1. The amount of dissolved oxygen found in water is much less than that found in the air.
2. So the fishes have to breathe more in order to get enough oxygen.
3. Therefore the rate of breathing will be more in fishes when compared to terrestrial organisms.

Partial pressure of oxygen and carbon di oxide (in mmHg) in comparison to those gases in the atmosphere. 

The five primary functions of the respiratory system are
(i) To exchange O₂ and CO₂ between the atmosphere and the blood.
(ii) To maintain homeostatic regulation of body pH.
(iii) To protect us from inhaled pathogens and pollutants.
(iv) To maintain the vocal cords for normal communication (vocalization).
(v) To remove the heat produced during cellular respiration through breathing.

The primary site for the exchange of gases is the alveoli. The uptake of O₂ and the release of CO₂ occur between the blood and tissues by simple diffusion driven by partial pressure gradient of O₂ and CO₂. Partial pressure is the pressure contributed by an individual gas in a mixture of gases. It is represented as pO₂ for oxygen and pCO₂ for carbon- dioxide. Due to pressure gradients, O₂ from the alveoli enters into the blood and reaches the tissues. CO₂ enters into the blood from the tissues and reaches alveoli for elimination. As the solubility of CO₂ is 20-25 times higher than that of O₂ the partial pressure of CO₂ is much higher than that of O₂.

When a person descends deep into the sea, the pressure in the surrounding water increases which causes the lungs to decrease in volume. This decrease in volume increases the partial pressure of the gases within the lungs. This effect can be beneficial, because it tends to drive additional oxygen into the circulation, but this benefit also has a risk, the increased pressure can also drive nitrogen gas into the circulation. This increase in blood nitrogen content can lead to a condition called nitrogen narcosis. When the diver ascends to the surface too quickly a condition called 'bends' or decompression sickness occurs and nitrogen comes out of solution while still in the blood forming bubbles. Small bubbles in the blood are not harmful, but large bubbles can lodge in small capillaries, blocking blood flow or can press on nerve endings. Decompression sickness is associated with pain in joints and muscles and neurological problems including stroke. The risk of nitrogen narcosis and bends is common in scuba divers.