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The Respiratory System (Wednesday, August 15, 2007)
1) Identify and describe the structure and function of the organs of the
respiratory system
2) comprehend ventilation and the exchange of gases between lungs, blood, and
cells
3) Describe the briefly on the control mechanisms for respiration.
Respiratory system of farm animals.
What is respiration? Why do animals respire and why is it important?
In animal physiology, respiration is the transport of oxygen from the ambient air to
the tissue cells and the transport of carbon dioxide in the opposite direction
This is in contrast to the biochemical definition of respiration, which refers to
cellular respiration: the metabolic process by which an organism obtains energy by
reacting oxygen with glucose to give water, carbon dioxide and ATP (energy).
Or
We can state respiration as the process by which animals take in oxygen necessary
for cellular metabolism and release the carbon dioxide that accumulates in their
bodies as a result of the expenditure of energy. When an animal breathes, air or
water is moved across such respiratory surfaces as the lung or gill in order to help
with the process of respiration.
Although physiologic respiration is necessary to sustain cellular respiration and thus
life in animals, the processes are distinct: cellular respiration takes place in
individual cells of the animal, while physiologic respiration concerns the bulk flow
and transport of metabolites between the organism and external environment.
In unicellular organisms, simple diffusion is sufficient for gas exchange: every cell
is constantly bathed in the external environment, with only a short distance for
gases to flow across. In contrast, complex multicellular organisms have a much
greater distance between the environment and their innermost cells, thus, a
respiratory system is needed for effective gas exchange.
The word respiration describes two processes.
The respiratory system’s primary function is the transport of oxygen and carbon
dioxide between the environment and the tissues. Defense is the next function.
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The regulation of body pH beside its essential requirement in
cellular metabolism;
The regulation of body pH is important because some organs, tissues, and various
types of cells are more affected by changes in pH than others. Therefore, within
an animal’s body are various mechanisms, including mechanisms at the cellular level
that regulate the body pH in order for the animal to maintain normal bodily
functions. For example, the regulation of body pH is needed in animals in order to
stabilize volume of hydrogen ions and to regulate enzyme activity. Within cells, pH
is regulated in order for cellular functions to proceed. At the tissue level, the body
has the ability to redistribute acid between body compartments because some
tissues have the ability to tolerate much larger fluctuations in pH than others do.
In general, animals have a body pH that is on the alkaline side of neutral, which
means that there is less hydrogen than hydroxyl ions in the body. Human blood
plasma, at 37° C (normal body temperature) has a pH of 7.4. Normal functioning can
be maintained in mammals at 37° C over a blood plasma pH range of 7.0-7.8.
Breathing regulates pH of the body. One of the main ways that a mammal
regulates pH is through the control of respiration. For example, if the body pH in a
mammal decreases, the respiration rate and depth of respiration increases in order
to get rid of the excess CO2, which brings H+ levels back down and brings pH back
up. Hence, when breathing is increased, CO2 levels in the blood decline and pH
increases. If pH increases, respiration rate decreases, thereby increasing CO2
levels, which forms more carbonic acid and brings pH back down.
In mammals, a stable body pH is achieved by adjusting the release of CO2 through
the lungs and excretion of acid or bicarbonate through the kidneys, so that acid
excretion and production are balanced. The collecting duct of the mammalian kidney
has acid-excreting and base-excreting cells, which can be altered to increase or
decrease acid or base excretion. In aquatic animals, the external surfaces have the
capacity to extrude acid in similar ways to the collecting duct of the mammalian
kidney. For example, a protein ATPase exists in the skin of frogs and gills of
freshwater fish which excretes protons on the apical surface of the epithelium.
Fish gills also have a HCO3-/Cl- exchange mechanism, which aids in the regulation of
body pH.
Respiration consists of:
1) inspiration, or the expansion of the chest or thorax (the part of the body
between the neck and abdomen containing the heart and lungs); and
2) Expiration or the expulsion of air from the lungs.
In examining respiration in an animal, check movement and sound at the nostrils
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and in the chest area.
Internal respiration is the process by which glucose or small molecules are
oxidized to produce energy. The 02 is used and CO2 is generated.
External respiration (breathing) involves simply the stage of taking from the
air and returning CO2 to it.
The essential organs of respiration are the lungs, in which gaseous exchange
takes place between the inspired air and the blood stream.
Variations in rate of respiration can be caused by many factors including body
size, age, exercise, excitement, environmental temperature, atmospheric
conditions, pregnancy, and fullness of the digestive tract. If variations in
respiration rates are encountered and if environmental conditions are
suspected as being a possible cause, it’s a good idea to check the rate of two or
three other animals for comparative purposes.
The normal range in respiratory rate
in mature animals at rest is:
Horse
Beef cow
Dairy cow
Sheep and Goat
Pig
8 to 16 per minute
10 to 30 per minute
18 to 28 per minute
12 to 20 per minute
8 to 18 per minute
Give attention to the following factors:
a. Rate –number of inspirations per minute.
b. Depth – the intensity or indication of straining.
c. Character – normal breathing involves an observable expansion and
relaxation of the ribs (costa) and abdominal wall. Any interference in
breathing that may show more or less effort in either of these areas
affects the character of the breathing.
d. Rhythm – change in duration of inspiration and expiration.
e. Sound – normal breathing is noiseless except when the animal is exercising
or at work. Snuffling, sneezing, wheezing, rattling, or groaning may indicate
something abnormal.
f. Dyspnea – laboured or difficult breathing.
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Organs of respiratory system
- Nostrils
- Nasal cavity
- Pharynx
- Larynx
- Trachea
- Primary bronchi
- Secondary bronchi
- Bronchioles
- Alveolus
The respiratory tract, where external respiration occurs, starts at the nose and
mouth. There is a brief complication where the air stream crosses the path taken
by food and water in the pharynx: air flows on down the trachea where food
normally passes down the oesophagus to the stomach / rumen. And respiratory
apparatus will be discussed as follows;
The nose (nasus) in the broad sense comprises the external nose, the paired
nasal cavities, and the paranasal sinuses (air-filled, mucus-lined cavities in the head
and cheekbones that drain into the nasal cavity. The largest are the two maxillary).
An external nose such as forms so conspicuous a feature of the human face is
hardly to be recognized in the domestic animals, in which it is merged within the
general contours of the muzzle.
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Nasal plate.
Fig:1
It is divided internally into two cavities, the nasal vestibules, each entered through
a nostril and leading through a region of constriction to much larger nasal cavity
placed beyond. The form and size of the nostrils, their orientation, and the nature
of the surrounding integument all show considerable species differences. The
integument around the nostrils is naked and sharply demarcated from the
unmodified skin in all domestic species other than the horse. According to its
extent, the modified region nostril is variously known as the nasal (carnivores, small
ruminants), nasobial (cattle), or rostral (pig) plate. The nasal plate may be divided
by a median groove or philtrum. The plate is kept moist in the ox, pig, and dog.
The nostril is round in pig, but most other species it is prolonged laterally by a slitlike extension. The form of nostril may be altered, principally by raising the lateral
“wing”, actively by certain facial muscles or passively when the air flow is increased
in strenuous breathing or when sniffing. These changes can be very pronounced in
the horse, leading to compression and almost obliteration of the diverticulum.
The integument is carried some distance.
There is difference in openings slightly anyhow the opening is smaller and the nasal
glands lying lateral discharges in this area. This arrangement aids humidification of
the incoming air.
The two nasal cavities occupy a large part of the face they extend caudally to the
transverse bony septum at the rostral end of the cranial cavity. The right and left
cavities are divided by the nasal septum which is largely cartilaginous but ossified in
its most caudal part. The septum meets the upper surface of the hard palate, which
separates the nasal and mouth cavities, but the details vary greatly between
species. In the horse the septum meets the whole length of the hard palate so that
each nasal cavity communicates with the pharynx through a separate opening
(choana). In other species (e.g. ox, dog) the caudal part of the septum fails to meet
the palate and there is a single opening shared by two sides.
The nasal sinuses are diverticula of the nasal cavity which excavate the skull bones.
The function of sinuses is obscure; they offer some thermal and mechanical
protection to the orbit and nasal and cranial cavities.
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The larynx- it forms the connection between the pharynx and the
tracheobronchial tree. It lies below the pharynx and behind the mouth, suspended
from the cranial base by the hyoid apparatus. Because of the connection with the
tongue and hyoid apparatus, the larynx shifts its position when the animal swallows.
The epiglottis regulates the food and air. The larynx originally developed as a
device to protect the lower respiratory inundation. Protection remains it’s primarily
role, although phonation (the production of voice) is the function that most often
comes to mind first.
The protection of the lower passages against the entrance of food and drink is
achieved in two ways. On swallowing, the larynx is drawn forward and the epiglottis,
tilted somewhat backward by coming against the roof of the tongue, forms a partial
cover to the laryngeal entrance. Solid foods are swiftly carried over the laryngeal
entrance muscles, whereas fluids are deflected by the epiglottis. Inhibition of
inspiration at this time further reduces the risk of food being drawn into the
larynx. In fact, food comparatively rarely “goes down the wrong way” but, when it
does, contact with the vestibular mucosa initiates reflex coughing.
During normal respiration there may be some slight abduction of the vocal of the
vocal folds that widens the glottis on inspiration, but this movement is pronounced
only when breathing movement is vigorous. Closure of the glottis also occurs in a
number of other functional contexts in which free passage of air to and from the
lungs must be prevented. A built-up of expiratory forces against a closed glottis
allows for a forceful expulsion when the air is eventually released; this is the
mechanism used when coughing to clear the lower passage of mucus accumulations
or foreign matter. Sustained closure with elevation of the intrathoracic pressure is
also used in activities involving straining-defecation, micturition and parturition; the
blockage of the escape route for air helps maintain the intrathoracic pressure and
by so stabilizing the diaphragm aids the action of the muscles of the abdominal wall.
The skeleton of the thorax can also be more effectively fixed to provide a firm
base for muscles attaching to the ribs when the glottis is closed. The production of
voice is very important function of larynx although the final form of sound is
modified and “coloured” by resonance chambers provided by other cavities of the
head. The air steam is made to vibrate as it passes through the glottis. The pitch
is controlled by thickness, the length, and the tension of the vocal folds and
individual features of laryngeal anatomy. The sounds of human speech are more
complex than those produced by in other species, although there is no greater
complexity of laryngeal structure.
The trachea- the trachea and bronchi forms a continuous system of tubes
conducting air between the larynx and the smaller passages (bronchi) in the lungs.
They have a very similar construction and together are sometimes termed the
“tracheobronchial”. The trachea leads from the larynx through the visceral space
of neck, enters the mediastinum at the thoracic inlet, and continues to its terminal
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bifurcation above the heart. The two chief bronchi diverge from the line of the
trachea to enter the corresponding lungs at their root. In ruminants and pigs a
separate tracheal bronchus arises proximal to the tracheal bifurcation and
separately aerates the cranial lobe of the right lung. The cervical part of the
trachea maintains a more or less median position, although its relationship to the
oesophagus alters at different levels and in different postures of the head and
neck. The thoracic part of the trachea is deflected slightly to the right where it
crosses the aortic arch. It is related ventrally to the cranial vena cava, to the
arteries arising from the aortic arch, and to various tributaries and branches of
these vessels; it is related dorsally to the oesophagus and related variously to
Mediastinal lymph nodes and, in young subjects, to the thymus. The bifurcation of
trachea lies in the region of the fourth to sixth intercostal spaces but varies with
the species and with the respiratory phase. The chief bronchi very quickly enter
the lungs, in which they ramify according to a pattern, relatively constant for each
species. The wall of the trachea is composed of an inner mucosa (trachea contains
both uni- and multi-cellular mucus glands, the excessive accumulation of mucus may
irritate, therefore stimulating coughing to clear the airway), a fibro-cartilaginous
middle layer (fibro-cartilaginous coat is composed of numerous strips of cartilage
that are bent to form “rings” that are incomplete dorsally where the ends may fail
to meet of may overlap. The edges of the strips are connected to each other by
sheets of rather elastic connective tissue continuous with the perichondrium- the
fibrous membrane that covers the surface of cartilage except at joints) and an
adventitia (in the neck) or serosa (in the thorax). The construction of the trachea
prevents it from collapsing and allows it to make the necessary adjustment in length
when the neck is extended and also when the diaphragm contracts. It is attached to
the diaphragm indirectly by pulmonary ligaments and Mediastinal connective tissue
and also, more effectively, by the negative intrapleural pressure that couples the
lungs to the chest wall, including the diaphragm. Variations in diaphragm are
regulated by the tracheal muscle.
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Pharynx
Nasal cavity
Oesophagu
s
Nostril
Trachea
Oral cavity
Epiglottis
Fig: 2 Longitudinal section of head (source:R. D. Frandson, T. L. Spurgeon)
The pleura.
Each lung is invested by a serous membrane, the pleura, which also line the
corresponding “half” of the thoracic cavity. The trachea (windpipe) extends from
the neck into the thorax, where it divides into right and left main bronchi, which
enter the right and left lungs, breaking up into smaller bronchi and bronchioles and
ending in small air sacs or alveoli, where gaseous exchange occurs. Each lung is
surrounded by a pleural cavity. Thus, there are two pleural membranes, each
arranged as a closed invaginated sac. The space between the right and left sacs
forms the mediastinum, a more or less median partition in the thorax in which the
heart and other thoracic organs are situated. The part of the pleura that surrounds
the lung directly is called as the visceral or pulmonary pleura. The pleura, has two
surfaces, which have a pleural cavity containing a thin layer of fluid which it
spreads over the pleural surface and facilitates the smooth movement of the lung
against the chest wall and of the lung lobe against each other. Each lung is enclosed
in a cage bounded below by the diaphragm and at the sides by the chest wall and
the mediastinum.
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The lungs
The right and left lungs (pulmones) are each invaginated into the corresponding sac
and are free, except at the roofs where they are attached to the mediastinum.
They have no fixed size or shape since they comply with changes in the dimensions
of the thorax with respiration. The lungs are normally kept expanded by the air
pressure within the respiratory tree and, being elastic, they recoil and collapse as
soon as air is admitted into the pleural cavities by trauma, surgery, or dissection.
They have a soft, spongy texture and the residual air they contain. The colour of
healthy lungs varies in intensify with the blood content and therefore with the
manner of death; it is a fresh pink in many slaughter-house specimens but a much
deeper red in lungs obtained from animals that were not bled. The lungs of animals
that spent their lives in heavily polluted atmospheres acquire a grayish tinge from
deposition of soot or other inhaled particles.
Anatomical descriptions are generally based upon specimens hardened in situ before
the chest was opened; at death such lungs retain their size, which is intermediate
between those adopted in full inspiration and full expiration. The two lungs are alike
forming mirror image with respect to its shape, the left being smaller to
accommodate the heart. Both the lungs are then divided into lobes (three on the
right, two on the left) supplied by lobar bronchi. Bronchi, pulmonary arteries and
veins (which supply deoxygenated blood and remove oxygenated blood), bronchial
arteries and veins (which supply oxygenated blood to the substance of the lung
itself) and lymphatics all enter and leave the lung by its root (or hilum). Each lobe
of the lung is further divided into a pyramidal bronchopulmonary segments as shown
below. (fig. 3)
In most species one or more fissures extend into the substance toward the root
(the root of lung, situated dorsal to the cardiac impression, is formed by the
bunching together of the chief bronchus and the pulmonary artery, veins,
lymphatics and nerves within a covering of pleura provided by the sign of the
Mediastinal pleura onto the lung), dividing each lung into parts that are commonly
equated with lobes. The lobes are properly defined by the ramification of the
bronchial tree. According to current knowledge and practice, the left lung consists
of cranial and caudal lobes, the right one of cranial, middle, caudal and accessory
lobes; however the cranial lobe is commonly subdivided by an external fissure,
whereas the right lung of the horse lacks a middle lobe. The fissures are much
deeper in the lungs of dog and cat than any other farm animals.
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Tracheal bronchus
Key
A = Apical / cranial lobe
C = Cardiac / middle lobe
I = Accessory / intermediate
Lobe
D = diaphragmatic lobe
R = Right lungs
L = left lungs
Figure 3.
The bulk of the lung substance is provided by the bronchi, pulmonary vessels, and
peri-bronchial and perivascular connective tissue. As stated earlier the right and
left chief bronchi arise at the tracheal bifurcation above the heart, and after
entering the lung at its root each detaches a bronchus to the cranial lobe before
continuing caudally as shown in figure 3. But the ramification is different for either
of the two lungs. The number of bronchial generations before the smaller bronchi
are succeeded by bronchioles varies between species and also between parts of the
one lung. The elasticity of lungs is due to presence of connective tissue which
expands on inspiration and collapse on expiration. Loss of the elasticity, which
occurs naturally with aging (but also in certain pathological conditions), reduces
respiratory efficiency.
The identification of the lungs of individual species is most conveniently based on
the degrees of lobation and lobulation. The lungs of horse show almost no lobation
and very conspicuous lobulation externally, whereas those of ruminants and pigs are
conspicuously lobated and lobulated (though not uniformly in sheep and goats),
whereas those of carnivores are very deeply fissured into lobes but show little
external evidence of lobulation.
The pulmonary arteries generally follow the bronchi while the pulmonary veins
sometimes run separately, alternating in position with the bronchoarterial
associations as shown in figure 5 .
Pulmonary arteries and veins (which supply deoxygenated blood and remove
oxygenated blood), bronchial arteries and veins (which supply oxygenated blood to
the substance of the lung itself) and lymphatics all enter and leave the lung by its
root (or hilum). The nerves to the lungs are delivered through a pulmonary plexus.
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Bronchi and bronchioles- on entering thoracic cavity, the trachea divides into
two primary bronchi. The right bronchus is slightly larger. The structure of the
larger bronchi is identical to that of trachea. On the smaller bronchi the cartilage
rings are gradually replaced by irregular plaques, and it is the shedding of the last
of these that defines the bronchobronchiolar transition. Variations in the diameter
of the bronchi and bronchioli are relatively greater and more significant than those
of the trachea.
Each primary bronchus enters the lung of its side and immediately divides into
secondary and tertiary or segmental bronchi. They further branches into
bronchioles, terminal bronchioles and then into respiratory bronchioles. A bronchus
with its branches is described as a bronchial tree. All other structures including
trachea, bronchi and bronchioles are lined with that of ciliated and goblet cells
which push the foreign particles into the pharynx and secretes mucous which
entangles dust particles or other foreign objects that happen to enter along with
inspired air respectively, but, the terminal or respiratory bronchioles are lined with
single layer of ciliated epithelium lacking goblet cells.
Alveolar sacs and alveoli- the respiratory bronchioles finally end in alveolar
ducts. Each alveolar duct is connected with several thin-walled alveolar sacs which
contains numerous alveoli or also called as air sacs.
Airs sacs are functional units of lung. These is where gas exchange between blood
and air.
Adaptation of lungs
Outside air varies in temperature from very dry to very humid. At the alveolar
surface it must be at body temperature. At the alveolar surface it must be
saturated with water vapour. Air contains dust and micro-organisms which must not
reach the alveolar wall.
So it must be filtered out of the inspired air and disposed off before they reach
the alveoli. The temperature and humidity of inspired air will increase as it passes
down a long series of tubes lined with a moist mucosa at body temperature. The
mechanisms for filtering are explained below.
Mucus
The respiratory tract, from nasal cavities to the smallest bronchi, is lined by a
layer of sticky mucus, secreted by the epithelium and small ducted glands. Particles,
which hit the sidewall of the tract, are trapped in this mucus. This is facilitated by:
(a) the air stream changing direction, as it goes in a continually dividing tube.
(b) Random (Brownian) movement of small particles suspended in the air stream.
Cilia
Once the particles have been sidelined by the mucus they have to be removed, as
indeed does the mucous. This is carried out by cilia on the epithelial cells which
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move the mucous continually up or down the tract towards the nose and mouth.
(Those in the nose beat downwards, those in the trachea and below upwards). The
mucus and its trapped particles or bacteria are then swallowed.
Length
The length of the respiratory tract helps in both bringing the air to the right
temperature and humidity but hinders the actual ventilation, as a long tract has a
greater volume of air trapped within it, and demands a large breath to clear out
residual air.
Protection
The entry of food and water into the larynx is prevented by the structure of the
larynx and by the complicated act of swallowing. Below the larynx the trachea is
usually patent i.e. open, and kept so by rings of cartilage in its walls.
Ventilation of the lung:
Ventilation is the movement of gas in and out of the alveoli through the
anatomical dead-space
The volume of air breathed per minute, minute ventilation (VE), is determined by
the volume of each breath, tidal volume (VT), and the number of breaths per
minute, respiratory frequency (f). The changes in minute ventilation that occur with
changes in metabolic rate can be brought about through changes in either tidal
volume or respiratory rate, or both.
Air flows into the alveoli through the nares (openings or passages leading out of the
nose or naval cavity.), nasal cavity, pharynx, trachea, bronchi and bronchioles. This
structure comprises the conducting airways, and because gas exchange does not
occur in these airways, they are also called as anatomical dead-space as shown
below schematic diagram.
Alveolar dead-space.
Equipment dead-space
Anatomical dead-space
Figure 4: Types of dead-space. (From: Cunninghamtext book veterinary physiology)
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A portion of each tidal volume and, therefore, of VE, ventilates the anatomical
dead-space, and a portion known as alveolar ventilation (participates in gas
exchange). Alveolar ventilation is regulated by control mechanism to match the O2
uptake and CO2 elimination which is necessary for metabolism.
Dead space ventilation can occur within the alveoli also called alveolar dead space
which is a result of alveoli poorly perfused with blood, where gas exchange cannot
occur optimally. Physiological dead space is a term used to describe the sum of the
anatomical dead space and alveolar dead space.
Ventilation requires muscular energy.
During inhalation, energy is provided by the skeletal muscle that makes air to enter
the lungs. During exhalation, much of the energy causing air to leave the lungs is
provided by the elastic force stored in the stretched lung and thorax. Therefore,
in most animals at rest, inhalation is an active process (as energy for inhalation is
provided by the contraction of muscles), whereas exhalation is passive. Anyhow
biology is also is a study of exception; horses have an active phase to exhalation
even at rest. During exercise or the respiratory abnormalities, exhalation may be
assisted by muscle contraction in most species. The diaphragm is the primary
inspiratory muscle, which is a domed-shaped musclotendinous sheet separating the
abdomen and the thorax. Its apex extends to seventh or eighth intercostal space
at the level the base of the heart. During contraction the done of the diaphragm is
pulled caudally, flattening and increasing the space above it i.e., enlarging the
thoracic cavity, which elevates intraabdominal pressure, which displaces the caudal
ribs outwards. The pressure in the lung decreases. The process is help by the ribs,
which move up and out. Between the ribs two sets of intercostal muscles, the
external intercostal running backward and downwards, the internal intercostal
running down and forward. These two muscle sheets thus run between ribs with
fibers roughly at right angle, when they contract each rib moves closed to its
neighbours. The ribs are all, therefore pulled up towards the horizontal, increasing
anterio-posterior and lateral thoracic diameter. During deeper breathing the
muscles of the neck (scalene) contracts raising the first ribs and hence the rest of
the rib cage.
The external intercostal muscles are active during inhalation. During contraction
the ribs move rostrally and outwards. The abdominal and intercostal muscles are
the expiratory muscles. Contractions of the abdominal muscles increase the
abdominal pressure, which forces the relaxed diaphragm forward and reduces the
size of the thorax. And also the internal intercostal muscles when contracts
decrease the size of the thorax by moving the ribs caudally and downwards.
During exercise respiratory muscle activity increases, and to supply the necessary
metabolic substrates; i.e., O2, the blood flow to the diaphragm increases over
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twenty fold in exercising animals. In running (cursovial) mammals the inhalation
occurs as the forelimbs are extended and the hind limbs are accelerating the animal
forward. Exhalation occurs when the fore limbs are in contact with the ground.
Mechanism of breathing
As stated earlier there are two respirations (external and internal) and here we are
mainly on the external respiration which is taking in oxygen and giving out the
carbon dioxide makes one breathing cycle. This is achieved by two processes
inspiration and expiration which is described as follows:
Inspiration
this works by making the rib cage bigger and the pressure in the lung is decreased,
so air is sucked in. The main component acting here is the diaphragm. This is a layer
of muscle, which is convex above, and squashed in the centre by the heart. When it
contracts it flattens and increases the space above it. The process is helped by the
ribs, which move up and out also increasing the space available. Between the ribs
run two sets of intercostal muscles, the external intercostals running backward and
downwards, the internal intercostals running down and forward. These two muscle
sheets thus run between ribs with fibres roughly at right angles. When they
contract each rib moves closer to its neighbours. The ribs are all, therefore pulled
up towards the horizontal, increasing anterio-posterior and lateral thoracic
diameters. During the deeper breathing the scalene muscles of the neck contract,
raising the first rib and hence the rest of the cage.
Expiration
Breathing out does the reverse of inspiration, diaphragm relaxes and it is pushed
forward by abdominal organs increasing the convexity, intercostals muscles relax
and the thoracic wall collapses. These two actions reduce the chest cavity and
increase pressure on lungs causing the expulsion of air out of the lungs.
In quiet respiration, say at rest, almost all movement is diaphragmatic and the
chest wall is still. The expansion of the lung deforms the flexible walls of the
alveoli and bronchi and stretches the elastic fibres in the lung. When the diaphragm
relaxes elastic recoil and abdominal musculature reposition the diaphragm again.
Gaseous exchange
Gaseous exchange relies on simple diffusion. In order to provide sufficient oxygen
and to get rid of sufficient carbon dioxide there must be a large surface area for
gaseous exchange a very short diffusion path between alveolar air and blood
concentration gradients for oxygen and carbon dioxide between alveolar air and
blood. Diffusion gradients are maintained by ventilation (breathing), which renews
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alveolar air, maintaining oxygen concentration near that of atmospheric air and
preventing the accumulation of carbon dioxide the flow of blood in alveolar
capillaries which continually brings blood with low oxygen concentration and high
carbon dioxide concentration. Haemoglobin in blood continually removes dissolved
oxygen from the blood and binds with it. Figure below show alveolar capillary
network.
Fig 5 : air sac showing its relationship with blood capillaries.
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Fig: 5 Image from Purves et al., Life: The
Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and WH
Freeman (www.whfreeman.com), used with
permission.
Summary .
1. Movement of an oxygen-containing medium so it contacts a moist membrane
overlying blood vessels.
2. Diffusion of oxygen from the medium into the blood.
3. Transport of oxygen to the tissues and cells of the body.
4. Diffusion of oxygen from the blood into cells.
5. Carbon dioxide follows a reverse path.
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The respiratory muscle generates work to stretch the lung and
overcome the frictional resistance to air flow.
At the end of a tidal exhalation, a volume of air remains in the lung. This air volume
is known as functional residual capacity (FRS). During inhalation the pressure in the
pleural cavity (Ppl) decreases as the thorax enlarges and the respiratory muscle
perform by overcoming the resistance provided by the air inside and tissues. You
can noticed that in resting animals, they breath slowly, air flow rate are low,
acceleration is minimal, and the primary work of the respiratory muscles is to
overcome the elasticity of the lung. When respiratory rate increase more energy is
used to generate sir flow against the frictional resistance of the airways according
to the need to produce the energy. Disease can cause change in both elasticity and
resistance and thereby increase the work of breathing.
Lung elasticity is due to tissue and surface tension forces (the property of liquids
that gives their surfaces a slightly elastic quality and enables them to form into
separate drops). When the thorax is opened and pleural pressure (pressure around
the lungs) becomes atmospheric, the lung collapses to its minimal volume. At minimal
volume some air remains trapped within the alveoli behind closed peripheral airways.
This collapse of the lung demonstrates its inherent elasticity.
It should be clear that air flow is opposed by the frictional resistance between air
molecules and the walls of the air passages, and also to a much lesser extent by the
viscous drag of the tissue. In resting animal, the nasal cavity, pharynx, and larynx,
which warm and humidify the air, provide approximately 50% of the frictional
resistance to breathing. Trachea, bronchi and bronchioles provides resistance 3050%.
The nasal resistance can be reduced, for example, during exercise, by dilating the
external nares (openings or passages leading out of the nose or naval cavity) and
vasoconstriction of vascular tissue in the nose. When airflow rates increases during
exercise, or when the nasal cavity is blocked, some species breathe through the
mouth or escape the high resistance nasal cavity. The tracheobronchial tree has up
to 24 branches lined by a secretory, ciliated epithelium. The larger airways, trachea
and bronchi, are supported by cartilage and supplied with secretory bronchial
glands as mentioned earlier. The wall of tracheobronchial has got smooth muscle
which regulates the diameter of the trachea to the alveolar duct. To smooth muscle
is provided with the ANS that administers the contraction of the muscle.
Even the lung aids resistance to the air flow.
Therefore, the air intake will be regulated by the ANS mainly, but, the distribution
of air depends on the local mechanical properties of the lung and the local changes
17
in pleural pressure. – Optimal gas exchange requires bringing together air and blood
at the alveolus, i.e., matching of ventilation and blood flow. Therefore, gas exchange
fails if an alveolus receives blood but no ventilation, or vice versa. Ideally, each
region of lung should receive equal amounts of ventilation, but this never occurs in
either animals or human.
Figure below shows how the mechanical properties of the lungs affect the
distribution of ventilation. The alveolus represented as C is a healthy piece of lung
with normal compliance and normal air way. Region B has a disease-like interstitial
pneumonia in which compliance is decreased, but air ways are normal. In region A
has a normal compliance but a narrowed airway, as many occur when airway smooth
muscle contracts. When the same decrease in pleural pressure is applied to each
region, A and C fill to the same volume, because they have similar compliance, but A
fills slowly because of hindered airway. Region B fills rapidly, but because of its
reduced compliance, achieves a lesser volume than regions A and C. When a model
lung is ventilated more rapidly at a time, the volume received by region A
decreases, because it has inadequate time to fill as region A has resistance to offer
to air that bypasses through them. In real, the small degrees of airway obstruction
may cause no signs of respiratory difficulty in the resting animal, anyhow may cause
hypoxemia (inadequate oxygen in the blood) when the animal exercises since uneven
distribution of aeration.
(A) High resistance
(B) Low compliance
(C) normal
Figure 6: Cunningham: text book of vet. Physiology, 1992: the effects of mechanical
properties of the lung on airway resistance.
18
Respiratory System of the Chicken
One adaptation to flight is the development of pneumatic bones.
Instead of being filled with marrow, pneumatic bones are hollow and act as
extensions of the respiratory system, which also includes the lungs, the tubes or
bronchi leading to the lungs and the air sacs.
The lungs are rather rigid, attached to the ribs in the upper portion of the
thorax, and do not expand or contract very much during breathing.
There are four pairs of air sacs which reach from the neck to the abdomen and
open into the pneumatic bones. The air sacs are delicate, thin walled and collapse
when the chest is opened, so they may be difficult to see.
Fig 7: respiratory apparatus of chicken.
The skeleton is compact, lightweight and quite strong. Many of the vertebrae are
fused, which provides sufficient strength to support wings for flight. The major
flight muscles, the pectorals, are much larger in birds, and therefore their bony
attachment, the sternum or breast bone, is also larger and stronger. In fact, the
sternum is so large that it forms much of the bird’s ventral body wall. When you run
your fingers along the ventral midline of the chest, you can feel the prominent
ridge of the sternum. The sternum of birds may be called the "keel" bone, because
it resembles the keel of a ship.
19
The nostrils are located at the base of the beaks. All are similar to that of
mammals. The larynx occupies a mound on the floor of the orpharynx, which is
supported by cartilages that differ from that of mammals but occupy similar
positions. The trachea, composed of tightly stacked, complete cartilaginous rings,
accompanies the oesophagus through the neck; it can be palpated on the right side.
The syrinx is formed by the terminal part of the trachea and the beginning of the
primary bronchi. The tracheal cartilages of the syrinx are well-built, whereas the
bronchial cartilages are largely lacking, although a short vertical (pessulus)
separates the bronchial openings. The lateral and medial walls of the bronchi are
membranous and produce the voice when caused to flutter.
Trachea
Tympanum
Lateral and medial
tympaniform membranes.
Pessulus
Fig: 8- semischematic
representation of
the opened
syrinx.
Primary
bronchi
The lungs are relatively small, unlobed, bright red, and soft and velvety to the
touch. They occupy the craniodorsal part of the body cavity, lying against thoracic
vertebrae and vertebral ribs. They fail to cover the lateral surfaces of the heart
as they do in mammals. The lungs are lightly attached to the body wall and to the
horizontal septum that confines them from below. There is not pleural cavity
corresponding to that of mammals and the capacity for expansion is negligible. The
primary bronchus enters the ventral surface, passes diagonally through the lung,
narrowing as it goes, and at the caudal border becomes continuous with the
abdominal air sac. Some eight mediodorsal bronchi then arise from the dorsal wall
of the primary bronchus; they have no connection with air sacs. There are usually
20
four medioventral bronchi. They arise just after the primary bronchus enters the
lung. About eight lateroventral bronchi arise opposite the preceding set. Lastly,
about 25 laterodorsal bronchi arise opposite the mediodorsal and lateroventral
groups. They are smaller than those of the three preceding groups and have no
direct connection with air sass.
Fig: 9 right
lung
(mediovent
ral view)
and related
air sacs.
Loops of parabronchi.
Medioventral bronchi
Lung
Mediodorsal bronchi
Cervical air
sac
Lateroventral bronchi.
Primary bronchus.
Abdominal air
sac
Clavicular air sac.
Cranial thoracic air
sac
Caudal thoracic air
sac
The secondary bronchi give off 400 to 500 parabronchi in whose relatively thick
walls gas exchange take place. The air capillaries are closely intertwined with blood
capillaries, the two networks constituting the bulk of the parabronchial wall. Gas
exchange takes place across the barrier. The air capillaries are similar but, the air
capillaries in chicken and any other birds the air capillaries are not terminations of
the respiratory tree but continuous channels that can receive oxygen-rich air
either direction.
21
The air sacs are blind, thin-walled enlargements of the bronchial system that
extend beyond the lung in close relationship to the thoracic and abdominal viscera.
Diverticula from these sacs various bones and even extend between skeletal
muscles. The chicken has eight air sacs: single cervical and Clavicular, and paired
cranial thoracic, caudal thoracic and abdominal sacs. The cervical sac consists of a
small central chamber ventral to the lungs from which long diverticula extend into
and along-side the cervical and thoracic vertebra. The Clavicular sac lies in the
thoracic inlet in which its thoracic part fills the space cranial to and around the
heart, and extends into the sternum. Its extrathoracic diverticula pass between
the muscles and bones of the shoulder girdle to aerate the humerus. Compound
fractures of the humerus may therefore introduce infection to the air sacs and
lungs. The paired cranial thoracic air sacs lie ventral to the lungs between the
sternal ribs and the heart and liver. The caudal paired sacs lie more caudally
between the body wall and the abdominal sacs. The paired sacs are the largest;
they occupy the caudodorsal parts of the abdominal cavity where they are in broad
contact with the intestine, gizzard, genital organs and kidneys. Their diverticula
enter recesses of the synsacrum and the acetabulum.
Air sacs function primarily in respiration, though there are poorly vascularized
walls deny them a role in gaseous exchange. They also lighten the body and, being
broadly dorsal in position, lower the centre of gravity, and also for improved
stability in flight.
It is primarily the change in pressure within the air sacs which allows air to pass
into and out of the lungs. This change in pressure is largely due to the sternum, or
keel bone, moving inward and outward during respiration.
Fig:10
(http://www.mie.utoronto.ca/labs/lcdlab/
biopic/fig/48.08.jpg)
WHEN RESTRAINING BIRDS, REMEMBER THAT THEY CAN SUFFOCATE,
IF THE BREAST BONE IS NOT FREE TO MOVE IN AND OUT!
22
Differences between avian and mammalian respiration
Respiration in birds is much different than in mammals.
Birds have a larynx, but it is not used to make sounds. Instead, an organ
termed the "syrinx" serves as the "voice box."
The air sacs of birds extend into the humerus (the bone between the shoulder
and elbow), the femur (the thigh bone), the vertebrae and even the skull.
Birds do not have a diaphragm; instead, air is moved in and out of the
respiratory system through pressure changes in the air sacs. Muscles in the
chest cause the sternum to be pushed outward. This creates a negative
pressure in the air sacs, causing air to enter the respiratory system.
Expiration is not passive, but requires certain muscles to contract to increase
the pressure on the air sacs and push the air out. Because the sternum must
move during respiration, it is essential that it is allowed to move freely when a
bird is being restrained. Holding a bird "too tight" can easily cause the bird to
suffocate.
Because birds have air sacs that reach into the bones, and have no diaphragm,
respiratory infections can spread to the abdominal cavity and bones.
Bird lungs do not expand or contract like the lungs of mammals. In mammalian
lungs, the exchange of oxygen and carbon dioxide occurs in microscopic sacs in
the lungs, called 'alveoli.' In the avian lung, the gas exchange occurs in the
walls of microscopic tubules, called 'air capillaries.'
The respiratory system of birds is more efficient than that of mammals,
transferring more oxygen with each breath. This also means that toxins in the
air are also transferred more efficiently. This is one of the reasons why fumes
from Teflon are toxic to chickens and other birds, but not to mammals at the
same concentration.
When comparing birds and mammals of similar weight, birds have a slower
respiratory rate.
Unlike mammals, Respiration in chicken requires two respiratory cycles
(inspiration, expiration, and inspiration, expiration) to move the air through the
entire respiratory system. In mammals, only one respiratory cycle is necessary.
o







Respiratory cycle of a chicken
1. During the first inspiration, the air travels through the nostrils, also called
nares, of a bird, which are located at the junction between the top of the
upper beak and the head. The fleshy tissue that surrounds them, in some
birds, is called the cere. As in mammals, air moves through the nostrils into
23
the nasal cavity. From there it passes through the larynx and into the
trachea. Air moves through the trachea to the syrinx, which is located at
the point just before the trachea divides in two. It passes through the
syrinx and then the air stream is divided in two as the trachea divides. The
air does not go directly to the lung, but instead travels to the caudal
(posterior) air sacs. A small amount of air will pass through the caudal air
sacs to the lung.
2. During the first expiration, the air is moved from the posterior air sacs
through the ventrobronchi and dorsobronchi into the lungs. The bronchi
continue to divide into smaller diameter air capillaries. Blood capillaries flow
through the air capillaries and this is where the oxygen and carbon dioxide
are exchanged.
3. When the bird inspires the second time, the air moves to the cranial air
sacs.
4. On the second expiration, the air moves out of the cranial air sacs, through
the syrinx into the trachea, through the larynx, and finally through the nasal
cavity and out of the nostrils.
Fig: 8 Diagram showing movement of
sternum and ribs during respiration:
A. Inspiration B. Expiration C. Sternum (keel
Types of breathing
Terminologies used to describe types breathing.
24
1. Costal respiration: In this type of respiration thoracic muscles are mainly
involved and the movement of the rib cage is more prominent. It is seen in dogs and
cats.
2. Abdominal respiration: This type of respiration is seen in ruminants viz cattle,
goat, sheep
and yak. Here the abdominal muscles are involved and movement of
the abdominal wall is noticed
3. Costo- abdominal respiration: In this type of respiration muscles of both thorax
and abdomen are involved so the movement of the ribs and the abdominal wall are
noticed.
4. Eupnea – normal quiet respiration
5. Dyspnea – difficult breathing
6. Apnea – Absence of breathing
7. Hypernea – increased depth or rate of breathing or both
8. Polynea – rapid shallow breathing
AN OVERVIEW
The importance of respiratory diseases in the overall perspective of pathology and
clinical medicine cannot be overemphasized. Primary respiratory infections such as
rhinitis, tracheitis, bronchitis, and bronchopneumonia occur with frequency. Also,
the respiratory apparatus, especially the lungs, is secondarily involved in the
terminal states of most diseases. Thus, regardless of the primary disease, the
immediate cause of death is often pulmonary embolism, pulmonary edema or
bronchopneumonia. Actually, it is quite rare to find the lung uninvolved at
postmortem examination.
In this section, the respiratory apparatus is discussed from an anatomic approach
beginning with the nasal cavity and ending with the lungs and pleura. Those diseases
and conditions encountered with reasonable frequency in veterinary medical
practice are discussed in some detail. Less common disease entities are given brief
consideration.
GENERAL CONSIDERATIONSThe respiratory apparatus of mammals consist of
the airways (nasal cavities, pharynx, larynx, trachea, and bronchi), lungs,
thorax, diaphragm, and muscles of the thorax. The exchange of oxygen and carbon
dioxide between body tissues and the environment is the primary function of this
apparatus.
The nasal mucosa is moist, highly vascular, and contains numerous glands, thus
adding warmth, and moisture to inspired air. The pharynx is a common passageway
for the respiratory and digestive tubes. The larynx is a musculocartilaginous
vascular structure that serves as the principal organ of phonation. The tracheal
25
mucosa has numerous glands and the epithelium is ciliated. The secretion of the
glands and the motion of the cilia help to clear this structure of foreign material.
Progressive branching of the major bronchi forms bronchioles. Further branching
of bronchioles lead to the terminal bronchioles called the acinus or the terminal
respiratory unit. Acini contain alveoli and are thus the site of gaseous exchange. An
acinus is composed of



(1) Respiratory bronchioles (emanating from the terminal bronchioles)
which gives off from their sides several alveoli; the respiratory
bronchioles then proceed into.
(2) Alveolar ducts which immediately branch and empty into.
(3) The alveolar sac (the blind end of the respiratory passage) whose
walls are formed entirely of alveoli. A cluster of 3 to 5 terminal bronchioles,
each with its appended acinus, is usually referred to as the pulmonary
lobule. Lobules are separated from each other by connective tissue septa.
Remember, The functional unit of the lungs in mammals is the acinus (not the
alveolus); thus, disease of the lungs usually affects acinar units rather than just
alveoli.
Remember, Alveolar walls are not solid but are perforated by numerous pores which
permit the passage of bacteria and exudate between adjacent alveoli.
The lungs are supplied by two rather efficient arterial systems (pulmonary and
bronchial arteries). In the absence of significant cardiac failure the bronchial
arteries of aortic origin can sustain the vitality of the pulmonary parenchyma when
the pulmonary blood supply is obstructed.
Most antibody activity in the respiratory apparatus is associated with IgA. IgA is
synthesized by plasma cells located in the lamina propria of bronchioles.
The respiratory system of birds is the most complicated and efficient in the
animal kingdom. In birds, the main bronchus does not ramify as in mammals but
passes through the entire lung giving off groups of secondary bronchi. However, the
most striking difference of the avian respiratory tract is the elaborate system of
air sacs and air spaces within the central areas of the bones. All of these sacs and
spaces connect to the lungs and on inhalation, air passes completely through the
lungs into the air sacs. On exhalation, air passes back again and more completely
fills the air channels in the lungs. Actually, a bird with a blocked trachea can still
breathe if a connection is made between one of its pneumatized bones and the
outside air.
26
Respiratory disorders
(non-malignant)
Asthma
Sensitising agents or irritants both recognised in this
regard and inherent in the work process
Byssinosis
Cotton, flax, hemp, sisal dust
Extrinsic allergic
alveolitis
Damp material of biological origin, such as mouldy hay,
straw, grain and feathers
Chronic obstructive
pulmonary disease
Coal, silica, cotton or grain dust
Silicosis
Silica
Asbestosis
Asbestos
Coal workers’
pneumoconiosis
Coal
Other pneumoconiosis
Exposures known to occasionally cause pneumoconiosis,
such as beryllium, tin, iron oxide, barium, aluminum,
cobalt, tungsten
27
Important Lung Worms
Dictyocaulus viviparus
Cattle
Aelurostrongylus abstrusus
Cat
Filaroides milski and F. osleri
Dog
Dictyocaulus arnfieldi
Horse
Dictyocaulus filaria
Sheep
Protostrongylus rufescens
Sheep
Muellerius capillaris
Sheep
Metastrongylus apri, M. salmi, and M. pudendotectus
Swine
-
Clinical importance
Trachea
It may be necessary to ensure that it is kept patent by passing a tube (endotracheal
intubation) to maintain the airway, especially post operatively if the patient has been
given a muscle relaxant. Another common surgical procedure, tracheotomy, involves a
small transverse cut in the neck. If this is done with anatomical knowledge no major
structure is disturbed and the opening may be used for a suction tube, a ventilator, or in
cases of tracheal obstruction as a permanent airway.
Faulty drenching
Faulty drenching in calves may lead into deposition of milk, liquid feed into lungs
causing instant death of aspiration pneumonia.
Parasitic Pneumonia
Inflammation of lungs caused due to the presence of lungworm in horse and cattle.
Gap worm: Respiratory distress due to presence of gap worm in trachea in chicken.
28
Asphyxiation
Strangulation to death while tethering
Artificial respiration
Need to be done during general anesthetic over dose in dogs. Alternately compressing
and releasing the thoracic wall.
References for respiratory system
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
14)
15)
Cunningham: text book of vet. Physiology, 1992, pp. 533-545.
Dr. Penjor and Mr. Nidup, Karma Hand outs for diploma.
Dyce, Sack< Wensing- text book of Vet. Anatomy, 1987.
http://www.cartage.org.lb/en/themes/sciences/zoology/AnimalPhysiology/Respiration/Respiration.htm.
http://images.google.com/imgres?imgurl=http://www.animalresearch.msu.edu/training/S
hared_Images/chicken_resp.gif&imgrefurl=http://www.animalresearch.msu.edu/training/p
oultry_module/poultry2_body_chicken_1.htm&h=356&w=340&sz=12&hl=en&start=2&um=1
&tbnid=Aajm2lgAM8aPtM:&tbnh=121&tbnw=116&prev=/images%3Fq%3Drespiration%2Bof
%2Ba%2Bchicken%26svnum%3D10%26um%3D1%26hl%3Den%26sa%3DN
http://www.vetmed.ufl.edu/courses/reep/RespNotes04.htm
http://www.copd-international.com/related_cond.htm
http://www.spectorfoundation.com/animal_welfare/farm_animals.htm
http://www.nohsac.govt.nz/reviewschedule2/index.php?section=sec6:s1:p052:
http://www.mie.utoronto.ca/labs/lcdlab/biopic/fig/48.08.jpg
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission
Information provided by: http://academics.smcvt.edu
Pathology of Domestic Animals, Vol. I, Jubb and Kennedy, - pp. 151-154.
http://www.earthlife.net/birds/breath.html
http://www.peteducation.com/article.cfm?cls=15&cat=1829&articleid=2721
29