Gas Transport

Oxygen and carbon dioxide are transported in blood through distinct mechanisms. Understanding these mechanisms is essential for interpreting arterial blood gas results and recognising disorders of gas transport.

Oxygen is transported in two forms: dissolved in plasma (a small fraction proportional to PO2) and bound to haemoglobin (the large majority). The haemoglobin-oxygen dissociation curve describes the relationship between PaO2 and haemoglobin oxygen saturation (SaO2). The curve is sigmoidal due to cooperative binding: as each oxygen molecule binds to haemoglobin, the affinity for subsequent oxygen molecules increases. This shape has physiological advantages — the flat upper portion means that SaO2 remains high even with moderate falls in PaO2, while the steep lower portion facilitates oxygen release at tissue level where PO2 is low.

The position of the curve is shifted by physiological factors. The Bohr effect describes a rightward shift (reduced O2 affinity) caused by increased PCO2, decreased pH, increased temperature, and increased 2,3-bisphosphoglycerate (2,3-BPG). A right shift increases O2 delivery to metabolising tissues. A left shift (increased O2 affinity, reduced delivery) occurs with the opposite conditions, as well as with carbon monoxide (CO) poisoning and foetal haemoglobin (HbF).

Oxygen content (CaO2) depends on both saturation and haemoglobin concentration: CaO2 = (Hb × SaO2 × 1.34) + (0.023 × PaO2). Anaemia reduces CaO2 even when PaO2 and SaO2 are normal, because there is less haemoglobin to carry oxygen. CO poisoning reduces CaO2 by binding haemoglobin with 250 times greater affinity than oxygen (forming COHb), causing a left shift and preventing O2 unloading — a double impairment.

Carbon dioxide is transported in three forms: dissolved in plasma (5–10%), bound to haemoglobin as carbamino compounds (10–20%), and as bicarbonate ions in plasma (70–80%). Carbonic anhydrase in red blood cells catalyses the rapid conversion of CO2 and water to carbonic acid, which dissociates to H⁺ and HCO3⁻. The Haldane effect describes how deoxyhaemoglobin carries more CO2 than oxyhaemoglobin; therefore, as Hb releases O2 at the tissues it picks up more CO2, and as it binds O2 at the lung it releases CO2.

Respiratory failure is classified by ABG findings: Type I (hypoxaemic) = low PaO2, normal or low PaCO2; Type II (hypercapnic) = low PaO2 + high PaCO2, indicating ventilatory failure. Cyanosis is the blue discolouration of skin/mucous membranes seen when deoxyhaemoglobin exceeds approximately 50 g/L.