The purpose of the respiratory system in all animals is to exchange gases both within the organism and its environment. There are several different forms and the procdure varies but the purpose remains the same.
In mammals the respiratory system is made up of nostrils, nasal cavity, the sinuses, the pharynx, larynx, the trachea, the bronchi and two lungs. Fish have gills that act the same as lungs. Butterflies do not have neither lungs nor gills.
In mammals and reptiles air is taken into the body through the nostrils into the nasal cavity, it permeates the sinuses, and follows the pharynx at the back of the mouth. The air flows through the larynx which allows for vocal function. Then follows the trachea to the bronchi which branches the trachea and the esophagus taking air through the trachea to the lungs where the transfer of oxygen to the blood takes place through the alveoli cells.
In fish water flows through the gills where the oxygen is transferred to the blood cells. Some insects like the butterfly do not have lungs but holes along their abdomen and thorax called spiracles. These spiracles connect to tubes called trachea which are interconnected and run throughout the body. The oxygen is absorbed directly into their organs by a process called diffusion. Frogs absorb oxygen through their skin.
This vital system is extremely delicate and is susceptible to toxins and diseases. Smoke clogs the alveoli decreasing the amount of oxygen that can be transferred to the blood cells. Also ammonia and other household cleaning products are toxic to the respiratory system. Pesticides act the same on mammals as they do on insects and bugs. These products can permanently destroy the cells in the lungs.