinforcing:

Bohr effect: rising CO₂/H⁺ in tissues → Hb unloads O₂ more readily
Haldane effect: deoxygenated Hb → better CO₂ carrier and H⁺ buffer

3.4 CO₂ Dissociation Curve

Unlike the O₂–Hb dissociation curve, the CO₂ dissociation curve is nearly linear over the physiological range:

Venous blood PCO₂ ≈ 46 mmHg, CO₂ content ~52 mL/100 mL
Arterial blood PCO₂ ≈ 40 mmHg, CO₂ content ~48 mL/100 mL
~4 mL CO₂/100 mL blood is exchanged per cycle

4. Key Clinical and Allied Science Pearls

Concept	Key Point
Cyanosis	Appears when reduced Hb in capillaries >5 g/dL; most visible in nail beds, lips, mucous membranes
Carbon monoxide poisoning	CO has ~250× higher affinity for Hb than O₂; shifts curve left (impairs O₂ unloading); treatment = 100% O₂
Anemia	Reduces CaO₂ despite normal PaO₂ and saturation; 2,3-DPG rises to compensate
High altitude	Reduced alveolar PO₂ → slower O₂ equilibration along pulmonary capillary; compensated by ↑ RBC/Hb and ↑ 2,3-DPG
Pulse oximetry	Measures SaO₂ (not PaO₂); falsely elevated in CO poisoning
Carbonic anhydrase inhibitors (e.g., acetazolamide)	Block CO₂→HCO₃⁻ conversion; used at altitude (stimulates ventilation via metabolic acidosis)
Banked blood	Low 2,3-DPG → left-shifted curve → impaired O₂ delivery; replenished after transfusion in ~24 h

Summary Diagram

LUNGS                           TISSUES
Alveolar PO₂ 104 mmHg          Tissue PO₂ 40 mmHg
       │ O₂ diffuses in              │ O₂ diffuses out
       │                             │
Hb + O₂ → HbO₂              HbO₂ → Hb + O₂
(loading)                    (unloading – Bohr effect)

Alveolar PCO₂ 40 mmHg         Tissue PCO₂ 46 mmHg
       │ CO₂ diffuses out           │ CO₂ diffuses in
       │                            │
HCO₃⁻ → CO₂ (expired)     CO₂ → HCO₃⁻ (chloride shift)
                            CO₂ + Hb → HbCO₂ (Haldane effect)

Sources:

Costanzo Physiology, 7th Edition, pp. 223–231
Ganong's Review of Medical Physiology, 26th Edition, pp. 641–643
Guyton and Hall Textbook of Medical Physiology, pp. 2614–2619
Harrison's Principles of Internal Medicine, 21st Edition, p. 2909

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