Transport of O2 and CO2 in the blood

Transport of O2
Only ~1% of oxygen in the blood travels as free O2. However, it is still important as an indicator for peripheral pO2 chemoreceptors.

However, most O2 is transported in the blood by binding to the heme moiety of hemoglobin.

Hemoglobin is a tetramer (adult: 2α, 2β; fetal: 2α, 2γ), which can be described differently, depending on the state of the heme:


 * Methemoglobin (Fe3+)
 * Carboxyhemoglobin (bound to CO)
 * Carbaminohemoglobin (bound to CO2)
 * Oxyhemoglobin (bound to O2)
 * Deoxyhemoglobin (not bound)

The 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.

The greater differences in the curves at intermediate oxygen amounts are necessary for normal physiological function.

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 how 85% of CO2 is transported.

Since HCO3- left, the charge needs to be balanced, and Cl- ions are pumped in – this is known as the chloride shift or Hamburger effect. This also brings in water; venous RBCs are therefore bigger.

CO2 is also transported in a chemical bond with Hb (10%), and dissolved as a gas (5%).

Defects
Hypoxia: decreased delivery or use of O2 by tissue, when pO2 < 21 kPa

Multiple types:


 * Hypoxic: decreased decreased pO2 → decreased heme saturation. Due to higher altitude (lower pO2 in atmospheric), hypoventilation (can be mechanical - broken ribs), pathophysiological causes in regulatory centers, neuromuscular diseases, defects of diffusion, V/Q ratio, right-to-left shunts
 * Anaemic: reduced transport capacity (ex: hemoglobin problems or RBC problems).
 * Ischemic: due to circulatory problems (ex: artery obstruction, heart failure)
 * 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