Partial pressures of gases: composition of air is: The partial pressure of a gas is the pressure that a particular gas exerts when it is in a gas mixture: 79. 04% nitrogen, 20. 93% oxygen, 0. 03% carbon dioxide. General forms of respiratory organs: ventilation can either be: There are numerous forms of gas exchange organs. Tidal ventilation involves moving the respiratory medium in and out of a respiratory organ (ex. Mammalian lung, air goes in and comes out along the same pathway): unidirectional ventilation involves moving the respiratory medium in one direction only (ex. In fish, water moves in a unidirectional manner, it enters through the mouth and out across the operculum): non-directional ventilation occurs with external gills that are free-floating in the respiratory media (ex. Respiratory systems: external gills protrude from the side of the animal, ventilation is generally passive and non-directional, oxygen diffuses from the water, across the gills into the blood.