Blood from the right atrium passes down to the right ventricle, and is pumped to the lungs to be oxygenated. The blood returns to the left atrium from the lungs, and then passes to the left ventricle. The left ventricle pumps the blood to the organs of the body, via the aortic arch. The oxygen-depleted blood finally arrives back at the right atrium, and the cycle starts again. Respiratory system Birds actually use a great deal of energy to fly, and they have developed a special mechanism to help them get the oxygen they need for flight.
Birds have a series of air sacs located around each lung.
This air flow through the bird lung is more efficient than that of the other vertebrates, and explains how birds can fly at high altitudes, and can get the oxygen necessary for them to keep the flight muscles working. Digestive system Birds do not have teeth. Modern birds do not have teeth, although their ancestors that lived a hundred million years ago did have teeth. Birds do have small tongues, and the bird mouth leads to a pharynx and esophagus, rather similar to our own. At the base of the esophagus is a soft-walled chamber called a crop, where food is stored and moistened.