Insects breathe through a special system of air tubes called tracheae that open to the air through spiracles. This allows air to get deep into the insect body and provide gas exchange to individual cells through tiny air capillaries . Water flow unidirectionally across the fish gills to provide fresh water continuously to the gas exchange areas. The gills have a very large surface area and bring the water very close to the fish blood. In addition the countercurrent flow of blood in the opposite direction of the water flowing across the gills, allows better equilibration between the fish blood and the water for oxygenation.
Birds also have unidirectional flow of air into and out of their lungs.
In addition, birds have air sacs at several locations. The bronchi in bird lungs divide into parabronchi and tiny airways called air capillaries run between the parabronchi. The air capillaries are tiny but numerous so they provide a huge gas exchange area . When birds breathe, they maximize gas exchange by keeping a single breath in their bodies for two cycles of inhalation and exhalation and using their air sacs as a bellows to maintain continuously the unidirectional flow of fresh air through the lungs. This is known as internal or cellular respiration .