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:
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 WeberianOssicles, 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.