plz..explain transport of oxygen & transport of carbon dioxide in our body???

Oxygen and carbon dioxide are transported in the body by the blood. Both oxygen and carbon dioxide bind with the haemoglobin of red blood cells to form intermediate compounds called oxyhaemoglobin and carbaminohaemoglobin. Carbon dioxide also dissolves in the plasma to some extent. The gases are exchanged between blood and cells depending upon their respective partial pressure. They flow from their higher partial pressure to lower partial pressure by detaching from the haemoglobin molecule.

View Full Answer

Carbon Dioxide Transport

Bicarbonate Buffer:

Carbon dioxide produced in the tissue cells diffuses into the blood plasma. The largest fraction of carbon dioxide diffuses into the red blood cells. The carbon dioxide in the red blood cells is transported as: dissolved CO₂, combined with hemoglobin, or as bicarbonate,(largest fraction).

The formation of bicarbonate ions, (HCO₃^- ) takes place by the following reactions:

Hydration of CO₂: CO₂ + HOH === H₂CO₃
Dissociation of H₂CO₃: H₂CO₃ === H⁺ + HCO₃^-

The H₂CO₃/HCO₃^- combination acts as the primary buffer of the blood. The hydration of carbon dioxide is a slow process but occurs rapidly in the red blood cells because a high concentration of the enzyme carbonic anhydrase catalyzes the reaction.

Bicarbonate diffuses out of the red blood cells into the plasma in venous blood and visa versa in arterial blood. Chloride ion always diffuses in an opposite direction of bicarbonate ion in order to maintain a charge balance. This is referred to as the "chloride shift".

The changes in concentration of CO₂ or HCO₃^- ion can influence slight pH changes in the blood even though it is buffered. At the same time the concentration of H⁺ ions will influence the concentrations of CO₂ and HCO₃^- ions.

Click for larger image

Combined Oxygen and Carbon Dioxide Transport:

The reactions for both oxygen and carbon dioxide are coupled together and work in cooperation with each other. The main reason for this "coupled" effect is that both systems are influenced by hydrogen ions and equilibrium principles.

At the lungs, the diffusion of oxygen into the blood triggers the reactions. The oxygen reacts with and attaches to hemoglobin. This oxygenation reaction with hemoglobin produces excess H⁺ ions which react with HCO₃^- to produce H₂CO₃. The carbonic acid decomposes to CO₂which diffuses out of the blood.

Oxygen Transport

General Cell Metabolism

Cell metabolism is based on the same general principle as the combustion of any fuel, whether it be in the automobile, power plant, or a home furnace. The general combustion reaction is:

CH₂O (fuel) + O₂ ===> CO₂ + HOH

The same reaction occurs in the cells. The "fuel" comes from food in the form of carbohydrates, fats, and proteins.

The important principle to remember is that oxygen is needed by the cell and that carbon dioxide is produced as a waste product of the cell. Carbon dioxide must be expelled from the cells and the body.

The lungs serve to exchange the two gases in the blood. Oxygen enters the blood from the lungs and carbon dioxide is expelled out of the blood into the lungs. The blood serves to transport both gases. Oxygen is carried to the cells. Carbon dioxide is carried away from the cells.

Click for larger image

Gaseous Diffusion:

Partial pressures are used to designate the concentrations of gases. Dalton's Law of Partial Pressures states that the total pressure of all gases is equal to the sum of the partial pressures of each gas. For example, the total atmospheric pressure of air is 760 mm Hg. In equation form:

P(total air) = P(O ₂) + P(N₂) + P(CO₂) + P(HOH)

760 = 160 + 594.7 + 0.3 +5.0

The partial pressures for oxygen and carbon dioxide in various locations are given in Figure 1. The movement or exchange of gases between the lungs, blood, and tissue cells is controlled by a diffusion process.

The gas diffusion principle is: A gas diffuses from an area of higher partial pressure to an area of lower partial pressure.

QUES. Starting with O₂ in inhaled air, describe in your own words the sequence of gas movements from the inhalation of oxygen to the exhalation of carbon dioxide.

Click for larger image

Oxygen Transport

In the lungs, oxygen diffuses from alveolar air into the blood because the venous blood has a lower partial pressure. The oxygen dissolves in the blood. Only a small amount is carried as a physical solution (0.31 ml per 100 ml). The remainder of the oxygen is carried in chemical combination with the hemoglobin in red blood cells (erthrocytes).

Hemoglobin (molecular weight of 68,000) is made from 4 hemes, a porphyrin ring containing iron and globin, a 4 protein chains. Oxygen is bound to the iron for the transport process. Hemoglobin (HHgb) behaves as a weak acid (K = 1.4 x 10^-8; pKa = 7.85). Oxyhemoglobin (HHgbO₂) also behaves as a weak acid (K = 2.5 x 10^-7; pKa = 6.6)

Because both forms of hemoglobin are weak acids, and a relationship of the numerical values of the equilibrium constants, the net reaction for the interaction of oxygen with hemoglobin results in the following equilibrium:

HHgb + O ₂ <===> HgbO ₂ + H⁺

If ₂ is increased in the blood at the lungs, the equilibrium shifts to
the right and H⁺ ions increase.

Oxyhemoglobin can be caused to release oxygen by the addition of H⁺ ions at the cells. The difference in pH (7.44) of arterial blood and venous blood (pH = 7.35) is sufficient to cause release of oxygen from hemoglobin at the tissue cells.