2018B02 Discuss the factors that affect oxygen transport from the alveoli to the tissues.



·         Intro

·         Alveolus -> pulmonary capillary

·         Pulmonary capillary -> systemic arterial blood

·         Systemic arterial blood -> tissues



O2 cascade

O2 content

·   CaO2 = [Hb] x SaO2 x 1.34 + 0.003 x PaO2 (per 100mL blood)

O2 delivery

·   DO2 = CaO2 x CO


Alveolus -> pulmonary capillary:

Fick’s law

·   Hence factors increasing O2 transfer rate: ↑P1, ↓P2, ↑area, ↓thickness


·   Alveolar gas equation: PAO2 (P1) = FiO2(Patm – PSVP-H2O) – PaCO2 / RQ)

o ↑FiO2 (i.e. supplementation)

o ↑Patm (i.e. hyperbaric)

o ↓PaCO2 (↑ratio alveolar ventilation: CO2 production)


·   ↑Cardiac output (Note: perfusion limitation if at rest, at sea level, healthy lungs)

·   ↑Pulmonary capillary blood volume

·   ↑[Hb]


·   i.e. lack of oedema, fibrosis


·   i.e. lack of

o Tissue loss

o Shunt (e.g. atelectasis)

o Dead space (e.g. pulmonary embolus)


Pulmonary capillary blood -> systemic arterial blood:

Venous admixture

The amount of mixed venous (pulmonary arterial) blood that would need to be added to end-pulmonary capillary blood to account for the observed ↓ pO2 from end-pulmonary capillary to systemic artery.

·   Comprises:

o Low VQ regions

o Shunt

·   Shunt equation: calculates venous admixture as a proportion

o Qs/Qt = (CcO2 – CaO2) / (CcO2 – CvO2)

o Normal: ~0.04

Low V/Q

·   Where a region of lung is hypoventilated relative to its perfusion

·   Physiological: V and Q scatter due to gravity

·   Pathological: lung disease (e.g. COPD), vasodilators impairing HPV


·   Where blood returns to LV without any oxygenation

·   Physiological: bronchial veins 1%, Thebesian veins 0.3%

·   Lung pathology: e.g. atelectasis

·   Cardiac pathology: e.g. VSD


Systemic arterial blood -> tissues:

Fick’s law

·   Hence factors increasing O2 extraction: ↑P1, ↓P2, ↑area, ↓thickness


·   ↑VO2 -> right shift oxyhaemoglobin dissociation curve -> ↑O2 offloading

o Bohr effect: ↑pCO2, ↓pH

o ↑2,3-DPG

o ↑temperature

·   ↑Hb -> ↑size of O2 reservoir

·   ↑Capillary blood volume -> ↑size of O2 reservoir

·   ↑Cardiac output -> ↑rate of O2 replenishment


·   ↑VO2

·   ↑[Myoglobin]


·   ↑Capillarisation (e.g. adaptation to aerobic exercise)

·   ↑Recruitment and distension (e.g. exercise)


·   ↑Capillarisation -> ↓diffusion distance



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