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.