Abstract: The present invention is an improved flow tube for passing inhaled and exhaled gases from a subject between a respiratory connector and a respiratory analyzer as the subject breathes into the respiratory connector for analysis of the user's breadth by the respiratory analyzer. The flow tube includes a housing having an outer shell, an inner flow tube within the outer shell defining a flow pathway for passing the inhaled and exhaled gases therethrough, such that the flow pathway is in fluid communication with the respiratory connector and also a source and sink for respiratory gases. The flow tube also includes an inlet conduit extending from the housing for removably connecting the respiratory connector to the respiratory analyzer that is in fluid communication with the flow pathway, and an engagement means integral with the outer shell for removably connecting the flow tube to the respiratory analyzer.