Respiratory System in Vertebrates

Every cell in a living organism oxygen (O²) during oxidation of substances resulting in the release of heat and energy and production of carbon dioxide (CO²). This CO² acts as a poison for protoplasm unless removed from the body . The term respiration is used in several senses. According to a simple definition , it means intake of O² and getting rid of CO² by a living organism. The system designed for gases(O² and CO²) between the organism and its environment is termed the respiratory system.

Respiratory Organs -

Exchange of O² and CO² in an organism takes place in two locations. During internal respiration, also termed cellular or tissue respiration, gaseous exchange occurs between blood and tissue 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 carring O² and CO² away from the body cells . The body structure 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 type of respiratory Organs : gills for aquatic respiration (in water) and lung 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 and the environment . Gills and lungs are derivatives of the embryonic pharynx.

Gills —

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 for loss or gain of water , and elimination of salts in marine teleosts. On the basis of their location, gills are of two general type : internal gills and external gills. In some animals, both internal and external gills are present.

(A) 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.

1. Gill slits - 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 skeleton structures of splanchnocranium, together forming the visceral skeleton.

2. 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 - 

a) Operculum - In a bony fish, a bony flap, called operculum or gill cover , aries 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.

b) 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.

c) 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 Spiracles. In chimaeras, lung fishes and teleosts, Spiracles become either closed or lost in the adult.

d) 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 Spiracles. However , the demibranch found on hybrid arch in elasmobranchs is lost in modern ganoids and teleosts which have only 4 holobranchs. Additional demibranchs are lost in some lung fishes. 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.

(B) External or Larval Gills

As against true gills, the external gills are formed as branching outgrowths form the exposed outer epithelium of gill arches and not from that of the pharyngeal of gill arches. 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), lingfishes (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 Anphiuma, gills are assume various shapes being pectinate, bipinnate, dendritic, leaf-like, etc. Each gill consist of a narrow main central axis bearing a double row of filaments. Thoroughly vascularized by aortic arches, external gills are simply waves in water, and no respiratory water current passes through gill-slits as in the case of true gills.