THE HUMAN RESPIRATORY SYSTEM
• The respiratory system consists of the nose,
pharynx (throat), larynx (voice box), trachea
(windpipe), bronchi, and lungs.
Structurally, the respiratory system consists of
1. The upper respiratory system includes the
nose, nasal cavity, pharynx and associated
2. The lower respiratory system includes the
larynx, trachea, bronchi and lungs.
1. The conducting zone consists of a series
of interconnecting cavities and tubes both
outside and within the lungs.
These include the nose, nasal cavity, pharynx,
larynx, trachea, bronchi, bronchioles, and
terminal bronchioles. Their function is to
filter, warm, and moisten air and conduct it
into the lungs
2. The respiratory zone consists of tubes
and tissues within the lungs where gas
These include the respiratory bronchioles,
alveolar ducts, alveolar sacs and alveoli. (Gas
exchange occur in these zone)
• It is a layer of pseudostratified ciliated
columnar epithelial cells that secrete mucus.
• Found in nose, sinuses, pharynx, larynx and
• Mucus can trap contaminants
–Cilia move mucus up towards mouth
• It provides an entrance for air in which air is
filtered by hairs inside the nostrils.
• It has two portions : the external and
– External nares (choanae) - opening to exterior
– Internal nares opening to pharynx
• The external portion is supported by a
framework of bone and cartilage covered
with skin and lined with mucous
• The internal portion is a large cavity in the
skull, merging with the external nose
anteriorly and communicating with the
• The entrance of the respiratory system that is
divided into an external portion and an internal
portion called the nasal cavity.
• Interior area of the nose; lined with a sticky
mucous membrane and contains tiny surface
hairs called cilia. It divided medially by the
• Particles trapped in the mucus are carried to
the pharynx by ciliary action, swallowed, and
carried to the stomach where gastric juice
destroys any microorganisms in the mucus.
• The anterior portion of the nasal cavity just
inside the nostrils, called the nasal vestibule,
is surrounded by cartilage.
• The superior part of the nasal cavity is
surrounded by bone.
• A vertical partition, the nasal septum, divides
the nasal cavity into right and left sides. The
anterior portion of the nasal septum consists
primarily of hyaline cartilage.
• Four bones of the skull contain paired air spaces
called the paranasal sinuses - frontal, ethmoidal,
• Add resonance to voice.
• Communicate with the nasal cavity by ducts.
• Lined by pseudostratified ciliated columnar
• The pharynx is a funnel-shaped tube
about 13 cm long that starts at the
internal nares and extends to the level of
the cricoid cartilage, the most inferior
cartilage of the larynx.
• The pharynx lies just posterior to the
nasal and oral cavities, superior to the
larynx, and anterior to the cervical
• It is a common passageway for air
• Its wall is composed of skeletal muscles and
is lined with a mucous membrane.
• The muscles of the entire pharynx are
arranged in two layers, an outer circular
layer and an inner longitudinal layer.
• Relaxed skeletal muscles help keep the
pharynx patent and contraction of the
skeletal muscles assists in deglutition
• Pharynx is divided into three regions
– Nasopharynx - uppermost portion
– Oropharynx - middle portion
– Laryngopharynx - lowermost portion
• Superior-most region of the pharynx.
• Covered with pseudostratified ciliated
– posterior to the nasal cavity
– superior to the soft palate.
– lateral walls of the nasopharynx connect to the
auditory/eustachian tubes which open into middle
• Normally, only air passes through.
• Material from the oral cavity and oropharynx is
typically blocked from entering the
nasopharynx by the uvula of soft palate, which
elevates when swallow.
• Posterior nasopharynx wall also houses a
single pharyngeal tonsil (commonly called the
• Through the internal nares, the nasopharynx
receives air from the nasal cavity along with
packages of dust-laden mucus. The cilia in
nasopharynx cilia move the mucus down
toward the most inferior part of the pharynx.
• The nasopharynx also exchanges small
amounts of air with the auditory tubes to
equalize air pressure between the pharynx and
the middle ear.
• The middle pharyngeal region.
• Immediately posterior to the oral cavity.
• Opens to the oral cavity via an archway called
– superiorly edge of the soft palate.
– inferiorly the hyoid bone.
• Common respiratory and digestive pathway
through which both air and swallowed food
and drink pass.
• Contains nonkeratinized stratified
• Lymphatic organs provide the first line of
defense against ingested or inhaled foreign
• Two pairs of tonsils, the palatine tonsils and
lingual tonsils are found in the oropharynx.
• Laryngopharynx or hypopharynx
• Inferior, narrowed region of the pharynx.
– Superiorly hyoid bone
• Inferior end it opens into the esophagus (food
tube) posteriorly and the larynx (voice box)
• Lined with a nonkeratinized stratified
• Permits passage of both food and air.
Lower Respiratory Tract
It includes conducting airway and respiratory
Composed of trachea, bronchial tree, lungs,
alveolus and alveoli.
Alveoli is the functional unit of lungs.
• Conducting airways (pharynx, trachea,
bronchi, up to terminal bronchioles).
• Respiratory portion of the respiratory system
(respiratory bronchioles, alveolar ducts, and
• Larynx or Voice box is a short, cylindrical
airway ends in the trachea. It is about 5 cm
– Superiorly it attaches to
hyoid bone and opens into th
– Inferiorly trachea.
– Posteriorly esophagus
• Position: It lies in the midline of the neck
anterior to the esophagus and the fourth
through sixth cervical vertebrae (C4–C6).
• It conducts air into the lower respiratory tract.
• Produces sounds.
• It is composed of a framework of nine pieces of
cartilage (three individual pieces and three
cartilage pairs) that are held in place by
ligaments and muscles.
• Except for the epiglottis, all laryngeal cartilages
are hyaline cartilages.
• Muscles of larynx: extrinsic muscles and
• The extrinsic muscles of the larynx connect the
cartilages to other structures in the throat.
• The intrinsic muscles connect the cartilages to
Cartilages of larynx:
• Nine c-rings of cartilage form a framework of the
• 3 unpaired
• 3 paired
• 3 unpaired cartilages
– Thyroid cartilage
– Cricoid cartilage
• 3 paired cartilages
– Corniculate cartilages
• Thyroid cartilage – (Adam’s apple) consists of
two fused plates of hyaline cartilage that form
the anterior wall of the larynx and give it a
triangular shape. The ligament that connects
the thyroid cartilage to the hyoid bone is called
the thyrohyoid membrane.
• Cricoid cartilage – ring-shaped, hyaline
cartilage that forms the inferior wall of the
• Epiglottis – large, leaf shaped piece of elastic
cartilage that is covered with epithelium.
• Arytenoid cartilages – are triangular pieces
of mostly hyaline cartilage located at the
posterior, superior border of the cricoid
cartilage. They form synovial joints with the
cricoid cartilage and have a wide range of
• Cuneiform cartilages - club-shaped elastic
cartilages anterior to the corniculate cartilages,
support the vocal folds and lateral aspects of
• Corniculate cartilages - horn-shaped pieces of
elastic cartilage, are located at the top of each
Epithelium of Larynx
• The lining of the larynx superior to the vocal
folds is non keratinized stratified squamous
epithelium. The lining of the larynx inferior
to the vocal folds is pseudostratified ciliated
columnar epithelium consisting of ciliated
columnar cells, goblet cells, and basal cells.
• The mucus produced by the goblet cells
helps to trap dust which is not removed in
the upper passages.
• The mucous membrane of
the larynx forms two pairs
of folds. A superior pair
called the ventricular folds
(false vocal cords) and an
inferior pair called the
vocal folds (true vocal
• The space between the
ventricular folds is known
as the rima vestibuli.
• Vocal cords contain elastic fibers and are
responsible for vocal sounds, which are
created when air is forced between these
folds, causing them to vibrate from side to
side. This action generates sound waves,
which can be formed into words by
changing the shapes of the pharynx and oral
cavity and by using tongue and lips.
• Glottis – a triangular slit opening containing
between the true vocal cords. Its closure
helps to prevent food or liquid from entering
Blood supply of larynx
• Blood is supplied to the larynx by the
superior and inferior laryngeal arteries and
drained by the thyroid veins, which join the
internal jugular vein.
Nerve supply of larynx
• The parasympathetic nerve supply is from
the superior laryngeal and recurrent
laryngeal nerves, which are branches of the
vagus nerves, and the sympathetic nerves
are from the superior cervical ganglia, one
on each side
• The epiglottis is a large, leaf shaped piece of
elastic cartilage that is covered with epithelium
• The “stem” of the epiglottis is the tapered inferior
portion that is attached to the anterior rim of the
thyroid cartilage and hyoid bone.
• The broad superior “leaf” portion of the epiglottis
is unattached and is free to move up and down
like a trap door. Prevents food and drink from
entering airway when swallowing
Parts of larynx
• Vocal cords
Flexible cylindrical tube - Size is 4 – 5 in long & 1 in diameter,
it is also called windpipe. Lined by ciliated pseudo
stratified columnar epithelium.
• Extends through the mediastinum and lies anterior to the
esophagus and inferior to the larynx.
• Anterior and lateral walls of the trachea supported by 15 to
20 C-shaped tracheal cartilages.
• Cartilage provide rigidity to the tracheal wall
• Posterior part of tube lined by trachealis muscle
Layers of trachea:
– Innermost layer (mucosa) = pseudostratified
columnar with cilia & goblet cells
– outer layer (submucosa) = loose connective tissue &
• At the level of the sternal angle, the trachea divides
into two smaller tubes, called the right and left
• Each primary bronchus projects laterally toward
• The inferior tracheal cartilage separates the
primary bronchi and forms an internal ridge called
It is a highly branched system of air-conducting passages that
originate from the left and right primary bronchi.
Incomplete rings of hyaline cartilage support the walls of the
primary bronchi to ensure that they remain open.
Right primary bronchus is shorter, wider, and more vertical
than the left primary bronchus.
• The left lung has two secondary bronchi. The right lung has
three secondary bronchi.
• They further divide into tertiary bronchi.
• Primary bronchi supply each lung
• Secondary bronchi supply each lobe of the lungs (3
right + 2 left)
• Tertiary bronchi splits into successive sets of
Intralobular bronchioles that supply each
bronchopulmonary segment ( right = 10, left = 8)
• Bronchioles split into Terminal bronchioles and
these split into Respiratory Bronchioles
• Respiratory Bronchioles splits into multiple Alveolar
ducts which end in an Alveolar sac
• Successive branching reduces the number
of cartilage decreases and the number of
smooth muscle increases, this allows for
variation in airway diameter, during
exertion and also sympathetic and
• The epithelial cells gradually changes from
ciliated pseudo stratified columnar
epithelium to simple cuboidal
epithelium in terminal bronchioles
• Lungs contain small saccular out pocketing called alveoli.
• They have a thin wall specialized to promote diffusion of
gases between the alveolus and the blood in the
• Gas exchange can take place in the respiratory
bronchioles and alveolar ducts as well as in the alveoli,
(range: 274–790 million). The spongy nature of the lung
is due to the packing of millions of alveoli together.
Cells in Alveolus
1. Type I alveolar cells – simple squamous cells
where gas exchange occurs
2. Type II alveolar cells (septal cells) – free surface
has microvilli – secrete alveolar fluid containing
surfactant (reduces surface tension). Hydrophilic
region and hydrophobic region)
3. Alveolar dust cells – wandering macrophages
Anatomy of the Lungs
• Lung occupies most of the space within the thoracic cavity.
It has a conical shape.
• Toward the midline, the lungs are separated from each
other by the mediastinum and this is called the
• The medial aspect of each lung and the site of entrance or
exit are called the hilum of lung.
• The structures connecting lung to the mediastinum are
called Root of the lung, it includes
– Bronchus ( Rt and Lt)
– Bronchial vessels
– Pulmonary artery
– 2 Pulmonary veins
– Pulmonary nerve plexuses
– Bronchopulmonary lymph nodes and lymphatics.
Costal surface of the lung.
• The relatively broad, rounded
surface in contact with the
thoracic wall is called the costal
surface of the lung.
• The lungs extend laterally
from the heart to the ribs on
both sides of the chest and
continue posteriorly toward
• The superior end of each lung forming the point of
the cone and the inferior end forming the base.
• The superior end of the lungs narrows to a
rounded tip known as the apex.
• The apex projects superiorly to a point that is
slightly superior and posterior to the clavicle.
• The inferior end of the lungs, known as the base.
• Divided into 2 lobes by oblique fissure
• Smaller than the right lung
• Cardiac notch accommodates the heart
• Weight- left lung - 550 gms
• Divided into 3 lobes by oblique and horizontal
• Located more superiorly in the body due to liver
on right side
• Weight- right lung - 600 gms
• Superior lobe
1. Apical segment
2. Posterior segment
3. Anterior segment
• Middle lobe
4. Lateral segment
5. Medial segment
• Inferior lobe
6. Superior segment
7. Medial-basal segment
8. Anterior-basal segment
9. Lateral-basal segment
• Superior lobe
segment(merger of "apical"
2. Anterior segment
• Lingula of superior lobe
3. Inferior lingular segment
4. Superior lingular segment
• Inferior lobe
5. Superior segment
6. Anteromedial basal segment
(merger of "anterior basal"
and "medial basal")
7. Posterior basal segment
8. Lateral basal segment
Pleura and Pleural Cavity
The rib cage is separated from the lung by a two-
layered membranous coating called the pleura.
The potential space between the serous membrane
layers is a pleural cavity.
• The pleural membranes produce a thin, serous
pleural fluid that circulates in the pleural cavity
and acts as a lubricant, ensuring minimal friction
• The outer surface of each lung and the adjacent
internal thoracic wall are lined by a serous
membrane called pleura.
• The outer surface of each lung is tightly covered by
the visceral pleura.
• while the internal thoracic walls, the lateral surfaces
of the mediastinum, and the superior surface of the
diaphragm are lined by the parietal pleura.
• The parietal and visceral pleural layers are
continuous at the hilum of each lung.
Blood supply of Lungs
• Pulmonary circulation – Supplies deoxygenated blood
pumped from the right ventricle and it is carried by
pulmonary arteries. When blood passes through the
capillaries the alveoli becomes oxygenated.
• Bronchial circulation – Supplies oxygenated blood
pumped from the left ventricle and it is carried by
bronchial arteries. This circulation is otherwise called
systemic circulation. The bronchial arteries supply blood
to the bronchi and connective tissue of the lungs. They
travel with and branch with the bronchi, ending about at
the level of the respiratory bronchioles. They anastomose
with the branches of the pulmonary arteries
Pulmonary arteries, the bronchial arteries supply nutrition
to the lungs.
A. Right clavicle
B. Right scapula
C. Right fourth anterior rib
D. Right eighth rib
E. Right costophrenic angle
F. Left lung apex
G. Aortic arch
J. Left lung base
K. Right hemidiaphragm
left lateral chest radiograph
C. lung hili
D. heart silhouette
E. lung apices
G. thoracic vertebra
H. thoracic intervertebral
I. superimposed posterior ribs
J. costophrenic angles
K. diaphragm (yellow arrows).