In unicellular animals like Ameba, gaseous exchange takes place directly through the plasma membrane and in its surrounding medium, but higher multi-cellular animals are provided with special organs such as skin, gills or lungs through which gaseous exchange takes place. Whatever may be the organ of respiration it is subject to the following conditions:

1. It should be naked namely without an exoskeleton of any sort.

2. It should have great absorptive surface exposed to the surroundings to absorb enough oxygen to fulfill demands of the body cells.

3. It should be covered with a thin membrane permeable to gaseous exchange .

4. It should be richly supplied with blood capillaries.

(A) Respiratory pathway and

(B) Respiratory organs (lungs).

(A) Respiratory pathway includes nasal cavities pharynx, larynx and trachea.

(i) Nasal cavities are a pair of cavities in the nose, separated by a vertical cartilaginous septum and lined by ciliated mucous membrane. While passing through these cavities air is filtered, moistened and warmed. The nasal cavities open posteriorly into the pharynx.

(ii) Pharynx serves as a common passage for the transport of air to the larynx, and food from the mouth to the esophagus.

(iii) Larynx (commonly known as "Adam’s apple") is situated in the neck just at the start of the trachea. It is guarded against the entry of food particles by a flap-like valve called the epiglottis. The larynx contains the vocal cords hence it also serves as the organ of phonation   (i.e. producing sound).