ABSTRACT
The natural lung is the organ in which blood exchanges oxygen and carbon dioxide with the body environment. In turn, blood brings oxygen to all body tissues, so as to oxidize the nutrients needed to sustain life. The end products of the chemical reactions that take place in tissues (globally referred to as metabolism) include carbon dioxide, water, and heat, which must all be eliminated. In mammals, oxygen is obtained from the air we breathe through diffusion at the level of the pulmonary alveoli and then carried to the tissues by the hemoglobin in the red blood cells. The carbon dioxide produced by living cells is picked up by the circulating blood and brought to the pulmonary capillaries from where it diffuses into the alveoli and is conveyed out by ventilation through the airways. These processes can be slowed down to a fraction of resting levels by hypothermia or accelerated up to 20-fold when the demand for fuel increases, as for instance with hypothermia, fever, and muscular exercise.
