STRUCTURE AND FUNCTION
a. Nose. Inhaled air is warmed, moistened, and filtered in the nasal cavities.
Filtering is done by the cilia (tiny hair-like projections) of the mucous membrane lining
the nasal passages.
b. Pharynx. The pharynx, or throat, connects the nose and mouth with the
lower air passages and the esophagus. It is divided into three sections, the
nasopharynx, the oropharynx, and the laryngopharynx. Both air and food pass through
the pharynx. Air passes from the nose and mouth to the larynx, while food passes from
the mouth to the esophagus. The walls of the pharynx contain masses of lymphoid
tissue called the adenoids and tonsils.
c. Larynx. The larynx, or voice box, connects the pharynx with the trachea.
Two membranous bands in the wall of the larynx are called vocal cords. Vibrations of
the vocal cords produce sound.
d. Trachea. The trachea, or windpipe, is a tube that carries air from the larynx
to the bronchi. It is held open by cartilaginous rings and is lined with cilia and mucous
glands to keep dust and dirt out of the lungs.
e. Bronchi. The trachea divides to form the two main bronchi. One bronchus
enters each lung and divides into many smaller air passages called bronchioles. The
bronchioles terminate in the final air spaces, called the alveoli.
(1) The lungs are the organs of respiration. They are elastic structures
contained within the thoracic cavity. The upper, pointed border of each lung, called the
apex, extends above the clavicle. The lower border, or base, rests upon the diaphragm.
Each lung is divided into sections called lobes. The right lung has three lobes, termed
upper, middle, and lower. The left lung has only two lobes, referred to as upper and
lower. Inside each lung, millions of tiny air sacs, called alveoli, are interlaced in a
network of capillaries. Certain cells in the alveolar walls secrete a lipid-rich material
called surfactant. Surfactant helps to maintain the elastic quality of the alveolar
membrane and assists with the transfer of gases.
(2) Oxygen-poor blood is pumped from the heart's right ventricle, through
the right and left pulmonary arteries, and into the lungs. Paralleling the branching of the
respiratory tree, the arteries divide and subdivide within the lungs. Arteries divide into
arterioles, and arterioles divide into the capillaries that surround the alveoli.
(3) Each lung is enclosed by a membranous sac formed of two layers of
serous membrane called pleura. One layer covers the lungs and is called the visceral
pleura. The other lines the chest cavity and is called the parietal pleura. The space
between the two layers, the pleural cavity, contains a small amount of fluid that