Respiratory System


Accessary Respiratory Organs in Fishes

Many species of fishes developed breathing organs other than gills for supplementing deficiency of oxygen in water. These are as follows:

Dendritic Organs

They are also called arborescent organs as they are highly vascularised tree-like, branched structures produced by the second and fourth gill arches and located in the suprabranchial chamber, posterior to the gills. Paired gill fans at the opening of branchial chamber force air over the dendritic organs as the fishes gulp air. Dendritic organs are found in catfishes such as Clarias.

Labyrinthine Organs

These are rosette-like concentric plates of tissue present in the suprabranchial chamber of climbing perch (Anabas), Trichogaster, Osphromanus and Polycanthus. Respiration takes place when these fishes gulp air. Perches can migrate from one pond to another by breathing air through labyrinthine organs and using pectoral fin spines to walk on land.

 Pneumatic Sac

It is a tube like extrabranchial diverticulum that extends up to tail in some cat fishes such as Heteropneustes, which can survive out of water for considerable time using these organs for air breathing.

Air Chamber

Air chamber is a small, highly vascularised sac located behind the gills of some fishes, e.g. Ophiocephalus, Macropodus and cuchia eel (Amphipnous). These fishes can gulp air and use it as air breathing organ.

Buccopharyngeal epithelium

Mud skippers (Periophthalmus, Balaeophthalmus) possess vascularised buccopharyngeal epithelium and also a respiratory tail. They skip around in swamy areas, breathing air by buccopharyngeal epithelium or keep their tail in water for aquatic respiration.

Integument

Eels (Anguilla) breathe through skin while migrating from the American and European rivers to Sargasso Sea in Bermuda. As much as 60% exchange of gases takes place through the highly vascularised skin.

Gut epithelium

Fishes such as Callichthys, Hypostomus, Doras, Misgurnus, Cobitis can suck and release water through anus and exchange of gases can take place in the rectal lining. In giant loach (Cobitis) and Misgurus lining of stomach and intestine is used as respiratory organ.

LUNGS

Lungs of Polypterus  and the ganoid fish Calamoichthys are asymmetrical and connected by pneumatic duct on the ventral side of pharynx. The blood is supplied to lung by pulmonary artery that emerges off the 6th aortic arch, but unlike in lungfishes venous blood returns to hepatic vein.

Lungs of Dipnoi (Choanichthys) are bilobed or paired as in Protopterus (African lung fish) and Lepidosiren (South American Lung fish) and are connected to oesophagus via a pneumatic duct. But the Australian lung fish (Neoceraodus) has a single lung that is used as hydrostatic organ.

In tetrapods, embryonic lungs arise from pharyngeal wall as a hollow mid-ventral evagination that subsequently bifurcates to form two lungs that carry an envelope of peritoneum.

AMPHIBIA

Lungs of amphibians are two simple sacs, narrow and elongated in urodeles and bulbous in anurans enclosed in a single peritoneal membrane and supplied by pulmonary arteries and drained by pulmonary veins. Left lung in limbless amphibians is rudimentary. Lungs are vestigial in salamanders inhabiting hill streams where in fast flowing water buoyancy would not be a desirable trait.

Amphibians lack ribs and hence use floor of the buccal cavity to force air in and out of the lungs. Frogs and toads modify 2nd, 3rd and 4th visceral arches to produce a plate-like hyobranchial apparatus that lies in the floor of oral cavity and is connected to squamosal bone of skull by petrohyal muscle and to sternum by sternohyal muscle. One breathing cycle is completed in four steps in anurans that is affected by contraction of these two muscles.

Breathing in frog requires much faster movement of hyoid plate as compared to lungs and considerable amount of gas exchange takes place in bucco-pharyngeal region too. Cutaneous respiration also contributes to major part of oxygen supply to the body of amphibians.

REPTILES

Lungs are narrow and elongated in snakes and lizards extending up to two-third of the body cavity but are more bulbous in turtles and crocodiles. The left lung is rudimentary in limbless lizards and snakes. There are well formed alveoli in lungs which are housed securely in a pair of pleuro-peritoneal cavities.

Breathing in snakes and lizards is carried out by a combination of hyoid plate, nostril valves and ribs or only by the movement of rib cage, while tortoises and turtles make use of muscles surrounding peritoneal membranes. Crocodiles are the only living reptiles that possess a muscular diaphragm for breathing as do the mammals.

BIRDS

Lungs are secured into pleural cavities and extend into membranous air sacs that occupy all available space in the body cavity and also penetrate into bone marrow cavities. This makes the bones pneumatic in birds and help to reduce body weight which is so necessary in flight. Majority of birds have 5 pairs of air sacs, namely, cervical at the base of neck; interclavicular often united across midline; anterior thoracic placed lateral to the heart; posterior thoracic within the oblique septum and abdominal within the abdominal cavity. Sometimes there are also axillary air sacs near the pectoral muscles. The flight muscles inflate and deflate the air sacs like bellows with each stroke of wings.

Air duct system is unique in birds as there are no alveoli as found in reptiles and mammals. Trachea divides into two bronchi which enter the lungs and branch into mesobronchi that again divide to form parabronchi. From each parabronchus, bunches of air capillaries arise which loop back into their own lumen to form anastomosis that eventually leads into the air sacs. Air capillaries are minute and only one cellular layer thick and contain respiratory epithelium and rich network of blood capillaries.

During inspiration and expiration, air passses through the air capillary anastomosis into the air sacs and back twice making it a double respiration.

MAMMALS

Mammalian lungs increase efficiency by increasing the surface area of respiratory epithelium of alveoli whose number goes up to millions and lungs become almost like semisolid sponge with little empty lumen inside. Lungs are enclosed in double peritoneal membranes, the outer parietalpleura and inner visceralpleura that enclose the fluid-filled pleuro-peritoneal cavity in between.

Trachea which is commonly known as windpipe opens in pharynx by a slit-like glottis and posteriorly divides into two primary bronchi that enter lungs and branch off to secondary and tertiary bronchi which ultimately lead to fine capillaries called bronchioles. Each bronchiole is connected to several alveoli by alveolar ducts. The number of alveoli in human lungs is estimated to be about 750 million which collectively carry an enormous surface area of about 100 m2 and if stretched.

A muscular diaphragm located between the thoracic and abdominal cavities moves in the antero-posterior direction and forces air in and out of the lungs. External costal muscles and internal costal muscles are attached between the ribs and sternum, the former increases the thoracic space while the latter decreases it to carry out what is known as thoracic respiration or common panting after strenuous physical exercise.

Whales possess enormous nasal chambers in the head that can store large quantity of air when diving deep in the sea. Passage of air from nasal chambers to lungs is controlled by a pair of valves.

SOUND PRODUCING ORGANS

Sound producing organs, larynx and syrinx, are associated with trachea through which air can be forced from lungs into the sound box to produce sound. Larynx in urodeles is so simple that it has only a pair of lateral cartilages, called Guardian cartilages that surround the glottis and this apparatus is incapable of producing any sound in these animals.

Larynx of frog is made of a cricoid cartilage which is a modification of the first tracheal ring and a pair of arytenoid cartilages, which support a pair of vocal cord that vibrates to produce sound. Males of frogs and toads in addition possess a pair of vocal sacs which are evagination of oral cavity and serve as resonance chambers to amplify sound.

Reptiles are silent animals but possess larynx, albeit without a vocal cord, in the absence of which they can at best produce a hissing sound.

Birds inherited a rudimentary larynx from their reptilian ancestors and hence evolved a secondary sound producing organ called syrinx located at the junction of trachea and bronchi and hence called bronchotracheal type. The tympanic chamber has a bony ridge at base called pessulus that supports the unpaired membranasemilunaris, which vibrates to produce sound.

In mammals larynx consists of paired arytenoid cartilages that support vocal cord. Base of the sound box is made of a ring-like cricoid cartilage and the unpaired thyroid cartilage forms the surrounding walls of the tympanic chamber. A pair of fleshy vocal cords is stretched across the cephalic part of the vocal chamber supported by the paired cartilages of Santorini. The cord vibrates to produce sound that is modulated in the oral cavity.

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