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

Fick’s law

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

↑P₁

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

o ↑FiO₂ (i.e. supplementation)

o ↑P_atm (i.e. hyperbaric)

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

↓P₂

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

·   ↑Pulmonary capillary blood volume

·   ↑[Hb]

↑Area

·   i.e. lack of oedema, fibrosis

↓Thickness

·   i.e. lack of

o Tissue loss

o Shunt (e.g. atelectasis)

o Dead space (e.g. pulmonary embolus)

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

Shunt

·   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

Fick’s law

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

↑P₁

·   ↑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

↓P₂

·   ↑VO₂

·   ↑[Myoglobin]

↑Area

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

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

↓Thickness

·   ↑Capillarisation -> ↓diffusion distance