Factors that contribute to homeostasis of the system occur throughout the entire day and night, without conscious thought. The respiratory system functions for life-sustaining activities and if the respiratory system’s tasks are interrupted for any reason for more than a couple of minutes, the body can seriously be damaged irreversibly. Damage to tissues can occur, followed by the failure of all body systems, and eventually leading to death. While the inhalation of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it has other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process necessary for the normal functioning of cells within the respiratory system. It also helps protects the body against pathogens and against toxic substances drifting in the air when inhaled. The respiratory system also houses the cells that detect smell through chemoreceptors, and assists in the production of sounds. The respiratory and circulatory systems work with one another to deliver oxygen to cells and remove carbon dioxide in a bi-phase process called respiration otherwise known as breathing. The first phase of respiration begins with inspiration or inhalation. Inhalation brings air from outside the body into the lungs through the nasal cavity to the larynx, through the trachea and finally to the lungs. Oxygen in the air moves from the lungs through the blood vessels leading to the heart, which pumps the oxygen-rich blood to all parts of the body through the cardiovascular system. Oxygen then moves from the bloodstream into cells and completes the first phase of respiration. In cells, oxygen is used as a separate process which produces energy called cellular respiration, which in turn produces carbon dioxide. The second phase of respiration begins with the movement of carbon dioxide from the cells to the bloodstream. The bloodstream moves carbon dioxide to the heart, which pumps the carbon dioxide-rich blood to the lungs. In the lungs, expiration, or exhalation, removes carbon dioxide from the body, which will complete the respiration cycle. The organs of the respiratory system start from the nose and extend to the lungs and are divided into the upper and lower respiratory tracts. The upper respiratory tract contains the nose, the pharynx, the nasal cavity, and the Para nasal sinuses.