Dr. P.B.Reddy provides an overview of the comparative anatomy of the vertebrate respiratory system. The document discusses why animals need to breathe and defines internal and external respiration. It then describes the key characteristics and functions of respiratory organs across different vertebrate groups, including fish, amphibians, reptiles and others. Specific structures are examined such as gills, lungs and the mechanisms of gas exchange that occur.

1. Dr. P.B.Reddy
M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM
PG DEPARTMENT OF ZOOLOGY
GOVERTNAMENT PG COLLEGE, RATLAM.M.P
reddysirr@gmail.com
Comparative anatomy of vertebrate respiratory system

3. Why do animals need to breathe?
Breathing is important to organisms because cells require
energy (oxygen) to move, reproduce and function. Breath also
expels carbon dioxide, which is a by-product of cellular processes
within the bodies of animals.
 Respiration is the process of releasing energy from food and
this takes place inside the cells of the body.
 The process of respiration involves taking in oxygen (of air) into
cells, using it for releasing energy by burning food, and then
eliminating the waste products (carbon dioxide and water) from
the body.
 Respiration is essential for life because it provides energy for
carrying out all the life processes which are necessary to keep the
organisms alive.
The energy produced during respiration is stored in the form of
ATP (Adenosine Tri- Phosphate) molecules in the cells of the body
and used by the organism as when required.

4. KEY POINTS
 Life started in an anaerobic environment in the so called ‘primodial broth’ (a mixture of
organic molecules.
 Subsequently, oxygen strangely enough became an crucial factor for aerobic metabolism
especially in the higher life forms.
 The rise of an oxygenic environment was an important event in the diversification of life.
 It evoked a dramatic shift from inefficient to sophisticated oxygen dependent oxidizing
ecosystems.
 Anaerobic fermentation, the metabolic process that prevailed for the first about 2 billion
years of the evolution of life, was a very inefficient way of extracting energy from organic
molecules. Ex: A molecule of glucose, e.g., produces only two molecules of ATP (≈ 15 kCal)
compared with 36 ATP molecules (≈ 263 kCal) in oxygenic respiration.
 Aerobic metabolism must have developed at a critical point when the partial pressure of
oxygen rose from an initial level to one adequately high to drive it passively across the cell
membrane.
Respiration is a complex and highly integrated biomechanical, physiological, and behavioral
processes.
The transfer of O2 occurs through a flow of tissue barriers and compartments by diffusion
down a partial pressure gradient, which drops to about zero at the mitochondrial level.
 Acquisition of molecular oxygen (O2) from the external fluid media (water and air) and the
discharge of carbon dioxide (CO2) into the same milieu is the primary role of respiration.
 The respiratory system is a biological system consisting of specific organs and structures.

5. Characteristics of respiratory organs
 Every respiratory device must conform to the following essentials features:
 Blood must be separated from the external environment that is air or water by a
thin epithelium.
 The epithelium must be permeable to permit diffusion of gases through it.
 The respiratory epithelium must always remain moist with a film of fluid to permit
osmosis of gases.
 The area of respiratory surface should be extensive to allow efficient absorption of
oxygen.
 Both the current of air or water outside and blood in capillaries must be made to
circulate constantly for quick replacement of gases.
All the respiratory organs should have large surface areas to get enough oxygen.
All the respiratory organs (like the skin, lungs, gills) have a rich supply of blood for
transporting respiratory gases.
 The thickness of the water/air–blood (tissue) barrier, the respiratory surface area, and
volume of pulmonary capillary blood are the foremost structural parameters which
determine the diffusing capacity of a gas exchanger for O2.
 Diffusion of oxygen from the medium into the blood.
 Transport of oxygen to the tissues and cells of the body.
 Diffusion of oxygen from the blood into cells.
 Carbon dioxide follows a reverse path.

6. External and internal respiration
Internal Respiration: Oxygen diffuses
out from the blood into tissue during
internal respiration. Internal
respiration, also known as cellular
respiration, is the process of energy
production via the breakdown of
glucose. Therefore, it occurs within the
cells. Internal respiration can be
aerobic respiration or anaerobic
respiration.
External Respiration: External
respiration is a physical process of
taking oxygen from the external
environment into the body and
expelling carbon dioxide from the
body to the external environment. It is
a vital process for life as it supplies
oxygen to extract energy from food via
internal or cellular respiration.
Additionally, it removes carbon
dioxide, which is a waste product of
internal respiration.

7. Respiratory System in Vertebrates
GILLS
Gills in Protochordates
 A large and sieve-like pharynx in majority of these animals performs dual function of
respiration and trapping food particles which are brought in through the current of water.
 The primitive pterobranch hemichordates (Cephalodiscus and Rhabdopleura) have either no
gill slits or have very few and sport tentaculated arms, which other than food gathering, also
function as efficient respiratory organs.
 Balanoglossus possesses a large pharynx having as many as 700 pairs of gill slits, which
appears to be a necessity in the burrowing habitat of the animal.
 The free-living urochordates, such as Salpa and Doliolum do not possess many stigmata or gill
slits as their entire body is permeable to oxygen but in the sedentary ascidians pharynx is
prominently enlarged and perforated with no less than 200,000 stigmata for filter-feeding.
 Cephalochordates use pharynx for both filter-feeding and respiration and hence carry 150-200
pairs of gill slits.

8. Respiratory organs of Cyclostomes
 Agnathans have 6-15 pairs of gill
pouches.
 They are lateral extensions of pharynx
and contain gill lamellae within.
 Cyclostomes are called
marsipobranchs, which means “pouched
gills”, since the gill lamellae are housed in
gill pouches.
 The hagfish, Myxine has only 6 pairs of
gill pouches whose ducts join together
and open to the exterior by a single pair
of openings, while Bdellostoma carries 6-
15 pairs of gill pouches that vary in
different species and open to the outside
independently.
The lamprey, Petromyzon, has 8
embryonic and 7 adult paired gill
pouches that open to the exterior by
independent openings.
Hagfish

9. .
In lower aquatic vertebrates the respiratory organs are not
connected to the olfactory organs, but in air-breathing vertebrates
there is a close association between the two.
In Chondrichthyes there is a direct connection between the olfactory
and respiratory organs in which the internal nares or choanae open
from the nasal cavities into the buccal cavity, but it is only in
tetrapoda that air enters through the nasal cavities into the buccal
cavity and then into the lungs.

10. Respiration in Fish
 Respiration in fish takes place with the help of gills.
 Most fish possess gills on either side of their head.
 Gills are tissues made up of feathery structures called gill filaments providing a large surface area for
exchange of gases.
 A large surface area is crucial for gas exchange in aquatic organisms as water contains very little amount
of dissolved oxygen.
 The filaments in fish gills are organized in rows in the gill arch.
 Each filament comprises lamellae, which are discs supplied with capillaries.
 Blood moves in and out of the gills through these small blood vessels.
 The gills provide extensive respiratory surface for the efficient gas exchange.
 Fish take in oxygen-rich water via their mouths and pump it over their gills.
 When water moves over the gill filaments, the blood within the capillary network takes up the dissolved
oxygen.
 Then, the circulatory system supplies oxygen to all tissues of the body and finally to the cells while taking
up carbon dioxide that is eliminated through the gills from the body.

11. Mechanism of Respiration:
1. Inspiration:
 The outer opening of the gill-chamber
remains tightly closed.
 The branchial-arches and the opercula
swell laterally increasing the volume of
bucco-pharyngeal cavity.
 Thus, the water flows in through the
opened mouth to fill the enlarged bucco-
pharyngeal cavity.
 2. Expiration:
 Mouth becomes closed by oral valves.
 Bucco-pharyngeal cavity contracts
exerting pressure on the contained water.
 In the meantime the opercula and
branchio-stegal membranes are lifted off.
 As a result, water from the bucco-
pharyngeal cavity passes out through the
gill-chamber, bathing the gill-filaments.
 Contraction and expansion of pharyngeal
cavity occurs by alternate retraction and
protraction of the hyoid arch which
supports the bucco-pharyngeal cavity.

12.  Amphibians utilize gills for breathing early in life
Most amphibians breathe through lungs and their
skin.
 Their skin has to stay wet in order for them to absorb
oxygen so they secrete mucous to keep their skin moist.
Cutaneous Respiration:
 The skin of frog is an important organ of respiration
(water).
 when frog undergoes summer sleep (aestivation) and
winter sleep (hibernation), the skin is the only organ of
respiration.
 The skin of frog is very much suited for the respiratory
function as it is very thin and richly supplied with blood
capillaries and remains moist with the water and also
mucus, secreted by mucous glands.
 During gaseous exchange the oxygen first dissolves in
the moisture present over the body and then diffuses
into the blood circulating in the blood capillaries, while
the resultant carbon dioxide passes out from the blood
into the surrounding medium (water) by diffusion. In
cutaneous respiration, no movements are needed
because skin always remains exposed to air or water.
Amphibia

13. Bucco-Pharyngeal Respiration or Buccal Respiration:
 The mucous lining of the buccal cavity is richly supplied with blood capillaries and remains moist by the
mucus.
 The buccal respiration occurs by lowering and raising of the floor of the buccal cavity, during the course of
which the air is constantly sucked into the buccal cavity and is drawn out through the external and internal
nares.
 In this type, the mouth and glottis remain closed.
 Thus, no air enters or goes out from the lungs.
 When the floor of the buccal cavity is lowered, the air enters the buccal cavity through the nostrils or the
nares. The oxygen of air dissolves in the layer of mucus and then goes into blood. At the same time carbon
dioxide is given out into the buccal cavity from the blood which is expelled along with residual air through the
nostrils when the floor of the buccal cavity is raised.
Pulmonary Respiration:
 Respiration on land in air with the help of lungs is the pulmonary respiration.
 In frog, lungs are poorly developed.
 The intake of oxygen by lungs is not sufficient to the body. Therefore, oxygen intake through moist skin and
buccal cavity is needed.
The lungs also hydrostatic organs as they enable frog to float in water when they are inflated. The air
enters and leaves the lungs through the respiratory fact.
(b) Lungs:
A pair of lungs are found in the anterior part of the body cavity, one on the either side of the heart.
They are ovoid, thin-walled, elastic sacs with shallow internal folds or septa that increase the inner surface to
form many chambers called alveoli. These are separated from each other through septa. The inner surface of
the aveoli is covered with a single layer of epithelial cells which are very thin and flattened except on the
edges of the septa where they are ciliated and cylindrical.

14. On the inner side of the epithelium there is aerolar type of connective tissue comprising blood
and lymph vessels and unstriped muscle fibres which give remarkable power of contraction
and expansion to the lungs. The outer surface of the lung is coated with coelomic epithelium
called peritoneum.
Mechanism of Pulmonary Respiration:
The incoming and outgoing of the air from the
lung is brought about by the action of the floor of
the buccal cavity.
The actions of the floor of the buccal cavity are
brought by two sets of muscles, the sternohyal
and the petrohyal muscles,
(i) Sternohyal muscles arise from the coracoid
and clavicle or sternum and attached to the lower
surface of the hyoid apparatus located in the floor
of the buccal cavity.
(ii) Petrohyal muscles are attached on one end
with the squamosal bone above and on the other
side with the upper surface of hyoid apparatus.

15. Respiratory organs:
Cutaneous respiration
Respiration through the skin can take place in air, water, or both
Most important among amphibians (especially the family Plethodontidae)
Gills
Cartilaginous fishes:
 5 ‘naked’ gill slits
 Anterior & posterior walls of the 1st 4 gill chambers have a gill surface
(demibranch). Posterior wall of last (5th) chamber has no demibranch.
 Interbranchial septum lies between 2 demibranchs of a gill arch
 Gill rakers protrude from gill cartilage & ‘guard’ entrance into gill chamber
 2 demibranchs + septum & associated cartilage, blood vessels, muscles, &
nerves = holobranch

16. •Bony fishes (teleosts):
•Usually have 5 gill slits
• operculum projects backward over
gill chambers
Interbranchial septa are very short or
absent
•Larval gills:
•External gills
•outgrowths from the external surface of 1
or more gill arches
•found in lungfish & amphibians
•Filamentous extensions of internal gills
•project through gill slits
•occur in early stages of development of
elasmobranchs
•Internal gills - hidden behind larval operculum of
late anuran tadpoles

17. Respiratory Structures in Reptilia
The respiratory organs include a pair of external nares, nasal chambers, internal nares,
glottis, larynx, trachea, bronchi and lungs.
 The external nares lie a little in front of eyes.
 They lead into nasal passages or chambers, which open into the roof of the buccal cavity.
 The glottis is located behind the tongue and it opens posteriorly into a short chamber, the
larynx.
 It is less prominently developed than in many of the Amphibia.
 Its walls are supported by a cricoid and a pair of arytenoid cartilages.
 The larynx opens into a narrow, elongated cylindrical tube, the trachea.
 Its wall is supported by a large number of small cartilaginous rings, the tracheal rings.
 The trachea is bifurcated into two narrow tubes in the thorax, the bronchi. Each bronchus
enters into a lung.
the lung.

18. Lungs:
The lungs are elastic, elongated sacs, the
right lung is slightly larger than the left
one.
The walls of the lungs are folded into
ridges giving the appearance of a
honeycomb.
These ridges are much closer and more
numerous towards the anterior end than
towards the posterior end of the lung.
These chambers are called alveoli where
gaseous exchange occurs. In the distal part
of the lungs, such chambers are absent.
This posterior part of the lung is considered
as reservoir for the residual air.
Respiratory movements are performed by
the intercostal muscles attached to the ribs.
Inspiration is caused by the movement of
intercostal muscles, raising the ribs that
increases the volume of the thorax and
reduces the lung pressure causing the
inflow of air into

19. Swim bladder & origin of lungs –
Most vertebrates develop an out pocketing of pharynx or esophagus that becomes one
or a pair of sacs (swim bladders or lungs) filled with gases derived directly or indirectly
from the atmosphere. Similarities between swim bladders & lungs indicate they are the
same organs.
Vertebrates without swim bladders or lungs include cyclostomes, cartilaginous fish, and
a few teleosts (e.g., flounders and other bottom-dwellers).
Swim bladders:
•may be paired or unpaired
•have, during development, a pneumatic duct that
usually connects to the esophagus. The duct remains
open (physostomous) in bowfins and lungfish, but closes
off (physoclistous) in most teleosts.
•serve primarily as a hydrostatic organ (regulating a fish's specific gravity)
.
•gain gas by way of a 'red body' (or red gland); gas is reabsorbed via the
oval body on posterior part of bladder
•may also play important roles in:
•hearing - some freshwater teleosts (e.g., catfish, goldfish, & carp)
'hear' by way of pressure waves transmitted via the swim bladder
and small bones called Weberian ossicles.
•sound production - muscles attached to the swim bladder contract
to produce sound.
•respiration - the swim bladder of lungfish has number subdivisions
or septa (to increase surface area) & oxygen and carbon dioxide is
exchanged between the bladder & the blood .

20. Lungs & associated structures
Larynx
Tetrapods besides mammals –
 2 pair of cartilages: artytenoid & cricoid
 Mammals - paired arytenoids + cricoid + thyroid +
several other small cartilages including the epiglottis
(closes glottis when swallowing)
 Amphibians, some lizards, & most mammals - also
have vocal cords stretched across the laryngeal
chamber
•Trachea & syrinx
•Trachea
•usually about as long as a
vertebrates neck (except in a
few birds such as cranes)
•reinforced by cartilaginous
rings (or c-rings)
•splits into 2 primary bronchi &,
in birds only, forms the syrinx
at that point

21. Lungs
•Amphibian lungs
•2 simple sacs
•internal lining may be smooth or have simple
sacculations or pockets
• air exchanged via positive-pressure
ventilation
Reptilian lungs
simple sacs in Sphenodon & snakes
Lizards, crocodilians, & turtles - lining is
septate, with lots of chambers &
subchambers
air exchanged via positive-pressure
ventilation
Avian Lungs:
modified from those of reptiles:
 air sacs (diverticula of lungs) extensively distributed
throughout most of the body
 arrangement of air ducts in lungs ---->
 no passageway is a dead-end
air flow through lungs (parabronchi) is unidirectional.
 Double respiration

22. Birds are different from other vertebrates, with
birds having relatively small lungs and nine air
sacs that play an important role in respiration.
The lungs of birds also do not have the
capacity to inflate as birds lack a diaphragm
and a pleural cavity. Gas exchange in birds
occurs between air capillaries and blood
capillaries, rather than in alveoli.
In addition to lungs, birds have air sacs inside
their body.
Air flows in one direction from the posterior air
sacs to the lungs and out of the anterior air
sacs.
The flow of air is in the opposite direction from
blood flow, and gas exchange takes place much
more efficiently.
This type of breathing enables birds to obtain
the requisite oxygen, even at higher altitudes
where the oxygen concentration is low. This
directionality of airflow requires two cycles of
air intake and exhalation to completely get the
air out of the lungs.

23. •Mammalian lungs:
•Multi chambered &
•usually divided into lobes
•air flow is bidirectional:
Trachea <---> primary bronchi <---> secondary bronchi <---> tertiary bronchi <--->
bronchioles <---> alveoli

24. The respiratory mechanism involves two phases:
(a) Inspiration and
(b) Expiration.
During pulmonary respiration the air is forced into the lungs. This is aided by the movable
premaxillae bones of the upper jaw situated just below the external nares and the foresaid
muscles.
During inspiration the frog closes the glottis and mouth, and the nostrils remain open. On the
contraction of the sternohyal muscles the floor of the buccal cavity along with hyoid is
lowered increasing the volume of the buccal cavity. Thus, the air enters the cavity through the
external nares.
(b) Expiration:
 Before expiration, when lungs are filled
with air, the glottis closes and the air is kept
in the lungs for a short time.
 During this period buccal respiration
occurs.
 Soon the glottis becomes opened and the
air from the lungs is expelled into the buccal
cavity by the contraction of the lungs and the
abdominal muscles and by lowering the floor
of the buccal cavity.
 Now the buccal floor is raised again, the
glottis closes and external nares are opened,
forcing the air out through the external

25. Mechanism of Breathing or Respiration:
Inspiration: Done by the contraction of the
obliquely arranged external intercostal muscles
dragging the ribs and the sternum forwards and
downwards. At the same time the radially
arranged muscles of the diaphragm contract,
thereby it becomes flattened. This results in the
increase of the internal capacity of the thoracic
cavity, thus, decreasing the pressure within the
thoracic cavity.
Expiration. It is a passive process. During
expiration, the internal intercostal muscles placed
at right angles to external intercostal muscles
contract and the later muscles also relax, thereby
the ribs and sternum attain their normal position.
At the same time the muscles of the diaphragm
relax, bringing it in its normal dome-shaped
position. Thus, the thoracic cavity and its volume
decreases, producing a huge pressure on the
lungs. Thus, the elastic walls of lungs shrink,
expelling the respired air out through the same
above path.