By folding the respiratory surface into the body, terrestrial animals increase the humidity of the environment of the respiratory surface, which will minimize evaporation and maintain moisture. Note that water loss via evaporation and perspiration accounts for the majority of water loss in terrestrial mammals.

Air enters the body of most terrestrial vertebrates through the nose or the mouth; the air then passes through the trachea to narrower tubes called the bronchi, to still narrower tubes called the bronchioles. The bronchioles "dead end" into structures called alveoli, which is where gas exchange of oxygen and carbon dioxide takes place with the blood in adjacent capillaries.

The lungs contain the bronchioles, the alveoli, and part of the bronchi. The trachea carries inhaled air into the bronchi, but it is not actually enclosed by the bronchi— the lungs only enclose structures that arise after the bronchi branch away from the trachea.

During inspiration, the lungs expand as the diaphragm contracts and internal intercostal muscles relax. As the volume of the thoracic cavity increases, its pressure decreases. This creates a pressure gradient, driving air from an area of high pressure (the environment) into the area of low pressure (the lungs).

When you are not breathing, your respiratory system cannot perform its function. Unwanted gases, such as carbon dioxide, cannot be removed from the system and necessary gases, such as oxygen, cannot enter the system. This causes a buildup of carbon dioxide in the body, which leads to acidosis. Carbon dioxide is converted to carbonic acid via carbonic anhydrase. This carbonic acid builds in the blood, lowering its pH.