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WHAT IS KNOWN AS AIR SAC?.
Any of the air-filled extensions of the breathing apparatus of many animals. Air sacs are found as tiny sacs off the larger breathing tubes (tracheae) of insects, as extensions of the lungs in birds, and as end organs in the lungs of certain other vertebrates. Our trachea and bronchi bring the air you breathe to our lungs.  Each bronchus is shaped like a tree, with lots of smaller and smaller branches. The smallest branches are called bronchioles and at the end of these are our air sacs (alveoli). Alveoli are filled with air and look like bunches of grapes!  They are about 600 million alveoli in our lungs and they are all covered with capillaries, which is where the oxygen gets into our blood.


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