Department Of Zoology
Dr. IrshadAhangar
Class 2ndYear
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) Mollusespossess 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.
- 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.
- 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
- 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 O2 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 physomationteleositsrespirationis 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 O2.
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.