(By Prof. Girish Chandra)
In higher animals, the main function of respiratory system is to convey oxygen from the external environment to the tissues where it is used up for oxidation of glucose to produce energy, and to carry carbon dioxide that is produced in the tissues and release it out of the body. The blood along with its haemoglobin serves to transport gases to and fro the sites of absorption of oxygen and release of carbon dioxide, which happen to be gills, lungs, bucco-pharyngeal epithelium, skin or other accessory respiratory organs.
Here we shall study the anatomy of different types of respiratory organs which have evolved in the vertebrate series according to the needs of different species and the environment in which they live. Every respiratory device must conform to the following essentials features:
ü Blood must be separated from the external environment that is air or water by a thin epithelium.
ü The epithelium must be permeable to permit diffusion of gases through it.
ü The respiratory epithelium must always remain moist with a film of fluid to permit osmosis of gases.
ü The area of respiratory surface should be extensive to allow efficient absorption of oxygen.
ü Both the current of air or water outside and blood in capillaries must be made to circulate constantly for quick replacement of gases.
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.
Respiratory organs of Cyclostomes
Agnathans have 6-15 pairs of gill pouches, which 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. In Myxine behind the gill pouches there is a single pharyngo-cutaneous duct on the left side, which is a modified gill pouch which drains excess water that fails to enter the gill pouches. The lamprey, Petromyzon, has 8 embryonic and 7 adult paired gill pouches that open to the exterior by independent openings.
Respiratory organs in elasmobranchs
Most elasmobranchs possess 5 pairs of gill slits and a pair of spiracles. There is no operculum covering the gill slits in cartilaginous fishes. A demibranch is a bunch of gill lamellae attached on one side of the interbranchial septum. Hence, there are altogether 9 pairs of demibranchs in elasmobranchs. Between the two demibranchs lies the interbranchial septum, which is supported by gill cartilages. Anterior to the first gill slit is a spiracle or pseudobranch. In free swimming sharks and dogfishes water generally enters through the mouth.
Blood to the gills is supplied by five pairs of afferent branchial arteries coming from ventral aorta and hence they bring deoxygenated blood from heart. Blood is then oxygenated in gills and is collected by the loops of four pairs of efferent branchial arteries and carried to the paired dorsal aorta, the two sides of which meet posteriorly to form single median dorsal aorta that supplies oxygen-rich blood to the whole body.
Gills of bony fishes
In bony fishes gills are covered with an operculum that is made of flattened skeletal plates and there is no spiracle as in elasmobranchs. There are 4 pairs of gill pouches, each containing two demibranchs, making the total number of demibranchs in bony fishes as 8 pairs or four pairs of complete gills or holobranchs. Teleosts always breathe with their mouth open and eject expiratory water by opening operculum. Gills in Chondrostei, Holostei and the lungfish Neoceratodus exhibit partial reduction in their interbranchial septa, which happens to be somewhat intermediate condition between elasmobranchs and teleosts.
External gills develop from the outer wall of pharynx or from the exposed portion of branchial arch. They occur in larval lampreys, few larval fishes, Polypterus, lungfishes, some larval teleosts and all larvae and some adults of amphibians. There is a single pair of larval gill in the chondrosteian bony fish, Polypterus, which has a long axis carrying gill lamellae. The African and South American lung fishes possess 4 pairs of feathery external gills. The larval forms of some amphibians and some adult urodeles possess external gills which arise simply as folds of skin on the surface of the III, IV and V branchial arches but weakly supported by the skeletal system. Perennibranch amphibians as Necturus and Proteus retain external gills throughout life along with 2 or 3 pairs of gill slits, which are functionless as the water does not pass through pharynx. Instead, gills are waved in water by means of muscles attached at the base of gill axis for respiration. The larvae of limbless amphibian, Caecilia, have a pair of exceptionally large leaf-like gills with profuse blood supply. Salamanders that inhabit hill streams, e.g. Eurycea and Salamandrina, which belong to family Plethodontidae have neither gills nor lungs for respiration and survive only on cutaneous respiration.
Barring agnathans, cartilaginous fishes and few bottom dwelling teleosts, all fishes carry a gas-filled air bladder on the dorsal side of the gut, which serves as hydrostatic organ. On the ventral side of the bladder there occurs a highly vascularised area called red gland that is supplied by intestinal artery and portal vein and which has unique capability of extracting free oxygen from the blood and release it into the air bladder in order to make it inflate. A small pouch-like diverticulum called oval that can be closed or opened by sphincter muscles is the site of reabsorption of gases. Secretion and absorption of gases in swim bladder occurs under the control of autonomic nervous system, based on the depth at which a fish is swimming.
In Cypriniformes (Teleostei), a series of four small bones (tripus, intercalarium, scaphium and claustrum), derived from the first three vertebrae and called Weberian Ossicles, connect the anterior end of air bladder with the sinus impar of membranous labyrinth. Sound vibrations received by air bladder from the surrounding water are conveyed to the internal ear through this unique apparatus to bestow some hearing ability to these fishes.
In some fishes as for example ganoids, carps and catfishes, a pneumatic duct connects the air bladder with oesophagus. Such condition is called physostomous (Gr. physo=bag; stoma=opening). Fishes which do not have such a pneumatic duct connecting the air bladder are called physoclistous (Gr. physo=bag; clista=closed).
The comparative study of air bladders in different groups of fishes and striking similarity between the swim bladder and lung suggest a phylogenetic relationship between the two. The conventional belief is that lungs evolved from the air bladder of fishes. However, recent evidences point to the contrary that lungs evolved first in fishes for supplementing oxygen from air and then they got transformed into swim bladder as the oxygen concentration in water increased.
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:
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.
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.
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 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.
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.
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.
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 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.
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.
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.
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.
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 parietal pleura and inner visceral pleura 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 membrana semilunaris, 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.