The human body is fueled by nutrients and oxygen (O2) that are metabolized to energy, carbon dioxide (CO2), and waste products (see Figure 25.1). The amounts of O2 consumed and CO2 produced reflect the rate of body metabolism and the types of nutrients metabolized. The tasks of the respiratory and cardiovascular systems are to ensure that the cells of the body receive sufficient O2 and adequate amounts of CO2 are removed. The result of these interactions is tight coupling between the respiratory, cardiovascular, and metabolic systems. Therefore, when interpreting measurements of CO2 production and O2 consumption, it is important to consider the interaction of these systems. The overall amount of O2 consumed and CO2 produced by the human body depends on the rate of metabolism, while the proportion of O2 consumed to CO2 produced depends on the type of nutrients being metabolized or synthesized. Each cell type and organ system has a different metabolic function and, as a result, has different metabolic rates and nutrient requirements. Therefore, measurements of whole-body O2 consumption and CO2 production reflect the sum of the quantity and types of O2-consuming and CO2-producing activities of the various cell and organ systems of the body. The cells of the body metabolize carbohydrates, lipids, and proteins to produce energy in the form of high-energy phosphates (adenosine triphosphate, ATP).