Transport of oxygen and carbon dioxide through blood during respiration


I. Transport of Oxygen:

The exchange of oxygen and carbon dioxide takes place in between the lungs and blood. The greater part of oxygen diffuses into the blood and at the same time, carbon dioxide diffuses out. Here the question is where the oxygen would go.

The most part oxygen (about 97%) is now carried by the erythrocytes or R. B. Cs. In which it combines with the hemoglobin, the iron containing respiratory pigment under high concentration forming loose chemical compound the oxy-hemoglobin.


Hemoglobin is purple colored but oxy-hemoglobin is of bright red color. Along the blood stream during circulation, the oxy-hemoglobin reaches the tissues, breaks up releasing most of its oxygen, and regains its normal purple color as hemoglobin, there by the blood acts as an efficient oxygen carrier.

A small portion of oxygen (about 3%) also dissolves in the plasma and is carried in the form of solution to the tissues blood stream. Now this free oxygen, before entering into the tissue proper first passes into the tissue fluid and then enters the tissue by diffusion. In return, the carbon dioxide is given out by the tissues, dissolves in the tissue fluid and finally passes into the blood stream and conveyed of blood is 10 to 26 volumes of oxygen per 100 volumes of blood.

The oxygen transport from lungs to tissues is achieved because hemoglobin has the highest affinity for oxygen at 100 mm Hg PO2 (which is almost present in the alveolar air) and low affinity for oxygen at 40 mm Hg PO2 which is prevalent in the tissues. So oxygen readily combines with the reduced hemoglobin of Venus blood in the lungs and it is readily given off to the tissues by the arterial blood. The release of oxygen from blood is further increased by the fall in pH increased CO2 tension, and rise in temperature etc.

II. Transport of Carbon Dioxide:


The resultant carbon dioxide, which is produced from metabolism and given out by the tissue, is passed into blood through the tissue fluid and conveyed back to the respiratory surfaces along with the blood stream. But by plasma and hemoglobin of blood. Blood transports carbon dioxide in three ways, namely:

(1) As carbonic acid

(2) As bicarbonates of sodium and potassium and

(3) As carbominohemoglobin


All these compounds are reversible compounds. About 10% of total carbon dioxide is carried by the blood in the dissolved state as carbonic acid (CO2 + H2O – H2CO3) but 80% of CO2 as Sodium bicarbonate in the plasma and as potassium bicarbonate in the plasma and as potassium bicarbonate in the corpuscles and the remaining 10% as carbamino-hemoglobin (a loose compound formed by CO2 + hemoglobin).

Now the question is if they are conveyed in compound from but not in Free State how does it become free. In the blood there is an enzyme called carbonic anhydrase formed in the erythrocytes. This enzyme enhances the conversion of bicarbonates into carbonates, carbon dioxide and water by catalytic apart from this enzyme, oxy-hemoglobin also helps in releasing the carbon dioxide from various compounds. Because oxy-hemoglobin is strongly acidic and the acidity causes the release of carbon dioxide from bicarbonates, carbonic acid and carbamonohemoglobin.

Therefore, the carbon dioxide so formed is removed by diffusion before the blood leaves the lung. This transportation of gases also comes under external respiration.

In short: from tissue – diffuses into – tissue


(CO2 high concentration)

From tissue fluid – diffuses into – blood

(Free CO2)

(i) By blood plasma:


As physical solution:

10% CO2 + H2O – Carbonic anhydrase – H2CO3

(Carbon dioxide) (Water) enzyme (carbonic acid)

(ii) By R. B. C. Hemoglobin as Carbamino Compounds:

10% carbon dioxide + hemoglobin – carbamino hemoglobin

(iii) As Bicarbonate compounds: 80% carbon dioxide – in plasma as sodium bicarbonate

2Na HCO3 – Na2 CO3 + H2O + CO2

(Sodium bicarbonate)

In corpuscles as potassium bicarbonate i.e. – 2 K. H.

(Potassium bicarbonate)

CO3 – K2CO2 + H2O + CO3

Release of CO2 at the Respiratory Surface:

Carbonic acid, bicarbonates of sodium and potassium and carbamino compounds are carried to the lungs where they breakdown under the influence of various factors and liberate free CO2.

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