It is important that you comprehend the main points because they ate essential for your understanding of respiratory physiology. To enter or leave the body, oxygen and carbon dioxide must cross a barrier of living tissue: In simplest animals gas exchange , takes place through the skin or through plasma membrane (as-in unicellular organism), but most multicellular animals have more elaborate respiratory mechanisms which ensure that blood and respiratory medium are brought in close contact over a large respiratory surface.
However, the most successful respiratory strategy is one that suits the medium in which the animal lives. We will deal with three contrasting mechanisms for respiration. Teleost gills, as examples of organs that take up oxygen dissolved in water, mammalian lungs as examples of air breathing organs and insect tracheal system as example of uptake and circulation-of gases without the involvement of a circulatory system. The movement of respiratory medium into and out of the respiratory organs will need to be regulated particularly when we note that the demand for oxygen increases occasionally.