Which gas is primarily used alongside medical air for varying concentrations?

The correct answer is nitrogen, which is used in medical settings alongside medical air for varying concentrations. Nitrogen serves as a diluent and helps achieve specific gas mixtures required for various therapeutic procedures. In medical applications, nitrogen can be used to control the concentration of oxygen in inspired gases, making it essential in the formulation of some anesthetic mixtures and other treatments.

Additionally, nitrogen plays a critical role due to its inert properties, ensuring that it does not react with other gases in the mixture, thereby allowing for predictable and controlled administration. This characteristic is key in applications such as creating hypoxic mixtures for certain respiratory therapies.

In contrast, the other options, while they have their own roles in medical practice, do not primarily function in conjunction with medical air to achieve varying gas concentrations in the same broad manner as nitrogen. Carbon dioxide has a more specific use, generally related to monitoring and treatment in respiratory care. Helium is often combined with oxygen for specific therapeutic uses, primarily in cases of obstructive airway diseases. Anesthetic gases have distinct applications and are focused primarily on providing sedation or anesthesia rather than serving as a diluent in gas mixtures alongside medical air.