Oxygen transport

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Oxygen Transport

Oxygen transport describes how O2 moves from the lungs to every metabolically active cell in the body. It involves two inseparable systems - the respiratory system (for gas exchange) and the cardiovascular system (for delivery).

1. Overview of Oxygen Delivery

Oxygen delivery (DO2) is defined as the volume of O2 delivered to the systemic vascular bed per minute:

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

The ability to deliver O2 depends on:

The amount of O2 entering the lungs
Adequacy of pulmonary gas exchange
Blood flow to the tissue (cardiac output + local vascular tone)
The capacity of blood to carry O2 (hemoglobin concentration and its affinity for O2)
Ganong's Review of Medical Physiology, 26e, p. 641

2. Forms of Oxygen in Blood

O2 is carried in blood in two forms:

A. Dissolved O2 (~2%)

Free in solution; follows Henry's Law: concentration = PO2 × 0.003 mL O2/100 mL blood/mmHg
At a normal PaO2 of 100 mmHg → only 0.3 mL O2/100 mL blood
Dissolved O2 is the ONLY form that exerts a partial pressure and drives diffusion
Alone it is grossly inadequate: at rest, O2 consumption ~250 mL/min; dissolved O2 can deliver only ~15 mL/min

B. O2 Bound to Hemoglobin (~98%)

Reversibly bound to hemoglobin inside red blood cells
Provides the large bulk of O2 transport
Costanzo Physiology, 7e, p. 223

3. Hemoglobin Structure and O2 Binding

Hemoglobin is a globular protein with 4 subunits, each containing:

A heme moiety (iron-binding porphyrin ring with Fe²+ in ferrous state)
A polypeptide chain (α or β)

Adult hemoglobin A = α₂β₂ - two α chains and two β chains.

Each subunit binds one molecule of O2, so one Hb molecule binds 4 O2 molecules total.

The stepwise binding:

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

Each reaction occurs in under 0.01 seconds.

Cooperativity (T-R model):

Deoxyhemoglobin = tense (T) configuration - low O2 affinity
After first O2 binds, bonds loosen into relaxed (R) configuration - exposes more binding sites
Net result: ~500-fold increase in O2 affinity with each successive binding
This cooperativity produces the characteristic sigmoid (S-shaped) dissociation curve
Ganong's Review, p. 641

4. O2-Binding Capacity and O2 Content

Quantity	Formula	Normal Value
O2-binding capacity	[Hb] × 1.34 mL O2/g	20.1 mL/100 mL blood
Arterial O2 content (CaO2)	(20.1 × 0.97) + 0.29	~19.8 mL/100 mL
Venous O2 content (CvO2)	(20.1 × 0.75) + 0.12	~15.2 mL/100 mL
O2 extraction per dL	CaO2 - CvO2	~4.6 mL/dL

At rest, 250 mL O2/min is transported from blood to tissues (cardiac output ~5 L/min × 4.6 mL O2/dL × 10 dL/L).

Ganong's Review, p. 641; Costanzo Physiology, p. 224

5. Oxygen-Hemoglobin Dissociation Curve (ODC)

The ODC plots % saturation of Hb against PO2:

Key points from the curve:

PO2 = 100 mmHg → SaO2 = 97.5% (arterial blood in lungs)
PO2 = 40 mmHg → SaO2 = 75% (venous blood at rest)
PO2 = 26 mmHg → SaO2 = 50% (this is the P50 - the standard reference point)

Clinical significance of the sigmoid shape:

The flat upper portion (PO2 > 60 mmHg) means Hb stays well-saturated even if alveolar PO2 falls moderately
The steep middle portion allows large amounts of O2 to be released with small PO2 drops in tissues

6. Factors Shifting the ODC

Right shift (↑ P50, ↓ O2 affinity - promotes O2 release to tissues):

Factor	Change	Mechanism
pH	Decreases	Bohr effect - H+ binds deoxyhemoglobin preferentially
Temperature	Increases	Thermal disruption of Hb-O2 bonds
PaCO2	Increases	CO2 → H+ → Bohr effect
2,3-DPG	Increases	Binds β chains, stabilizes T state

Left shift (↓ P50, ↑ O2 affinity - promotes O2 loading in lungs):

Alkalosis (↑ pH), hypothermia, low 2,3-DPG, fetal hemoglobin (HbF)
Carbon monoxide (CO) - shifts curve left AND reduces O2-carrying capacity

The Bohr Effect in context: In metabolically active tissues, CO2 and lactic acid accumulate → pH falls → curve shifts right → more O2 released exactly where it is needed.

Ganong's Review, p. 642

7. Role of 2,3-DPG

2,3-Diphosphoglycerate (2,3-DPG) is formed in red cells via glycolysis (Embden-Meyerhof pathway). It binds the β chains of deoxyhemoglobin, stabilizing the T (low affinity) state:

HbO2 + 2,3-DPG ⇌ Hb-2,3-DPG + O2

Increased 2,3-DPG (right shift) occurs with:

Chronic hypoxia (high altitude, anemia, COPD)
Exercise (within 60 min)
Thyroid hormones, growth hormone, androgens

Decreased 2,3-DPG (left shift) occurs with:

Acidosis (inhibits glycolysis)
Stored bank blood (key clinical consideration in transfusions)
Ganong's Review, p. 642

8. Hemoglobin Variants Affecting O2 Transport

Variant	Defect	Effect on O2 transport
Methemoglobin	Fe²+ oxidized to Fe³+ (by nitrites, sulfonamides)	Cannot bind O2; also shifts ODC left in remaining Hb
Carboxyhemoglobin (HbCO)	CO binds with 210× affinity of O2	Blocks O2 binding; left shift of ODC
Hemoglobin S (HbS)	β-chain mutation (αA₂βS₂); polymerizes when deoxygenated	Lower O2 affinity; sickling causes vascular occlusion
Fetal Hemoglobin (HbF)	γ chains replace β chains; lower 2,3-DPG binding	Higher O2 affinity - facilitates O2 transfer from mother to fetus

9. O2 Delivery at the Tissue Level

In pulmonary capillaries:

Venous blood arrives with PO2 ~40 mmHg; alveolar PO2 ~104 mmHg
O2 diffuses down its gradient; Hb becomes ~97.5% saturated
Due to ~2% shunt (bronchial circulation), systemic arterial PO2 ~95 mmHg

In peripheral capillaries:

Arterial PO2 = 95 mmHg; interstitial fluid PO2 ~40 mmHg; intracellular PO2 ~23 mmHg
O2 diffuses from capillary → interstitial fluid → cell
Tissue factors (↑ CO2, ↓ pH, ↑ temperature) enhance unloading via Bohr effect
Guyton and Hall Textbook of Medical Physiology, p. 522

10. Myoglobin - Intracellular O2 Storage

Myoglobin in skeletal muscle resembles hemoglobin but:

Has only 1 subunit → binds 1 O2 molecule
No cooperativity → hyperbolic (not sigmoid) dissociation curve
Higher O2 affinity than Hb → curve is shifted LEFT of Hb curve
Releases O2 only at very low PO2 (e.g., during intense exercise when blood flow is compressed)
Acts as an O2 reservoir and facilitates intracellular O2 diffusion in sustained contractions

11. Summary: O2 Transport at a Glance

Lungs (PO2 ~104 mmHg)
    → O2 loads onto Hb (97.5% saturation)
    → Arterial blood carries ~19.8 mL O2/dL
    → Cardiac output delivers O2 to tissues
Tissues (PO2 ~40 mmHg; ↑ CO2, ↑ temp, ↓ pH)
    → Bohr effect + 2,3-DPG shift ODC right
    → O2 unloads (~4.6 mL O2/dL extracted at rest)
    → Venous blood returns (~15.2 mL O2/dL, 75% saturation)

Parameter	Normal Value
Arterial PO2 (PaO2)	95 mmHg
Venous PO2 (PvO2)	40 mmHg
SaO2	97%
SvO2	75%
Hb concentration	15 g/dL (men ~16, women ~14)
O2 carrying capacity	20.1 mL/100 mL blood
O2 consumption at rest	~250 mL/min
P50	~26-27 mmHg

Sources: Ganong's Review of Medical Physiology 26e; Costanzo Physiology 7e; Guyton and Hall Textbook of Medical Physiology

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