Final answer:
Air flows through the air sacs of the avian respiratory system in a unidirectional manner, while blood flows in the opposite direction, creating a counter-current exchange for efficient gas exchange.
Step-by-step explanation:
Air flows in only one direction through the air sacs of the avian respiratory system, with blood flowing in the opposite direction, known as counter-current exchange. This highly efficient system enables birds to meet the high metabolic demands of flight. The unidirectional airflow is achieved through a complex system of air sacs that effectively separate the function of ventilation from gas exchange. Air enters the posterior air sacs, moves through the lungs where gas exchange takes place, and then exits via the anterior air sacs during exhalation. This type of system ensures that the oxygen concentration in the lungs remains high, facilitating a more effective diffusion of oxygen into the blood.