 Dear students, in this topic we shall discuss the functional anatomy of mammalian lungs. The mammalian lungs are located in thoracic cavity or chest cavity. Air enters through the nostrils then passing through the pharynx it enters the trachea. Trachea say a system of branching ducts conveys air to the inner of the lungs. This branching network is divided into trachea and it forms bronchi. These bronchi have a lot of branching profusely and they form after this branching the terminal bronchioles. Terminal bronchioles are divided further and they form respiratory bronchioles. Respiratory bronchioles are connected to a cluster of alveolar ducts or sacs which are called alveoli. These terminal clusters are like this. Like in any fruit, if we look at grapefruits, branches are moving but at the end a cluster of small fruits is attached to the branch. Similarly, these alveoli are attached to alveolar ducts with respiratory bronchioles. These alveoli are present. Then the respiratory portion of the lung constitutes the terminal bronchioles, respiratory bronchioles, alveolar ducts and alveolar sacs. These are the places where gas transfer occurs. Most of the gas transfer occurs in alveolar sacs. But alveolar ducts or respiratory bronchioles or terminal bronchioles may be a little bit of gas exchange because they are involved tubules. The extensive branching in the airways which is found in the lungs that results in a decrease in the diameter of individual air ducts from the trachea to the terminal bronchioles. The higher the division, the more individual airways are reduced so that the most diameter is of trachea and the most work is of the terminal bronchioles. But along with this, where the diameter of the tubules is decreasing, the total cross-sectional area of the airways is increasing rapidly. The gas transfer surface of the lungs has been involved alveoli. The airways which lead to the terminal bronchioles, terminal bronchioles, the airways first constitute the non-respiratory portion of the lung. The airways which lead to the respiratory portion of the lung contain either cartilage or a little smooth muscle along with cartilage or only the smooth muscle. These features, the presence of cartilage and the presence of smooth muscle, they prevent the airways from collapsing even in unconscious states. The airways do not collapse because of the presence of cartilage or smooth muscles which are involuntary.