Comparative Study of Respiratory System of Vertebrate
Exchange of 02 and C02 in an organism takes place in two locations. During internal respiration, also termed cellular or tissue respiration, gaseous exchange occurs between blood and tissues or cells of the body. During external respiration, gaseous exchange takes place between blood and the external environment. Blood serves as a transportation medium for carrying 0 2 and C0 2 away from the body cells. The body structures which are needed for gaseous exchange between the blood and the surrounding medium are known as respiratory organs. Depending on the type of medium, vertebrates have two principal types of respiratory organs : gills for aquatic respiration (in water) and lungs for terrestrial respiration (in air). The same animal may have both gills as well as lungs. Accessory respiratory organs are also present in some vertebrates. All respiratory structures consist of a moist, semipermeable and highly vascularized membrane, exposed to the external medium, so that exchange of gases takes place by diffusion between the body blood and the environment. Gills and lungs are derivatives of the embryonic pharynx.
Gills or branchiae are the aquatic respiratory organs of fishes and amphibians. Amniotes do not utilize gills at any time in their embryonic or adult life. In addition to gas exchange, gills may serve f water, and elimination of salts in marine teleosts. On the basis of their location, gills are of two general types : internal gills and external gills. In some animals, both internal and external gills are present.
Internal or true gills
Internal or true gills are characteristic of fishes. They are located in the gills slits and attached to the visceral arches. In amniotes, embryonic pharyngeal pouches do not open by gill slits to outside in the adults, so that no gills are present in them.
Gill slits are one of the most fundamental traits of the Chordata. In the embryo, the pharyngeal cavity is connected to the outside by a series of lateral openings, known as pharyngeal clefts or simply gill slits. These persist in the adult state of protochordates, cyclostomes, fishes and certain amphibians, but become reduced, abolished or modified in higher vertebrates. The number of gill slits varies in different chordates— 140 in amphioxus, 6-14 pairs in cyclostomes, 5 pairs in most elasmobranchs, 6 pairs in Hexanchus, 7 pairs in Heptanchus, 4 pairs in chimaeras, 5 pairs in most bony fishes, and 4 pairs in some teleosts. The gill slits are separated from one another by partitions called visceral or gill arches. The arches are supported by skeletal structures of splanchnocranium, together forming the visceral skeleton.
Structure or a true gill
True gills are developed on the walls of some gill clefts or gill arches. Typically, a gill is composed of two rows of numerous gill filaments or lamellae. These are derived from epithelium on either side of an interbranchial septum containing arteries and supported by the branchial cartilage or bone of a gill arch. A single row of lamellae on one side of branchial septum forms only half the gill, called a demibranch or hemibranch. A septum with two attached demibranchs comprise a complete gill or holobranch. Gill filaments are richly supplied with blood capillaries and it is here that exchange of gases with water takes place.
Gills of elasmobranchs (e.g. dogfish) are generalized in structure and relationships. Gills of bony fishes are also basically similar but show the following differences :-
- Operculum :- In a bony fish, a bony flap, called operculum or gill cover, arises from the hyoid arch and covers the gills in a common opercular cavity which opens by a single slit-like crescentic external gill opening behind.
- Interbranchial septum :- The median septum is best developed in elasmobranchs. It is reduced in some intermediate fishes like chimaeras. It is greatly reduced or virtually absent in teleosts.
- Spiracles :- In elasmobranchs and ganoids, the first gill slit, between mandibular and hyoid arches, bears a reduced pseudobranch and opens to outside through a small opening, the spiracle. In chimaeras, lung fishes and teleosts, spiracles become either closed or lost in the adult.
- Reduction in number of demibranchs :- Number of gills greatly varies among fishes. There are 7 pairs in Heptanchus, 6 pairs in Hexanchus and 5 pairs in most elasmobranchs in addition to spiracle. However, the demibranch found on hyoid arch in elasmobranchs is lost in modern ganoids and teleosts which have only 4 holobranchs. Additional demibranchs are lost in some lungfishes. The extreme case of reduction is found in the eel Amphipnous in which first and fourth branchial arches are without gills, while the second arch retains only a demibranch.
External or larval gills
As against true gills, the external gills are formed as branching outgrowths from the exposed outer epithelium of gill arches and not from that of the pharyngeal pouches. They are ectodermal in origin, and usually temporary organs found only in larval stages, hence also termed larval gills. They occur in the larvae of lampreys, a few bony fishes including Polypterus (bichir), lungfishes (e.g. Lepidosiren), and all amphibians including caecilians. In amphibians, larval external gills are absorbed at the time of metamorphosis, but in water-living perennibranchiate urodeles, both external gills and gill slits persist during adult life. In Amphiuma, gills are absorbed but gill slits remain. Gills assume various shapes being pectinate, bipinnate, dendritic, leaf-like, etc. Each gill consists of a narrow main central axis bearing a double row of filaments. Thoroughly vascularized by aortic arches, external gills are simply waved in water, and no respiratory water current passes through gill slits as in the case of true gills.
Lungs and Ducts
Lungs are the essential respiratory organs of land vertebrates or tetrapods and lung fishes. They are very elastic and distensible. Phylogenetic history of the development of lung is still obscure. Most accepted theory regarding the origin of lung was forwarded by Goethe. He believes that they are derived from the last pair of gill pouches which do not open to exterior through gill slits. Lungs receive blood supply from 6th aortic arch also strongly supports this view. In tetrapod embryos, lungs arise as a single midventral diverticulum (lung primordium) from the floor of pharynx. It soon bifurcates into right and left lung buds. The undivided common portion develops into windpipe or trachea and larynx and opens into pharynx through glottis. Each lung bud branches repeatedly and grows posteriorly into coelom, invested by mesoderm. Thus, each lung has an inner endodermal lining derived form embryonic gut, an outer visceral peritoneum and in between the two a mesodermal mesenchyme containing lymph and blood vessels, nerve and smooth muscle fibres and connective tissue.
Beginnings of larynx are seen in Amphibia. In its simplest condition (Necturus), it is supported by a pair of lateral cartilages, bounding the slit-like glottis. In other amphibians, each lateral cartilage is divided into a dorsal arytenoid and a ventral cricoid. Sometimes both the cricoids fuse to form a cartilaginous ring (frog). It is suggested that these skeletal parts have evolved from the modification of vestiges of branchial arch (probably VI). It is further supported that this region is innervated by a branch of vagus (the X cranial nerve). In Anura, inner lining of laryngotracheal chamber forms two muscular bands, or vocal cords, which vibrate to produce various calls. Larynx is scarcely more developed in reptiles. Cricoid in case of reptiles is more differentiated than amphibians and in many cases gives off process with which arytenoids are movably articulated. It is small and rudimentary in birds in which another organ, the syrinx, located at the lower end of trachea, is responsible for sound production. In most common type of syrinx (.Bronchotracheal type), there is tympanum formed of last tracheal cartilage. Besides this, into this box like structure certain membranes project from the walls of bronchi viz., membrana tympaniformis interna and membrana tympaniformis externa. However, in singing birds there is also another paired vibratory membrane called membrana semilunaris which extend dorsoventrally near the junction of bronchi and trachea. A bony ridge pessulus support these membranes. Larynx reaches its greatest point of evolution in mammals. Besides paired arytenoid and single cricoid, a single thyroid cartilage is added on ventral surface of larynx. Although, thyroid cartilage was paired initially as in monotremes it is made of two plates rather than one. Moreover, in all mammals it develops from remains of paired branchial arch embryologically. The vocal cords reach maximum differentiation in mammals. They are two pairs band like folds on the inner walls of larynx, extending between arytenoids and thyroid cartilages, one above the other. The upper is called false and lower one is called true vocal cord. Elephants do not have false vocal cord and hippopotamus has no vocal cords. A flap-like muscular epiglottis is present in front of glottis and is characteristic of mammals.
Part of air duct between larynx and lungs is termed trachea. Its wall is prevented from collapsing due to a series of usually incomplete cartilaginous rings arranged in various ways. Lower end of trachea bifurcates forming two bronchi, lined with cilia, and each entering a lung. In Anura, trachea is extremely short or absent, merging with larynx to form a laryngo-tracheal chamber. A definite trachea is differentiated in Siren, Amphiuma and Gymnophiona only. It reaches to a length of 4-5 cm. The walls are further strengthened by a series of small, irregular cartilages, which are usually united to form bands. Length of trachea varies in reptiles depending upon that of the neck. The cartilage rings of trachea gradually become more solid and complete but are generally incomplete dorsally. In lizards and snakes the anterior cartilage round the trachea forms complete rings. In birds, trachea is unusually elongated, and tracheal rings are complete and ossified. In mammals, trachea is variable.
Swimming bladders of lung fishes (Protopterus) are better lungs than those of most amphibians. Two lungs of modern Amphibia are simple, hollow sacs with a wide central cavity, and suspended freely into peritoneal body cavity. They are elongate in urodeles but bulbous in anurans. Left lung is usually longer in urodeles but rudimentary in caecilians. Internal lining of lungs may be smooth (Necturus), or may have simple sacculations proximally (Siren, Amphiuma). In frogs and toads (anurans), lung wall may be divided peripherally by a network of folds or trabeculae into air sacs or alveoli. They are richly vascular, lined with mucous epithelium and their inner edges bearing tall ciliated columnar cells.
Lungs of reptiles are more complicated and also abdominal in location. In Sphenodon and snakes, lungs remain simple thin-walled sacs. In legless lizards and snakes, left lung may be rudimentary or absent. In Boa and Python both the lungs are functional but left one is slightly smaller. In lizards and turtles, wall of lung is considerably thickened due to inclusion of greater amount of highly vascularized connective tissue in partitions, so that the whole lung becomes spongy. The crocodilian lung most nearly approaches the condition found in mammals. Reptilian lungs also hang freely in the body cavity. In chamaeleons, several long, thin-walled, sac-like diverticula or air sacs arise from distal portion of lungs. With the help of these they use to swell to some extent which frightens the predator and helps it to escape.
Structure and function of lungs in birds are not wholly understood. Avian lungs are unique in architecture and greatly modified due to their aerial mode of life. They show many peculiarities not found in other groups. Lungs are small, compact, spongy and only slightly capable of contraction and expansion. They are placed outside coelom in pleural cavities. The lower or ventral surface of each lung is closely invested by thin fibrous membrane called pulmonary. Several muscle bands called, costo-pulmonary muscles which arise from the vertebral ribs are inserted into pleura which are supplied with inter-costal nerves. They give out several thin-walled membranous air sacs (= Cellulae aereae) that invade most parts of the body. The bronchus after entering the lung, divides repeatedly forming a network of anastomosing air capillaries which do not terminate blindly. As a result of these novelties, avian lungs become highly efficient organs.
Mammalian lungs are also highly developed, spongy and very elastic. They lie protected in special chambers, called pleural cavities, which are separated from rest of perivisceral body cavity by a muscular diaphragm. The pericardium containing heart lies between the pleural cavities. In most mammals, lungs are subdivided externally into lobes, usually more on the right. Thus while there are only 2 left lobes, the right lung has 3 lobes in man and 4 lobes in rabbit. In certain mammals such as whales sirenians, elephants, hyrax, and many perissodactyles, lungs are simple and without lobes. In monotremes and rats, only the right lung is lobulated. The mammalian lung consists like an elaborate branched respiratory tree. The bronchus divides repeatedly inside the lung ultimately resulting into a large number of terminal grape-like clusters of air-sacs or alveoli. Being terminal and blind, they always retain a certain amount of residual air after every expiration. In mammals, intercostal muscles, ribs, diaphragm, sternum and abdominal muscles, all aid in breathing.
Accessory Respiratory Organs
Although gills serve as chief respiratory organs of aquatic vertebrates, and lungs serve terrestrial vertebrates in a similar way, other structures present may also provide accessory respiratory mechanism, for taking 0 2 directly from water or air.
- Yolk sac and allantois :- Practically all embryonic vertebrates use yolk sac with its vitelline circulation for gaseous exchange in addition to absorbing yolk which is used as food. Yolk sac of dogfish embryo and yolk sac placenta of the marsupials, in contact with uterine wall, serve as respiratory devices. In reptiles, birds and mammals, allantois and allantoic (umbilical) vessels also become temporary respiratory organs during embryonic life.
- Skin :- In amphibians, respiration is common via moist and naked skin which is highly vascular. Lungless salamanders or plethodonts rely entirely on skin for respiration, since larval gills disappear at metamorphosis and adults fail to develop lungs. Vascular hairy cutaneous projections in the male of so-called African hairy frog, Astylosternus serve a respiratory function. The vascular caudal fin of mud-skipper Periophthalmus, which remains submerged, also functions as a breathing organ.
- Lining epithelium :- In some fishes and aquatic amphibians, the lining of cloaca, rectum, gut or bucco-pharyngeal epithelium is highly vascular and aids in respiration.
- Cloacai bladders :- Reptilian skin is cornified and useless in respiration. But, in some aquatic turtles, a pair of thin-walled, lateral and greatly vascular cloacai bladders are continually being filled and emptied of water through vent. These accessory bladders serve as important respiratory organs, especially during submergence for longer durations.
- Pelvic gills :- In American lung fish Lepidosiren, the bushy, filamentous vascular gills attached to the pelvic fins of male provide fresh oxygen to the guarded eggs.
- Opercular gills :- In some fishes with an operculum such as Acipenser, Lepidosteus, Polyodon, Polypterus and many teleosts, a series of vascular lamellae with a respiratory function develop on the inner surface of operculum.
- Pseudobranchs :- Pseudobranchs in spiracles of elasmobranchs and also in some teleosts are homologous with true gills and regarded as demibranchs of the mandibular arch. However, they are not respiratory as they receive already arterial blood.
- Pharyngeal diverticula :- The vascular posterior extensions of pharynx in Periophthalmus, Amphipnous and Channa (= Ophiocephalus) serve to breathe atmospheric air during aestivation and emergence out of water for short periods.
- Branchial diverticula :- The vascular outgrowths of branchial chamber in Heteropneustes (= Saccobranchus), Clarias and Anabas form more complicated aerial accessory respiratory organs.
- Swim bladders :- Another important structure serving as a lung in some lower fishes is the swim bladder or air bladder.