Download The Human Respiratory System

THE HUMAN RESPIRATORY SYSTEM
The Upper Respiratory Tract
Air enters the body either by the
or by the
. In
the latter case, it moves through nasal passages coated with
that have fine hair-like
and lined with cells
projecting into the airways. The mucous traps
,
, and other foreign materials. The constant
motion of the cilia then carries this mucous away from the lungs toward the pharynx, where it is
. (Sneezing and coughing can also expel some of this foreign material from
the body.) The air is
and
as it goes through the nasal
passages. This conditioning helps protect the delicate tissues of the lungs and the ciliated cells.
After leaving the nasal passages or the mouth cavity, the air passes into the pharynx which
connects with two passages, the
tissue, the
and the
. A flap of
, prevents food from entering the trachea. The walls of the trachea
are reinforced by rings of
which hold it open at all times. Most of the air that is
inspired (breathed in) enters the
, the enlarged upper portion of the trachea. To
get into the larynx, the air passes through the
, a slit which can open or close to
varying degrees. The larynx contains the
formed from folds of mucous membrane.
- thin strong fibers of tissue
The Lower Respiratory Tract
The trachea continues down into the
or chest cavity. Like the nasal
passages, its walls are lined with cells, some that have cilia and others that secrete mucous.
At its lower end, the trachea divides into two
. Each
then divides into smaller and smaller
. The very small bronchioles lack
the supporting rings of cartilage present in the trachea. These small bronchioles finally lead to
, each of which ends in a group of alveolar ducts
and sacs called
. The air passages perform two tasks. The bronchi and
bronchioles conduct air in and out, and the alveoli are involved in the exchange of
and
between the air and the blood.
** Each lung is enveloped by a double layer of membrane,
membrane.
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