respiratory system

Description
Lungs are a pair of elastic bag like structures, which has NO muscular attachment. Inside lungs are sacs called Alveoli (There are about 150 million Alveoli in Human Lung). These very tiny sacs, which are arranged in grape like clusters, are the smallest and basic component of the respiratory system. Small air passages called Bronchioles connect the alveoli to the bronchus. Bronchioles help in transfer of air between alveoli and bronchus.
Tiny blood vessels reach upto each bunch of alveoli. So they can be denoted as air-blood barriers.
Lungs are covered by a membrane. This membrane folds back upon itself to form two layered membrane around the lung. These membranes are called Pleurae. A small amount of a fluid called Pleural Fluid (which protects the lungs from external blows) is present between the pleural membranes.