In this article we will discuss about the need for the exchange of gases in respiration.
1. Transport of oxygen in the air or the oxygen dissolved in water to organs of respiration, gills or lungs. In cases, the body surface functioning as the respiratory organ, it is in direct contact with the oxygen in water or air.
2. Diffusion of oxygen through a moist respiratory membrane to reach the transport medium.
3. Transport of oxygen from the respiratory membrane to the cells and its participation in biological oxidation.
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4. Transport of carbon dioxide produced in the cells during chemical reactions, to the exterior following the reverse route.
The gas can take part in a chemical reaction only in a dissolve state.
The quantity of a gas available for respiration varies with the solubility of the gas in water, which in turn depends on the:
(a) concentration (pressure) of the gas,
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(b) availability of the gas and
(c) temperature.
The amount of gas dissolved in water is important in respiration of aquatic animals. Dry air contains about 21 per cent O2 and 0.04 per cent CO2. Since O2 is one of the constituents of the air, the availability of O2 is expressed in terms of partial pressure (Dalton’s law of partial pressure).
The atmospheric pressure at sea level is 760 mm mercury (Hg). The percentage of O2 in air being 21, the partial pressure of O2 at sea level is 760 x 0.21 = 159.60 mm Hg. It may be said that 159.60 mm Hg at sea level is the tension of O2 in water. Presence of water vapour reduces the percentage of O2 and consequently O2 pressure. In the lung, saturated with water vapour, the partial pressure of O2 is 104 mm Hg.
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The partial pressure determines the gas to be dissolved in water. O2 is sparingly soluble in water, the amount of O2 going to solution in fresh water at 0°C is 1 ml in 100 ml; at 15°C 0.7 ml; at 20°C 0.6 ml and at 37°C about 0.5 ml. The solubility for gases decreases with the increase in temperature.
The solubility decreases also with the fall of partial pressure. Decrease in atmospheric pressure with higher altitude is associated with reduced solubility of a gas. At 6,000 in the atmospheric pressure is half of that at sea level and the amount of O2 going in solution is also one-half.
CO2 is much more soluble in water, about 30 times more than O2. It forms carbonic acid (H2CO3), which form salts — carbonates and bicarbonates. Since the solubility of O2 in water is very low, the amount of gas received by aquatic animals (water-breathers) is less than that received by those breathing air.
Both water-breathing and air-breathing animals need a constant renewal of O2 supply for respiration. Aerial respiration is superior to aquatic respiration, since O2 concentration is high in air. One-fifth of the air is O2 and the amount of air to be moved in and out of respiratory structures is five times the weight of O2.
Contrast to this, a large amount of water is to be moved in aquatic animals. At 15°C, 100 ml water saturated with O2 contains only 0.7 ml O2 (about 1 mg) and the amount of water to be moved is 1,00,000 times the weight of O2.
In land forms, O2 first dissolves in moist surface of the body or the respiratory organ. In aquatic forms, O2 dissolved in water is used. Passing of a gas across the respiratory membrane and the plasma membrane of the cell is by diffusion. The mechanism involved in the supply and removal of the gases to and from the membrane is known as ventilation.
From the respiratory organ the O2 enters the body fluid till an equilibrium is reached. The state in which the number of gas molecules passing to the liquid equals the number of gas molecules moving out from liquid is known as equilibrium.
Oxygen combined with respiratory pigment is carried by the blood or haemolymph (haemocoelomic fluid) and released in the interstitial (intercellular) fluid, from where it enters the cells through diffusion across the plasma membrane.
Respiration is a continuous process. Since the oxygen used in respiration is obtained from the surroundings, air or water, the concentration of the gas in the cells is always less than that in the source, i.e. the medium from where the gas is received. On the other hand, continuous release of carbon dioxide in the cells during respiration always leads to much higher concentration of the gas in the cells than that in the surroundings.