COMPARATIVE ACCOUNT OF RESPIRATORY SYSTEM WITH SPECIAL

REFERENCE TO THE PHARYNX

Respiration is the process of exchange of gases (O2 and CO2). The oxygen is taken inside the

body from the environment for oxidation of food to release energy & CO2 so produced is

expelled out. The respiratory organs in vertebrates are either the gills or the lungs which are

associated with the pharynx.

The basic necessities of any respiratory organ are

1. The respiratory organs must be thin walled so that there is easy diffusion of gases

2. It must be richly supplied with blood enable exchange of gases

3. It should have large area for contraction and expansion.

Respiratory organs

1. Gills : Fish, larval amphibians, adult urodeles

2. Swim bladder: Fishes

3. Lungs: Tetrapods (amphibian, reptilia, birds &mammals)

I. GILLS

In fishes, gills are the main respiratory organs. In amphibian’s larval forms (tadpole) and in the

adult urodels gills are the respiratory organs.

Development of gills

a) In the embryo, the pharynx develops paired pouches due to evagination or pushing out.

At the same time skin develop grooves due to invagination or pughing in when the pouch

and the groove meet and the membrane between the two disintegrates, a slit develops to

form gill slit

b) The gill cleft or slit contains gills which are present in a gill chamber

c) Each gill chamber has an internal branchial aperture opening to pharynx

d) An external branchial aperture opens to external

e) The gill chambers are separated by inter branchial septa and supported by gill rays

f) Each half of the gill filament is called Demibranch.

g) The demibranch on either side of interbranchial septum, gillrays, connective tissue and

associated blood vessels with nerves form a holobranch.

h) The demibranch at the anterior end is called pre-trematic demibranch and at the

posterior end is called post trematic demibranch.

GILLS IN FISHES: are internal and vary in number depending on the classes they belong to.

Note: Operculum is a bony flap like covering that is seen over the gill slits on either sides in

bony fishes; they develop from the hyoid arch an extends backwards or posterior, covering the

gill slits

Difference between bony and cartilaginous fishes

Cartilaginous fishes

(Elasmobranches or Chondrichthyes)

Bony fishes

(Teleosts/ Osteichthyes)

There are five pairs of gill slits There are four pairs of gill slits

There are a pairs of spiracle in front of the

hyomandibular arch

Spiracles are absent

Operculum is absent Operculum is present

Gills open to the exterior by inter branchial aperture Gills open to the opercular chamber

opercular chamber is absent opercular chamber is present

They have 1 demibranch & 5 Holobranches They are 4 holobranch, the demibranch

of the first gill is lost

GILLS IN AMPHIBIANS: The larval forms called tadpoles are purely aquatic and they need to

utilize oxygen in the water. This can be done only with the help of gills. Initially three pairs of

external gills (gills that are constantly bathed in water) will be developed which will be replaced

by internal gills as the development proceeds. In most of the amphibians these internal gills will

be replaced by lungs during metamorphosis. Only in Urodeles like Salamanders and Necturus, 3

pairs of external gills persist throughout the life.

II. SWIM BLADDER/AIR BLADDER

Swim bladders or air bladders are paired or unpaired structures arising from the pharynx or

oesophagus of bony fishes. The air bladder arises as an outgrowth of the pharynx on either side,

initially lateral in position then becomes dorsal. It lies below vertebral column and outside

coelom.

Connection between between the pharynx and airbladder is called pneumatic duct. Swim

bladder serves both as respiratory and hydrostatic organ. In lung fishes the Swim bladders

resembles the lungs. They arise from the ventral part of the pharynx. The lining of air bladder

shows several air sacs. Arteries arising from 6th embryonic arch supply the airbladder.

Oxygenated blood is returned to the left atrium of heart.

When the fish gulps in air, it enters inside through mouth and pharynx, Oesopharyngeal pump

forces air into pneumatic duct. There is exchange of gases inside airbladder. When air bladder is

compressed, gas exchange & CO2 is expelled through the mouth as bubbles. In lungfishes like

Protopterus and Lepidosiren, swim bladder is a better respiratory organ that makes respiration

more effective eventhough they have lesser number of demibranches.

III. LUNGS

All air breathing organisms possess lungs. From amphibians to mammals, all terrestrial

organisms posses lungs that arise from pharynx.

Formation of lungs

1. Lungs develop from the floor of pharynx at its posterior end as

a single small bud called lung bud through single evagination.

2. The lung bud slightly elongates & bifurcates into two

3. The opening of lung bud into the pharynx develops a small slit

& form the glottis. The part after glottis forms the larynx

(voice box)

5. The elongated part of the lungs before bifurcation forms

Trachea (wind pipe)

6. The bifurcated part becomes the bronchi

7. The ends of bifurcated parts of the lung buds expand to form

lungs.

8. The lungs push backwards & come to lie on either side of the

heart. Lungs get surrounded by coelomic epithelium

LARYNX

Larynx is the part between glottis and upper end of trachea, well developed in tetrapods

a. Larynx in Amphibia

a. In amphibians like anurans there is a laryngo-tracheal chamber for production of sound

(urodeles and apodans do no not produce sound)

b. Laryngo-tracheal chamber has three cartilaginous structures to keep it stretched-one ring

like cricoid cartilage and two semicircular arytenoids cartilages

b. Larynx in Reptiles

Larynx in reptiles is similar as in frog. But there is a well developed hyoid cartilage to hold

larynx in position

c. Larynx in Birds

In birds larynx is simple and there is another organ for sound production called Syrinx

d. Larynx in Mammals

In mammals Larynx is Highly developed with 3 cartilages

I. Thyrenoid/ thyroid cartilage which forms a half ring like structure at the anterior end of

larynx

II. Dorsal to thyroid cartilage is the arytenoids

III. Below arytenoids are the crinoids, followed by trachea.

Anterior to glottis is an epiglottis which is a fold of mucous membrane of pharynx supported

by an internal cartilage. The epiglottis kept erect while breathing, but while feeding it is pulled

down to closes the glottis. A pair of vocal cords is found between thyroid & arytenoids

cartilages to produce sound.

TRACHEA

Lungs in amphibians

Amphibian lungs are not very efficient (hence they rely on cutaneous respiration).

Amphibian lungs vary in different orders,

Urodela Anura (eg: frog)

1.Lungs are less functional, when compared to gills

1. Lungs are comparitvely more functional, no gills

2. Lungs of Siren is as long as body cavity 3. Lungs of Necturus is long and poorly vascularised

2. Lungs are not long. They are thin walled sac like structures covered by peritoneum. Internal linings of lungs are folded to increase the surface area for exchange of gases 3. The space inside is divided into smaller areas called alveoli, which are highly vascularised, Inner lining has mucous glands.