Carriage and Transfer of CO2 by Blood

17.3.2.3 Tidal Volume of CO₂

= Volume of CO₂ added by the tissues to every 100 ml arterial blood.

= CO2 of venous blood - CO2 of arterial blood.= 5%.

17.3.2.4 Forms of CO₂ in Arterial Blood

(I) In physical solution (5%)

(II) In chemical combination (95%)

Free CO2 gas dissolved in plasma.

H2CO3 or its dissociation products

(HCO3^- and H⁺)

(89%)

NaHCO3 in plasma and KHCO3 in RBCs., in a ratio of 3:1

(6%)

In direct combination with hemoglobin forming carbaminohemoglobin and with plasma proteins forming carbaminoproteins.

17.3.2.5 Carriage of Tidal CO₂

1. In physical solution = 5%

2. In chemical combination = 95%

a) buffered by plasma = 10%:

- Proteins: Na proteinate (basic) + H2CO3 → NaHCO₃+H Proteinate (Proteinic acid)

- Phosphates: Na₂HPO₄ (basic) + H₂CO₃ → NaHCO₃ + NaH₂PO₄.

b) buffered by Hb = 85%

a. 40% of tidal CO₂ is buffered through indi­rect reaction with K+ released from KHbO₂ At the tissue level: CO₂ produced as a result of tissue activity diffuses into plasma with water then into RBCs via its semi-permeable membrane.

- Bicarbonate formation CO₂ + H₂O₂ → H₂CO₃ → HCO₃^- + H⁺.

17.3.2.6 Inside RBCs

KHCO₃ + H⁺, then

Hb (reduced Hb) combines with H⁺ → H.Hb (very weak acid)

N.B.:

• Reduced Hb (= Hb) and oxy Hb (=HbO₂) act as weak acids in the alkaline pH of blood. They com­bine with K⁺ to form K salts.

• HbO₂ is a stronger acid (holds more K⁺) than Hb. So, on reduction of HbO₂ at the tissue level, Hb cannot hold all K⁺ of HbO2; consequently, K⁺ combines with the already formed H₂CO₃ inside RBCs to form KHCO3.

a) 25% of tidal CO₂ is buffered by the direct reaction between H₂CO₃ and K salts of Hb acids inside RBCs:

• K. Hb (very weak acid) + H₂CO₃ → H HbO₂ + KHCO3

• KHbO₂ (stronger acid than reduced Hb) + H₂CO₃ → HHb + KHCO₃

b) 20% of tidal CO₂ combines directly with the free amino group of the Hb molecule to form carbamino Hb. (= Hb. NH.COOH):

Hb. NH₂ + CO₂ → Hb. NH. COOH Hb's affinity to carry CO₂ as carbamino Hb is dou­ble that of HbO₂.

17.3.3