Gas exchange is the process by which oxygen and carbon dioxide (the respiratory gases) move in opposite directions across an organism's respiratory membranes, between the air or water of the external environment and the body fluids of the internal environment.
niche. The function or position of a species within an ecological community. A species's niche includes the physical environment to which it has become adapted as well as its role as producer and consumer of food resources.
Companion professions. The idea of companion skills is to use one profession or skill to complement another. Typically, this is a crafting profession and the corresponding gathering profession that gathers the bulk of the materials for the crafting profession.