During normal breathing, what is the primary gas that must be expelled?

During normal breathing, the primary gas that must be expelled is carbon dioxide (CO2). This is because, during the process of respiration, oxygen is inhaled into the lungs and delivered to the body's tissues for use in cellular metabolism. As cells utilize oxygen to produce energy, they generate carbon dioxide as a waste product.

It is essential for the body to remove this carbon dioxide efficiently, as elevated levels can lead to respiratory acidosis and hinder the body’s ability to maintain a stable pH balance. The respiratory system is specifically designed to facilitate the exchange of gases; inhalation brings in oxygen while exhalation primarily removes carbon dioxide.

Oxygen is not expelled in significant quantities during normal breathing because the body requires it for metabolic processes, while carbon monoxide is typically not present in the air we breathe and poses a different set of risks, primarily related to poisoning. Nitrogen, which makes up the majority of inhaled air, is largely inert and does not directly participate in metabolic processes, thus it is not a primary concern for expulsion during normal respiration.