Q.What are the major transport mechanisms for CO2? Explain.

Carbon dioxide produced by the tissues is carried back to the lungs in three ways, in very different amounts.

• As bicarbonate — about 70 per cent. Carbon dioxide entering the blood is converted, with the help of the enzyme carbonic anhydrase, into bicarbonate and hydrogen ions at the tissues; at the alveoli the reaction reverses to regenerate carbon dioxide.
• Bound to haemoglobin — about 20 to 25 per cent, carried as carbamino-haemoglobin. It binds where carbon dioxide pressure is high (the tissues) and is released where it is low (the alveoli). …

Cells constantly produce carbon dioxide as they break down nutrients, and this waste gas has to be carried back to the lungs to be breathed out. Unlike oxygen, which relies almost entirely on one carrier, carbon dioxide is moved in three distinct forms, and the proportions are worth knowing.

Bicarbonate — the major route (about 70 per cent).

Most of the carbon dioxide is carried in this form. The key player is an enzyme called carbonic anhydrase, present in very high concentration inside the red blood cells and in small amounts in the plasma. This enzyme speeds up the interconversion of carbon dioxide and water into bicarbonate and hydrogen ions, and it works in both directions depending on local conditions:

• At the tissues, where carbon dioxide pressure is high because of catabolism, the gas diffuses into the blood and is converted into bicarbonate and hydrogen ions.
• At the alveoli, where carbon dioxide pressure is low, the reaction runs in reverse, regenerating carbon dioxide and water so the gas can be exhaled.

Carbamino-haemoglobin — bound to the pigment (about 20 to 25 per cent).

A further portion binds directly to haemoglobin to form carbamino-haemoglobin. This binding is governed by the partial pressures of the gases: in the tissues, where carbon dioxide pressure is high and oxygen pressure is low, more carbon dioxide binds; in the alveoli, where carbon dioxide pressure is low and oxygen pressure is high, it is released. …

Method: Working Out How Much of a Gas Travels by Each Transport Route

Use this approach for any question asking how a gas (O2 or CO2) is carried in the blood, and in what proportion.

Steps

Step 1: List every possible carrier for the gas

For CO2 there are three: dissolved directly in plasma, bound to haemoglobin as a carbamino compound, and converted into a chemical derivative (bicarbonate) with the help of an enzyme.

Step 2: Explain the enzyme-mediated route's reversibility …