Transport of O2 and CO2 in the blood

Transport of O2
For the most part, O2 is transported in the blood via binding to the heme moiety of hemoglobin. Hemoglobin is tetramer, whose four chains participate in cooperative binding to O2 depending on its partial pressure. Their affinity for O2 increases for greater pO2 values. Hemoglobin is fully saturated if pO2 = 100%; at pO2 = 50%, about half of the chains are bound to O2.

This is visualized by the dissociation curve.

Increased PCO2 decreases the pH, decreases the affinity of heme to O2, and shifts the curve right – Bohr effect. This helps release oxygen in regions where it is deficient.

Transport of CO2
CO2 is produced in the tissues, diffuses in the blood. Inside erythrocytes, it reacts with water to form HCO3- and is released back into the plasma – this is the primary form in CO2 is transported.

Defects
Hypoxia: decreased delivery to or use of O2 by tissue. Multiple types:


 * Hypoxic: decreased PAO2 → decreased pO2 → decreased heme saturation. Due to higher altitude (lower pO2 in atmospheric), hypoventilation, neuromuscular diseases, defects of diffusion, V/Q ratio, right-to-left shunts
 * Anaemic: reduced transport capacity (hemoglobin problems or blood loss).
 * Ischemic: due to circulatory problems
 * Histotoxic: due to cellular inability to take up or use O2 (ex. cyanide poisoning).

Hypercapnia: abnormal retention of CO2 leads to increased partial pressure (5.3-6.65 kPa). Can be due to decreased respiration or extension of dead space.


 * mild (5.3-6.65 kPa): causes stimulation of the respiratory center (therapeutic use: pneumoxid = mixture of oxygen + 2-5% CO2 )
 * around 10 kPa: CO2 narcosis. Respiratory depression; preceded by dyspnea, headache, and confusion
 * acute (over 12 kPa): significant respiratory depression, leads to coma and death