Organization of the respiratory system. The ribs have been removed
in front, and the lungs are shown in a way that makes visible the
major airways within them.
structures for the entire animal to exchange oxygen and carbon dioxide with the external environment.
In humans and other mammals, the
includes the oral and nasal cavities, the lungs,
the series of tubes leading to the lungs, and the chest structures responsible for moving air into and out
of the lungs during breathing.
In addition to mediating gas exchange with the environment, the respiratory system serves other
functions, as listed in
Organization of the
There are two lungs, the right and left, each divided into lobes.
The lungs consist mainly of tiny air-containing sacs called
), which number approximately 300 mil-
lion in an adult. The alveoli are the sites of gas exchange with
the blood. The
are the tubes that air ﬂ ows through
from the external environment to the alveoli and back.
(inhalation) is the movement of air from
the external environment through the airways into the alve-
oli during breathing.
(exhalation) is movement
in the opposite direction. An inspiration and an expiration
During the entire respiratory
cycle, the right ventricle of the heart pumps blood through
the pulmonary arteries and arterioles and into the capillaries
surrounding each alveolus. In a normal adult at rest, approxi-
mately 4 L of fresh air enters and leaves the alveoli per min-
ute, while 5 L of blood, virtually the entire cardiac output,
ﬂ ows through the pulmonary capillaries. During heavy exer-
cise, the air ﬂ ow can increase twentyfold, and the blood ﬂ ow
ﬁ ve- to sixfold.
The Airways and Blood Vessels
During inspiration, air passes through either the nose or mouth
a passage common to both air and food
). The pharynx branches into two tubes: the
esophagus, through which food passes to the stomach, and
which is part of the airways. The larynx houses the
two folds of elastic tissue stretched horizontally
across its lumen. The ﬂ ow of air past the vocal cords causes
them to vibrate, producing sounds. The nose, mouth, phar-
ynx, and larynx are collectively termed the
The larynx opens into a long tube, the
in turn branches into two
of which enters each lung. Within the lungs, there are more
than 20 generations of branchings, each resulting in narrower,
shorter, and more numerous tubes; their names are summa-
. The walls of the trachea and bronchi
contain rings of cartilage, which give them their cylindri-
cal shape and support them. The ﬁ rst airway branches that
Functions of the Respiratory System
1. Provides oxygen.
2. Eliminates carbon dioxide.
3. Regulates the blood’s hydrogen ion concentration (pH) in
coordination with the kidneys.
4. Forms speech sounds (phonation).
5. Defends against microbes.
6. Inﬂ uences arterial concentrations of chemical messengers
by removing some from pulmonary capillary blood and
producing and adding others to this blood.
7. Traps and dissolves blood clots arising from systemic veins
such as those in the legs.