The lungs are spongy and porous, and their tissues are very elastic.

They are covered with a delicately constructed but strong sac, known as the pleural sac, one wall of which closely adheres to the lung, and the other to the inner wall of the chest, and which secretes a fluid which allows the inner surfaces of the walls to glide easily upon each other in the act of breathing.
The Air Passages consist of the interior of the nose, pharynx, larynx, windpipe or trachea, and the bronchial tubes.

When we breathe, we draw in the air through the nose, in which it is warmed by contact with the mucous membrane, which is richly supplied with blood, and after it has passed through the pharynx and larynx it passes into the trachea or windpipe, which subdivides into numerous tubes called the bronchial tubes (bronchia), which in turn subdivide into and terminate in minute subdivisions in all the small air spaces in the lungs, of which the lungs contain millions.

A writer has stated that if the air cells of the lungs were spread out over an unbroken surface, they would cover an area of fourteen thousand square feet.
The air is drawn into the lungs by the action of the diaphragm, a great, strong, flat, sheet-like muscle, stretched across the chest, separating the chest-box from the abdomen.

The diaphragm's action is almost as automatic as that of the heart, although it may be transformed into a semi-voluntary muscle by an effort of the will.
When it expands, it increases the size of the chest and lungs, and the air rushes into the vacuum thus created.