Why do birds not suffocate during flight?

The flight of a bird is accompanied by a large expenditure of energy and oxygen. To provide the cells of organs and tissues of the bird with the necessary amount of oxygen, the blood is saturated with it on inhalation and exhalation. The mechanism of this process is as follows. When the wings are raised, the bird’s chest expands, the pressure in it decreases and the air flow enters the spongy lungs and simultaneously into the air sacs. In the lungs, blood is enriched with oxygen and carbon dioxide is removed. When the wings are lowered, the chest is compressed, air from the air sacs enters the lungs and the blood is again saturated with oxygen. The essence of double breathing consists in oxygenation of blood during inhalation and exhalation. The more often a bird flaps its wings, the more oxygen enters its body. Therefore, the birds do not suffocate during flight.