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CO2 is mainly generated during cellular respiration, a process where cells generate ATP (energy molecules) by breaking down glucose and other organic molecules. In this process, CO2 is produced as a waste product and needs to be removed from the cells. The main destinations for CO2 are the lungs, where it is exchanged for O2 and expelled as we breathe. O2 is mainly sourced from the air we breathe, which contains around 21% oxygen. The major destination for O2 is cells throughout the body, where it is used in cellular respiration to produce ATP.

The transportation of CO2 and O2 in the blood involves two main components: hemoglobin (a protein found in red blood cells) and blood plasma. About 98.5% of O2 is transported in the blood bound to hemoglobin, which picks up O2 in the lungs and releases it into tissues. Only 1.5% of oxygen is dissolved in the plasma. CO2 transport is more complex. Approximately 10% of CO2 is dissolved in plasma, while 30% is bound to hemoglobin. The remaining 60% is transported as bicarbonate ions (HCO3-) that form through the following reaction: \[ CO_{2} + H_{2}O \rightleftharpoons H_{2}CO_{3} \rightleftharpoons H^{+} + HCO_{3}^{-} \] This reaction is accelerated by carbonic anhydrase, an enzyme found inside red blood cells.

Gas exchange primarily occurs in the lung's alveoli and the surrounding capillaries. Oxygen diffuses from the alveoli into the blood, where it binds to hemoglobin in red blood cells. Simultaneously, CO2 diffuses from the blood into the alveoli to be exhaled. This process is facilitated by the difference in partial pressures of O2 and CO2 between the air in the alveoli and the blood. In the body tissues, gas exchange happens at the capillaries, where O2 unbinds from hemoglobin and diffuses into the cells. At the same time, CO2 produced during cellular respiration diffuses out of the cells and into the blood. This process relies on the difference in partial pressures of O2 and CO2 between the blood and the tissues.

Gas exchange is necessary for maintaining cellular function and overall homeostasis in the body. The main physiological process that makes gas exchange necessary is cellular respiration. In this process, cells use O2 to generate ATP and produce CO2 as a waste product. It is essential that the cells continuously receive O2 and get rid of CO2 to maintain their metabolism and cellular functions. Gas exchange ensures that the blood continuously transports O2 from the lungs to the cells and CO2 from cells to the lungs. This process is vital for maintaining proper oxygenation and pH balance in the body. Without adequate gas exchange, cells would not be able to generate energy or perform essential functions, leading to tissue damage and ultimately organ failure.