The purpose of the respiratory system is breathing, also called ventilation. The process of cycling air in and out of the lungs so that alveolar air that has given up oxygen and absorbed carbon dioxide can be exchanged for fresh air from the atmosphere. Ventilation is achieved by the system that alternately shifts the direction of the pressure gradient for air flow between the atmosphere and the alveoli through the expansion and retraction of the lungs. The contraction of the inspiratory muscles indirectly produces the inflation and deflation of the lungs by continuously expanding and compressing the thoracic cavity with the lungs following its movements. Since energy is required to contract these muscles, inspiration 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 by relaxing the inspiratory muscles without energy. Ventilation involves two aspects, both subject to neural control. First, the rhythmic cycle between inhalation and exhalation and the regulation of the amount of ventilation, which depends on the control of the respiratory rate and the depth of the tidal volume. The lungs can be stretched to varying degrees during the inhalation process and thus have the ability to shrink back to their size. before inspiration due to their elastic behavior. Lung compliance is the term used to indicate the dispensability of the lungs and their ability to stretch in response to a given change in transmural pressure gradient. Elastic recoil refers to the ability of the lung to return to its resting position during exhalation. The lungs can be filled to about 5.5 liters during maximum inhalation, and on the other hand emptied to... middle of paper... breathlessness. In conclusion, the respiratory system contributes to homeostasis by obtaining O2 and eliminating CO2 from the external environment. All cells in the body require an adequate amount of O2 to use to oxidize nutrient molecules to generate ATP. For example, brain cells, which are highly dependent on continuous oxygen supply, die if they are deprived of O2 for more than 4 minutes. It is clear that the precious respiratory system can be damaged by a change in chemicals in the atmosphere. This can range from car fumes to household cleaning products. Clearly cigarette smoke is a clear threat to the respiratory system due to tar and other constituents entering 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.