RESPIRATORY SYSTEM
The University of Nottingham
School of Veterinary Medicine and Science
Created by Chloe Hughes
Veterinary Student
Nares (nostrils)
The nares are surrounded by hairless skin and
supported by nasal cartilages
Species differences:
• Carnivores: a nasal plate that is divided by a
medium groove called the philtrum.
• Bovine: a nasolabial plate that has cornified
stratified epithelium and serous glands
• Equine: have an incomplete cartilaginous ring
allowing distensible nostrils
• Avian: slit-like openings with an overhanging
operculum.
• Porcine: snout contains os rostrale (bone)
Upper Respiratory System
Consists of the nasal cavity, paranasal sinuses, nasopharynx and larynx.
Functions: heating/moistening air and defence against incoming particles.
Nasal Cavity
Nasopharynx
Hard Palate
Oesophagus
Oropharynx
Larynx
Oral Cavity
Trachea
Tongue
Laryngopharynx
Epiglottis
Soft Palate
Nasal Cavity
o Paired chambers separated by a nasal septum.
o Connected anteriorly to the nares and posteriorly to the nasopharynx through
choanae.
o Divided into meatus by nasal conchae that consist of turbinate bone and the
covering respiratory mucosa (pseudostratified columnar epithelium and the
underlying lamina propria)
o Allows a defence (sneeze reflex and cilia) and warming and moistening function
Dorsal
conchae
Caudal
conchae
Vestibule
Choanae
Ventral
conchae
Nasal Cavity
Dorsal meatus
Middle meatus
Ventral meatus
Common meatus
Nasal Septum
The nasal conchae divide the nasal cavity into meatus.
In the horse:
• Dorsal meatus leads to the olfactory mucosa
• Middle meatus leads to the paranasal sinuses
• Ventral and the common meatus lead to the pharynx
The nasal cavity anatomy is fairly simple in horse compared to the complex
structure of the canine nasal cavity.
Vomeronasal Organ
In addition to the olfactory cells at
the end of the dorsal meatus, there
is also an accessory olfactory
structure, the vomeronasal organ.
The vomeronasal organ consists of
paired blind ending ducts contained
within the hard palate. It connects
the nasal and oral cavities and
contains unique chemoreceptors.
Functions:
• Pheromone detection
• Flehmen response
Vomeronasal Organ
Paranasal Sinuses
Air filled sinuses connected to the nasal cavity through the middle meatus. Functions
include resonating (voice), insulation and cooling of the brain and light weight skull
construction. The frontal and maxillary sinuses are common to all species.
The frontal sinus drains into the ethmoidal meatus (except in the horse). It has a
rostral and caudal part, with the caudal part communicating with horn in cattle.
Maxillary sinus is divided into two compartments and drains into the middle meatus. It
contains the cheek teeth (molars and premolars).
Cattle have palatine, sphenoid, lacrimal and palatomaxillary sinuses
Birds have an infraorbital sinus below the eye
Frontal sinus
Rostral
maxillary sinus
Sphenopalatine sinus
Caudal maxillary
sinus
Pharynx
The pharynx connects the oral cavity and nasal cavity to the trachea
and oesophagus. It functions as a passageway for air and food but also
as a resonating chamber. It is split into three regions, the nasopharynx,
oropharynx and laryngopharynx.
Nasopharynx
Laryngopharynx
Oropharynx
Eustachian tube and Guttural Pouch
The Eustachian tube connects the nasopharynx to the middle ear. In
the horse, an out pouching forms the guttural pouch. The guttural
pouch is approximately a 300-500cm³ space in the equine skull,
between the skull base, atlas, pharynx and oesophages. It is divided
into medial and lateral compartments by the stylohyoid bone. It
functions in cooling of the blood entering the brain.
Larynx
The larynx is a structure connecting the pharynx to the trachea and
oesophagus. It is suspended by the hyoid bones that articulate with the
base of the skull.
Functions:
• Phonation
• Swallowing
• Breathing
It consists of hyaline and elastic cartilages: cricoid, thyroid, epiglottis
and 2 arytenoid cartilages.
The larynx contains the vocal cords required for phonation.
In birds, the larynx consists of only cricoid and arytenoid cartilages. It
does not have vocal folds therefore sound is created by the syrinx at
the tracheal bifurcation
Larynx and Hyoid Apparatus
Equine larynx and
hyoid apparatus
Arytenoid
cartilage
Epiglottis
Stylohyoid
Epihyoid
(vestigial)
Cricoid
cartilage
Ceratohyoid
Basohyoid
Thyrohyoid
Thyroid
cartilage
Epiglottis
The epiglottis channels food
towards the epiglottis during
swallowing, preventing
ingesta entering the lungs
through the trachea
The larynx, and most of the
epiglottis, is lined with stratified
squamous epithelium up to the vocal
cords . The rest of the larynx is lined
by pseudostratified columnar
epithelium, like the respiratory tract.
Lower Respiratory System
The lower respiratory system consists of the trachea down to the alveoli.
Functions:
• Gas exchange
• Metabolic functions (surfactant synthesis, histamine synthesis,
activation of angiotensin)
• Body temperature regulation
• Acid-base regulation
• Vocalisation
Consists of two types of airways:
1) Conducting – carry, warm, filter and saturate air going to respiratory
airways
2) Respiratory – undergo gaseous exchange
Lungs
The lungs obtain their appearance through the deep fissures.
Canine:
• the right lung is larger and is divided into cranial, middle, caudal and
accessory lobes.
• The left lung is divided into a cranial and caudal lobe.
Equine:
• The lungs are more equal in size.
• There is no external lobation other than the accessory lobe of the right
lung.
Bovine:
• The right lung is significantly larger than the left
• The left lung is divided into a cranial and caudal lobe, with the cranial
further divided into two parts.
• The right lung is divided into cranial, middle, caudal and accessory lobes
Lower Respiratory System
Trachea
Bronchus
Conducting
System
Bronchiole
Respiratory
Bronchiole
Alveolar duct
Alveolar sac
Fallow deer lung cast
Alveolus
NB. The upper respiratory system is also part of the conducting system
Respiratory
System
Trachea
Cartilaginous C- shaped
rings
• Flexible tube held open by
cartilaginous c-shaped rings
• Rings are joined by connective
tissue
• Trachealis muscle runs along
the dorsal aspect
• lined by ciliated columnar
epithelium that functions as a
‘mucociliary escalator’
• Mucus traps small particles
which are moved towards the
pharynx by the cilia, removing
debris from the lungs.
• Various goblet cells line the
trachea, producing the mucous
coat.
Trachea
Ciliated epithelium
Glands
Cartilage
Conducting System
Bronchi
• Lined by pseudostratified epithelium and goblet cells
• Developed spiral bands of bronchial smooth muscle regulate the
volume of the conducting zone
• Outside the smooth muscle are irregular plates of bronchial
cartilage
• Surrounded by a highly vascular peribronchial sheath that contains
lymphatic vessels
Bronchiole
• The epithelium is reduced to cuboidal with no mucus secreting
glands
• Although cartilage is absent, the spiral bronchial muscle is well
developed in this region
• There is collateral ventilation (connections) between bronchioles,
making them less vulnerable to blockage
• Lacks the vascular sheath
Conducting System
Bundles of smooth
muscle
Cartilage (hyaline)
Bronchus
Blood vessel
Respiratory System
Respiratory Bronchiole:
• Has a few alveolar scattered along its wall
• Lined by typical bronchiolar epithelium, but changes to simple squamous
at alveolus entrance
• The alveoli entrance is guarded by bronchial smooth muscle
Alveolar duct:
• Have alveoli on its walls with the openings guarded by smooth muscle
Alveolar sac:
• Usually there are clusters of alveolar sacs on the end of the alveolar duct
Alveolus:
• Minute polygonal chambers whose diameter changes with breathing
• The wall is a thin irregular sandwich with the two outer surfaces formed
by epithelial cells and an inner network of capillaries
Respiratory System
Alveolar Sac
Bronchiole
Alveolar Duct
Thoracic cavity and pleura
Intercostal muscles:
• External – run in a caudoventral direction and
are responsible for inspiration
• Internal – run in a caudodorsal direction and
consists of 2 components: the interosseus,
responsible for expiration, and interchondral,
responsible for inspiration.
Pleura:
• A serous membrane lining the contents of the thoracic cavity
• Consists of a single layer of flat epithelium and underlying propria
• The thoracic cavity is lined by parietal pleura.
• Each lung is covered in visceral pleura, arranged as closed pleural sacs.
• The space between the right and left pleural sacs is the mediastinum
• The narrow space between the parietal and visceral pleura is the pleural
cavity. This is under sub-atmospheric pressure.
Diaphragm
Muscular
part
Tendinous
part
o
o
o
o
A dome shaped structure separating the thoracic and abdominal cavities.
Convex on its cranial surface.
Has a muscular peripheral part and a tendinous central area.
Muscular part has sections arising from the xiphoid process of sternum,
vertebral column and caudal ribs.
o Supplied by the phrenic nerve
o During inspiration, the diaphragm contracts to increase the volume of the
thoracic cavity, thus decreasing its pressure to draw air in.
o Vice versa occurs for expiration with the diaphragm relaxing.
Avian Respiratory Tract
The avian tract differs in the following ways:
• A choana (opening) connecting the nasal and oral cavity
• The larynx does not have an epiglottis, so muscles
constrict, closing the glottis, to allow passage of food
• The trachea is longer and wider than in mammals, and
consists of interlocking rigid cartilages.
• A syrinx is located at the trachea bifurcation and is
responsible for sound production
• Air sacs, acting as bellows, may store air during
respiration, allowing the lungs to be constantly supplied
by fresh air.
Parabronchi
• The lungs are stiffer and do not expand like mammalian
lungs due to more cartilage being present
• There is a counter current exchange in the lungs of the
bird, allowing for efficient exchange
• Birds experience a continuous unidirectional air flow
through the lungs