Anatomy of the Respiratory System
Nose and Nasal Cavity
The nose and nasal cavity form
the main external opening for the respiratory system and are the first section
of the body’s airway—the respiratory tract through which air moves. The nose is
a structure of the face made of cartilage, bone, muscle, and skin that supports
and protects the anterior portion of the nasal cavity. The nasal cavity is a
hollow space within the nose and skull that is lined with hairs and
mucus membrane. The function of the nasal cavity is to warm, moisturize, and
filter air entering the body before it reaches the lungs. Hairs and mucus
lining the nasal cavity help to trap dust, mold, pollen and other environmental
contaminants before they can reach the inner portions of the body. Air exiting
the body through the nose returns moisture and heat to the nasal cavity before
being exhaled into the environment.
Mouth
The mouth, also known as the oral
cavity, is the secondary external opening for the respiratory tract. Most normal
breathing takes place through the nasal cavity, but the oral cavity can be used
to supplement or replace the nasal cavity’s functions when needed. Because the
pathway of air entering the body from the mouth is shorter than the pathway for
air entering from the nose, the mouth does not warm and moisturize the air
entering the lungs as well as the nose performs this function. The mouth also
lacks the hairs and sticky mucus that filter air passing through the nasal
cavity. The one advantage of breathing through the mouth is that its shorter
distance and larger diameter allows more air to quickly enter the body.
Pharynx
The pharynx, also known as the throat, is a
muscular funnel that extends from the posterior end of the nasal cavity to the
superior end of the esophagus and larynx. The pharynx is divided into
3 regions: the nasopharynx, oropharynx, and laryngopharynx. The
nasopharynx is the superior region of the pharynx found in the posterior
of the nasal cavity. Inhaled air from the nasal cavity passes into the
nasopharynx and descends through the oropharynx, located in the posterior of
the oral cavity. Air inhaled through the oral cavity enters the pharynx at
the oropharynx. The inhaled air then descends into
the laryngopharynx, where it is diverted into the opening of the larynx by
the epiglottis. The epiglottis is a flap of elastic cartilage that
acts as a switch between the trachea and the esophagus. Because the pharynx is
also used to swallow food, the epiglottis ensures that air passes into the
trachea by covering the opening to the esophagus. During the process of
swallowing, the epiglottis moves to cover the trachea to ensure that food
enters the esophagus and to prevent choking.
Larynx
The larynx, also known as the voice
box, is a short section of the airway that connects the laryngopharynx and the
trachea. The larynx is located in the anterior portion of the neck, just
inferior to the hyoid bone and superior to the trachea. Several
cartilage structures make up the larynx and give it its structure. The
epiglottis is one of the cartilage pieces of the larynx and serves as the cover
of the larynx during swallowing. Inferior to the epiglottis is the thyroid
cartilage, which is often referred to as the Adam’s apple as it is most
commonly enlarged and visible in adult males. The thyroid holds open
the anterior end of the larynx and protects the vocal folds. Inferior to the
thyroid cartilage is the ring-shaped cricoid cartilage which holds the larynx
open and supports its posterior end. In addition to cartilage, the larynx
contains special structures known as vocal folds, which allow the body to
produce the sounds of speech and singing. The vocal folds are folds of mucous
membrane that vibrate to produce vocal sounds. The tension and vibration speed
of the vocal folds can be changed to change the pitch that they produce.
Trachea
The trachea, or windpipe, is a 5-inch long
tube made of C-shaped hyaline cartilage rings lined with pseudostratified
ciliated columnar epithelium. The trachea connects the larynx to the bronchi
and allows air to pass through the neck and into the thorax. The rings of
cartilage making up the trachea allow it to remain open to air at all times.
The open end of the cartilage rings faces posteriorly toward the esophagus,
allowing the esophagus to expand into the space occupied by the trachea to
accommodate masses of food moving through the esophagus.
The main function of the trachea is to
provide a clear airway for air to enter and exit the lungs. In addition, the
epithelium lining the trachea produces mucus that traps dust and other
contaminants and prevents it from reaching the lungs. Cilia on the surface of
the epithelial cells move the mucus superiorly toward the pharynx where it can
be swallowed and digested in the gastrointestinal tract.
Bronchi and Bronchioles
At the inferior end of the trachea, the
airway splits into left and right branches known as the primary bronchi. The
left and right bronchi run into each lung before branching off into smaller
secondary bronchi. The secondary bronchi carry air into the lobes of the
lungs—2 in the left lung and 3 in the right lung. The secondary bronchi in turn
split into many smaller tertiary bronchi within each lobe. The tertiary
bronchi split into many smaller bronchioles that spread throughout the
lungs. Each bronchiole further splits into many smaller branches less than a
millimeter in diameter called terminal bronchioles. Finally, the millions of
tiny terminal bronchioles conduct air to the alveoli of the lungs.
As the airway splits into the tree-like
branches of the bronchi and bronchioles, the structure of the walls of the
airway begins to change. The primary bronchi contain many C-shaped cartilage
rings that firmly hold the airway open and give the bronchi a cross-sectional
shape like a flattened circle or a letter D. As the bronchi branch into
secondary and tertiary bronchi, the cartilage becomes more widely spaced and
more smooth muscle and elastin protein is found in the walls. The bronchioles
differ from the structure of the bronchi in that they do not contain any
cartilage at all. The presence of smooth muscles and elastin allow the smaller
bronchi and bronchioles to be more flexible and contractile.
The main function of the bronchi and
bronchioles is to carry air from the trachea into the lungs. Smooth muscle
tissue in their walls helps to regulate airflow into the lungs. When greater
volumes of air are required by the body, such as during exercise, the smooth
muscle relaxes to dilate the bronchi and bronchioles. The dilated airway
provides less resistance to airflow and allows more air to pass into and out of
the lungs. The smooth muscle fibers are able to contract during rest to prevent
hyperventilation. The bronchi and bronchioles also use the mucus and cilia of
their epithelial lining to trap and move dust and other contaminants away from
the lungs.
Lungs
The lungs are a pair of large,
spongy organs found in the thorax lateral to the heart and superior
to the diaphragm. Each lung is surrounded by a pleural membrane that provides
the lung with space to expand as well as a negative pressure space relative to
the body’s exterior. The negative pressure allows the lungs to passively fill
with air as they relax. The left and right lungs are slightly different in size
and shape due to the heart pointing to the left side of the body. The left lung
is therefore slightly smaller than the right lung and is made up of 2 lobes
while the right lung has 3 lobes.
The interior of the lungs is made up of
spongy tissues containing many capillaries and around 30 million tiny sacs
known as alveoli. The alveoli are cup-shaped structures found at the end
of the terminal bronchioles and surrounded by capillaries. The alveoli are
lined with thin simple squamous epithelium that allows air entering the alveoli
to exchange its gases with the blood passing through the capillaries.
Muscles of Respiration
Surrounding the lungs are sets of muscles
that are able to cause air to be inhaled or exhaled from the lungs. The
principal muscle of respiration in the human body is the diaphragm, a thin
sheet of skeletal muscle that forms the floor of the thorax. When the diaphragm
contracts, it moves inferiorly a few inches into the abdominal cavity,
expanding the space within the thoracic cavity and pulling air into the lungs.
Relaxation of the diaphragm allows air to flow back out the lungs during
exhalation.
Between the ribs are many
small intercostal muscles that assist the diaphragm with expanding
and compressing the lungs. These muscles are divided into 2 groups: the
internal intercostal muscles and the external intercostal muscles. The internal
intercostal muscles are the deeper set of muscles and depress the ribs to
compress the thoracic cavity and force air to be exhaled from the lungs. The
external intercostals are found superficial to the internal intercostals and
function to elevate the ribs, expanding the volume of the thoracic cavity and
causing air to be inhaled into the lungs.
