The somewhat cone-shaped lungs are inside the thorax, or chest, in
 the cavity framed by the rib cage. One lung is on either side of the
     heart. The right lung has three lobes, or rounded divisions; the left has
 two lobes. The lungs are covered by a thin membrane called the
 pleura. The base of each lung rests on the diaphragm, a strong sheet
of muscle that separates the chest and abdominal cavities. In normal
breathing the diaphragm and the muscles between the ribs
automatically contract and expand in a rhythmic cycle. As the
diaphragm contracts, it moves downward and increases the volume of
the chest. The chest muscles pull the ribs outward, further increasing
the volume. The expanded space creates a partial vacuum that draws
in, or inhales, outside air. Then the muscles relax, reversing the
 process, and the lungs contract, pushing out, or exhaling, the air.

Air inhaled through the nose or mouth enters the trachea, or windpipe.
About halfway down the chest, the windpipe divides into a right and
 left bronchus, or branch. Each enters a lung, where it divides into
 smaller and smaller branches called bronchioles. The arrangement
 resembles an upside-down tree, with the trachea as the trunk.

Each bronchiole ends in a cluster of tiny air sacs called alveoli. The
lungs have about 300 million such clusters. Wrapped around each
    alveolus are pulmonary capillaries, the smallest blood vessels in the
  lungs. The capillaries are so narrow that red blood cells must pass
through in a single line. Here is where gas exchange takes place. Each
 red blood cell expels carbon dioxide and absorbs oxygen through the
thin walls of the capillary and alveolus (see Blood ).

 The cleaning system of the lungs has four main components. Lining the
 trachea and bronchial tree are cells that secrete mucus, which traps
pollutants and bacteria. Also in the bronchi are cells containing tiny
hairlike lashes, called cilia, that project into the blanket of mucus and
with constant wavelike motions push the mucus up out of the airways.
Irritating chemicals, stagnant and excessive mucus, and large bits of
 foreign matter are forcibly ejected from the bronchi by a cough. This
third important cleaning device--like breathing, under partial voluntary
control-- is a rapid muscle contraction and bronchial-tube constriction
that generates a wind force far stronger than a tornado. Small harmful
substances that make their way into the alveoli are destroyed by the
fourth line of defense, the macrophages. These are patrolling cells that
"swallow up" foreign particles or destroy them with enzymes.