Contents of CO2 in Blood of Human Beings | Respiration | Biology

In this article we will discuss about the contents of CO₂ in blood of human beings with the help of suitable diagrams.

CO₂ Content and Tension of Blood:

Arterial blood contains about 48 volume of CO₂ and venous blood 52 volume of CO₂ per 100 ml, tension of CO₂ being 40 mm Hg and 46 mm Hg in the arterial and venous blood respectively.

CO₂ is carried in blood in:

(a) Physical solution, and

(b) As chemical compounds.

The Table 8.12 summarises the situation.

Both the corpuscles and plasma carry CO₂, the former carries about one-fourth of the total.

i. CO₂ in Physical Solution:

Roughly about 5% of CO₂ is carried in physical solution in blood. In solution CO₂ exists mostly as H₂CO₃ according to the following equation –

Since the reaction is reversible it is clear that H₂CO₃ always remains in equilibrium with small amount of molecular O₂ gas which is responsible for CO₂ tension of blood.

The ingress of CO₂ into the blood in the tissue capillaries is not accompanied by any significant in H⁺ concentration because of the effective ‘buffering’ action of the blood and of haemoglobin in particular. The buffering property of haemoglobin is due to imidaxole group of histidine linked with haem which contains an ionisable H⁺ ion. On oxygenation, histadine parts with its H⁺ ion thus act as an acid.

On reduction the H⁺ ion is mostly linked with N and thus acts as a weaker acid. Reduced Hb, therefore, is potentially H’ ion acceptor. 1 m Mol of reduced Hb can accept 0.7 m Mol of H⁺ ion without any change of blood reaction. It has been suggested that the H⁺ of the — NH, radical of haem linked valine is also oxylabile like that of histadine and thus confers upon haemoglobin its remarkable buffering property. This explains why venous blood with reduced Hb contains more CO₂ than arterial blood at a given PCO₂ and the dissociation curve of CO₂ for venous blood is situated at higher level.

ii. CO₂ as Bicarbonate:

From the Table 8.12 it is clear that more than 80% of the CO₂ is carried as bicarbonate in the blood and that the major fraction of the carbonate is present in plasma. However, most of the bicarbonates found in plasma are primarily formed within the RBC and then shifted to plasma.

Carbon Dioxide Dissociation Curves:

With a comparative study of the CO₂ contents of reduced blood, oxygenated blood, bicarbonate solution and water, under different pressures of CO₂, important facts have been known regarding the behaviour of CO₂ of blood under different physiological conditions. The results when plotted in the form of curves constitute the CO₂ dissociation curves (Fig. (8.29).

The following facts are seen:

i. In a vacuum, CO₂ content of blood is nil.

ii. At any given CO₂ tension, reduced blood takes up larger amount of CO₂ than oxygenated blood. So that in the body, reduction of bloods in the tissue capillaries increases the degree of CO₂ uptake from the tissues.

iii. Oxygenation of blood causes evolution of CO₂. This happens in lungs.

iv. As the CO₂ tension is increased, the total amount of CO₂ taken up by blood also rises. As the CO₂ tension falls, CO₂ content also diminishes.