Oxygen enters these spiracles and diffuses directly into the tracheal system, delivering oxygen to cells and tissues, bypassing the need for a circulatory system to transport oxygen.
There's a magnificently well preserved dinosaur from Italy, which includes the cartilaginous tracheal rings, still preserved in the throat, and muscle fibers at the base of the tail.
Finally, if the pleural effusion is large enough, it can start to push against the lung not letting it fully aerate and even causing the trachea to shift away from the side of the effusion --called tracheal deviation.