The role of the respiratory system is breathing, also called ventilation. The process of cyclically moving air in and out of the lungs so that alveolar air that has given up oxygen and captured carbon dioxide can be exchanged for fresh air from the atmosphere. Ventilation is achieved from the system that alternately shifts the direction of the pressure gradient for airflow between the atmosphere and the alveoli through the expansion and recoil of the lungs. Contraction of the inspiratory muscles indirectly produces inflation and deflation of the lungs by continually expanding and compressing the thoracic cavity, with the lungs following its movements. Since energy is required to contract these muscles, inhalation is known as an active process. Exhalation is known as a passive process during calm breathing because it is accomplished by the elastic recoil of the lungs during relaxation of the inspiratory muscles without energy. Ventilation involves two aspects, both subject to neural control. First, a rhythmic cycle between inspiration and expiration and the regulation of the magnitude of ventilation, which depends on the control of respiratory rate and depth of tidal volume. The lungs can be stretched to varying degrees during the process of inspiration and then have the ability to return to their full size. before inspiration due to their elastic behavior. Pulmonary conformation is the term used to refer to the dispensability of the lungs and their ability to stretch in response to a given change in the transmural pressure gradient. Elastic recoil refers to the ability of the lung to return to the resting position during expiration. The lungs can be filled to about 5.5 liters at maximum inspiration, and on the other hand empty halfway through......shortness of breath.In conclusion, the respiratory system contributes to homeostasis by obtain O2 and eliminate CO2 in the external environment. All cells in the body need an adequate amount of O2 to use in oxidizing nutrient molecules to generate ATP. For example, brain cells, which rely heavily on a continuous supply of oxygen, will die if deprived of O2 for more than 4 minutes. It is clear that the valuable respiratory system can be damaged by a variety of chemicals in the atmosphere. This can range from automobile fumes to household cleaning products. It is clear that cigarette smoke poses a clear threat to the respiratory system due to the tar and other constituents that enter the lungs and should be avoided. The respiratory system should never be taken for granted, as it is one of the most important systems in our body..