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Department Of Zoology
Dr. Irshad Ahangar
Class 2nd Year
Topic :- Respiratory System:Cutaneous Respiration:- The respiration or exchange of gases occurring
through the skin, outer body wall is called skin or cutaneous respiration.
There are a good number of animals, particularly aquatic where
integument or skin functions as respiratory surface. In these animals, skin
is moist or made moist so that oxygen from the outside can enter the body
through simple diffusion.
Amoeba, sponges, coelenterates and flatworms (planaria) are
chief examples which require no special respiratory system or organs.
Amongst annelids, chaetopterus possess modified parapodia to
move the water over the skin for respiration. Aquatic Drilocrius draws air
bubbles from the surface of water into the grooves, present on the dorsal
side of the body for the purpose of making skin oxygenated.
Similarly body surface of aquatic gastropods (limnae) can allow
gaseous exchange to some extent. Young larvae of some insects (blood fly
larvae) can respire through thin cuticle, specially through membranous
arthrodial membranes.
In the Eel fish, 60% of the oxygen requirement is fulfilled
through the skin.
In frog skin is very thin and richly supplied with blood
capillaries and remain moist due to secretion of mucous from mucous
glands. During gaseous exchange O2 just dissolves in moisture present
over body and then diffuses into blood, while CO2 passes out from blood
into surrounding medium by diffusion. Frogs, respire only through skin
when in water. During aestivation and hibernation. skin is the only organ
of respiration. Cutaneous respiration can be successful only when surface
are thin, wet and easily permeable. Soft covering possess risk of predators
and they tear easily. A more permeable surface present greater problems
of transport of electrolytes and of water leading to fall in respiration.
Coelenterates, sponges have a variety of water circulatory devices (canal
system) which help to increase the O2 diffusion. In higher organisms;
circulatory fluids contain respiratory pigment, which carry the gasses.
Gill Respiration (Branchial Respiration):Gills are found in aquatic worms, tube dwelling annelids, certain
crustaceans, mollusks, certain insects larvae, fishes and tadpole larvae.
Gills have evolved independently in each of these groups and
vary considerably from group to group. Typically, gills are filamentous
structures richly supplied with the blood capillaries. Gases are exchanged
between the circulating blood of gill filaments and surrounding water. In
polychaetae worms gill structure is parapodia (neries) branchial tufts
(sabelle) and gills (aranicole)
In echinoderms dermal branchial sub serve the function of gills.
In crustaceans gills may be podobranchis attached to bases of limbs);
arthrobrranch (attached to arthrodial membrane connecting appendage to
thorax); pleurobranch (attached to lateral wall of segments above limb
articulations). Gills may be branched (devdrobranch) unbranched
(Trichobrach) or with broad plates (phyllobranch). Respiratory chamber in
which gills are lodged are ventilated by fanning action of scephognathite.
Aquatic insects and their larvae have abdominal and caudal gills for
for respiration (blood gill or canal gill, tracheal gill; cuticular or spinacular
gill) Molluses possess a variety of gills, true gills or etenidia and
secondary gills (cerrate, pallial gills). In chiton there are 10-80 gills in each
pallial groove. In unio there is a pair of gills on each side of body.
Among protochordates , there are pharyngeal gills in tunicatus,
there are a large number of these gills and in Amphibians as many as 90
pairs of gills.
In fishes true gills are found. In contrast to invertebrates gills;
fish and cyclostomes gills are covered and ventilated by movements of
mouth and operculum, causing water to pass over flannels. Cyclosdomes
have 6-14; Fishes have 5-7 pairs of gills pouches which bear gills.
Amphibians and Reptiles develop 5 pairs; birds and mammals 5 or 4 pairs
of gill pouches in embryonic stage. These lack gills and disappear except
first pair which gives rise to Eustachian tube and middle ear on each side.
In some amphibians 2nd -4th pairs of pouches change into clifts, bear fills
and ersist in the adult.
Extensions of gills may be plate like landla (Landliform) or rod
like called filaments (filiform or pectinate). If the filaments are overlapping,
gill is called plicete gill. Landele on one side of interbranchial septum form
half gill or hemibranch. Two hemi branches with their intervening septum
constitute a complete gill or holobranch. In hippocampus and syngetum
gills consists of tufted process instead of planets called as Lophobranches.
Many bony fishes possess additional respiratory organs to
supplent or replace gill respiration at tissues. In claries these are branched
extensions of the gill arches bearing numerous papilac (arborescent
organs). In ophicephelus Supra branchial chamber has a number of fole
which increase the epithelial surface. These are richly vascularised. In
anabes, these are present helicoidel structures serving same purpose
(Labyrinthrine organs) these fishes require additional respiratory surface
through which air can be breathing directly. If the air is denied these
fishes become asphyxiated. If kept in water they may thrive. It is just
possible that their gills are less efficient and are incapable of fulfilling the
oxygen need of fish.
Q. Pulmonary (or lung) respiration:- Lungs include a variety of
structures, most of which are filled with air but some are water filled. They
are virtually outgrowths of alimentary canal and richly vascularised.
I.
Water lungs:- Include a range of dissimilar structures alternately filled
or emptied of water e.g. respiratory tract in Holothuria and water lungs
of pulmonate snails (lymneae and planorbis). Here water is drawn in
respiratory cavity where exchange of gases occurs and then it is
expelled out.
II.
Air lungs:- They have moist surfaces across which O2 diffuses and
water evaporates. Therefore, they are protected against evaporation
and open by small pores to the outside
III.
Diffusion lungs:- e.g. in book lungs of scorpions and spiders opening
to exterior through a narrow pore (spiracle) and have tubes for aeration
of blood. They are best known in pulmonate snails, where mantle cavity
is modified as a lung. These lungs are filled with air and simple
diffusion occurs bag-like strucute of variable shape is found above the
oesophagus and below vertebral column. This bag-like structure is
called ”air bladder” or “gas bladder”. It is primarily a hydrostatic organ
but in some fishes it serves as respiratory organ as it contains much O 2
which is usually used in hypoxic conditions. Deponi (lung fishes) use
the lungs as the main respiratory organ. It is believed that it is a fore
runner o the vertebrate lung. Infishes the air bladders are of two types.
a) Physostomteleosts (open types) in which air bladder retains its
connection with pharynx by a preuntic duct e.g. dipnoi, lower telecasts.
b) Physoclistous (closed type) in which preuntic duct is lost and air
bladder is a closed sac.
Both types can secrete into the bladder from the red gland of bladder. Red
glad makes O2, CO2 and N2 by removing them from blood and putting them
in bladder. In physochistone fishes help in buoyancy. In physomation
teleosits respirationis only probable function which can fill the bladder by
gulping air.
Gas diffusion:- Alveolar diffusion:- The passage of gases across the
blood-gas barrier in alveoli is by simple diffusion.
Fick’s law states, “volume of gas per unit time that diffuse
across the tissue barrier is directly proportional to the surface area, a
diffusion constant and the partial pressure difference across the tissue
and is inversely proportional to the tissue thickness.”
Vgas = As. D. (P1- P2)/T
A2 = Surface are, D= diffusion constant, P1 – P2 = partial pressure
difference.
Diffusion constant (D) of the gas is proportional to solubility and inversely
related to square root of molecular weight of gas.
D= solubility/ √MW
From above equation it is clear that heavy gases move more slowly as
compared to lighter molecular weight gases CO2 diffuses 85% as fast as O2
as shown below:
Rate of CO2/ Rate of O2 = √MW of O2/√MW of CO2 = √32/√44=0.85
When gas diffuses through liquid phase, rate is dependent not only
molecular weight of gas but also on its solubility in liquid.
In gaseous phase, cone of gas is directly proportional to partial
pressure only.
In liquid phase, cone of gas is directly proportional to partial
pressure + solubility
Thus a gas mmore soluble has a greater cone difference than a
gas less soluble and will diffuse more easily. CO2 is 23 times more soluble
in plasma than O2. Diffusibility of two gases is as:
DCO2/DO2 = 32/44 = 20/1
Therefore CO2 diffuses 20 times more faster than O2 in liquid
phase but in gaseous phase it diffuses only 0.85 times as easily as O 2.
2. Air sacs in birds:-
In birds around the lungs and
connected with main branchial branches are remarkable thin walled, non
muscular and non vascular bags called as Air sacs. They lie among the
viscera and even extend into larger bones. They arise from the secondary
bronchi except the abdominal air sacs which arise at the posterior end of
mesobronchi. Opening of bronchi into air sacs is termed as Ostia. Except
cervical air sacs, all air sacs rejoin the bronchi through recurrent bronchi or
saccor bronchi, Air sacs e.g. There are 9 major air sacs in pigeon. They are
named according to their location in the body such as 1. Interclavicular, 2
Cervical, 2 Anterior thoracic, 2 Posterior thoracis and 2 abdominal.
Functions of Air sacs:- The air sacs are thin reservoirs of air which
communicate with bronchi on theone side and with the pneumatic cavities
of bones on the other side.
1. Air sacs are not respiratory organs but help in respiration. They act as
bellows forcing their air into lungs for ventilation at each expiration to
completely renew the air in lungs. Thus, there is no dead space in lungs.
2. the air sacs also act as buoyant organs and reducing specific gravity of
bird due to warm air in sacs.
3. The air sacs also help to maintain and regulate body temperature acting
as cooling devices by loosing body heat through internal evaporation i.e.
water vapour diffuse from blood into air sacs and pass out through lungs
accompanied by loss of body heat.