Molecules of oxygen gas (O2) enter the internal environment through the respiratory organs and are transported throughout the body by the blood, especially by red blood cells. Within cells, organelles use oxygen to release energy from nutrient molecules. The released energy is used to drive the cell's metabolic activities. A continuing supply of oxygen is necessary for cell survival and, ultimately, for the survival of the organism.

NO (nitric oxide) and CO (carbon monoxide) are two small chemicals with bad reputations. NO is found in smog, cigarettes, and acid rain. CO is a colorless, odorless, lethal gas that is notorious for causing death when it leaks from home heating systems or exhaust pipes in closed garages. However, NO and CO are important in physiology as biological messenger molecules. NO is involved in digestion, memory, immunity, respiration, and circulation. CO functions in the spleen, which recycles old red blood cells, and in the parts of the brain that control memory, smell, and vital functions.

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