<P> In most fish, and a number of other aquatic animals (both vertebrates and invertebrates) the respiratory system consists of gills, which are either partially or completely external organs, bathed in the watery environment . This water flows over the gills by a variety of active or passive means . Gas exchange takes place in the gills which consist of thin or very flat filaments and lammelae which expose a very large surface area of highly vascularized tissue to the water . </P> <P> Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange . Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals . The respiratory system in plants includes anatomical features such as stomata, that are found in various parts of the plant . </P> <Ol> <Li> Trachea </Li> <Li> Mainstem bronchus </Li> <Li> Lobar bronchus </Li> <Li> Segmental bronchus </Li> <Li> Bronchiole </Li> <Li> Alveolar duct </Li> <Li> Alveolus </Li> </Ol> <P> In humans and other mammals, the anatomy of a typical respiratory system is the respiratory tract . The tract is divided into an upper and a lower respiratory tract . The upper tract includes the nose, nasal cavities, sinuses, pharynx and the part of the larynx above the vocal folds . The lower tract (Fig. 2 .) includes the lower part of the larynx, the trachea, bronchi, bronchioles and the alveoli . </P>

The human respiratory system is an example of what level of organization