gills lungs tracheae

There are three general types of respiratory systems, these are: gills, lungs, tracheae. a gill is an evaginated extension of the body surface that can be highly folded to increase the surface area. ıt is covered by a special structure called operculum. An internal circulatory system distrubutes blood through gill and body. The external circulation of water over the gill can be accomplished by pumping movements, by the action of surface ciliated cells, or by moving the gills through water.

a lung in contrast to gill, is an invaginated internalized surface. Lungs are ofter highly folded to maximize their surface area for gas exchange. Trachea are a third type respiratory system used by many air breathing invertebrates particulary arthropods. The trachea branch internally to form numerous small tubes whose blind ends lie close to all cells of the body. http://respiratory-system.blogspot.com

Gills often have a flow of water over their surface that is in the opposite direction to blood flow; ths is countercurrent flow. Birds are very specialized exchange system; there is a one way flow of air through the lungs but blood flow is a one-way essentially crosscurrent rather than countercurrent or co-current.