A nasal/oral cannula for collecting a flow of exhaled gases and its method of manufacture are disclosed. The cannula comprises an elongated tubular body having a first and a second end portion, a surface and an internal volume; a wall internally disposed within said tubular body, said wall defining a first subvolume of said internal volume in the lengthwise direction of the tubular body; and an inlet through said surface, for introducing exhaled gases into said first subvolume. The first end portion defines an exit port for exhaled gases from said subvolume, and said wall is arranged directly adjacent to said inlet.