Unit 4 Respiratory System UNIT 4 RESPIRATORY SYSTEM Structure 4.1 Introduction 4.4 Respiration by Lungs Objectives Respiratory System of Amphibians 4.2 Salient Features of Respiratory Respiratory System of Reptiles System of Vertebrates Respiratory System of Birds 4.3 Respiration by Gills Respiratory System of Mammals General Gill Structure 4.5 Summary Respiratory System of 4.6 Terminal Questions Cyclostomes 4.7 Answers Respiratory System of Fishes Accessory Respiratory Organs in Fishes Respiration in Amphibian Larvae using Gills 4.1 INTRODUCTION In the previous unit you have learnt about the structure of various types of digestive systems that occur in vertebrates and their role in providing nutrients to the cells of the body. These nutrients are essential for fulfilling the energy requirements of the animal cells. The nutrients produce energy when they undergo oxidation during the process of respiratory metabolism. The end products generated by oxidation are energy, water, and carbon dioxide. Thus vertebrates consume oxygen for oxidation of nutrients present within the cells. This oxygen needs to be replenished and the waste byproducts like heat and carbon dioxide produced during oxidation metabolism must be removed in order to survive. The sequence of events that result in exchange of oxygen and carbon dioxide between an organism and its environment is known as respiration. This is done primarily by the respiratory system. The gas exchange between the environment and blood via the respiratory surface is referred to as external respiration. The utilization of oxygen for oxidation of nutrients within the cells and tissues may be termed as internal respiration. In this unit you will study the major respiratory structures i.e. gills, and lungs that facilitate respiration in both aquatic and terrestrial vertebrates. 109 Block 1 Comparative Anatomy of Vertebrates-I ------------ Objectives After studying this unit you should be able to: describe the structure and function of gills which form the respiratory system of aquatic vertebrates like fishes and amphibian larvae to breathe in water and air and explain how they are used; describe the structure and function of accessory respiratory organs in fishes; and describe the structure and function of respiratory organs in terrestrial vertebrates. 4.2 SALIENT FEATURES OF RESPIRATORY SYSTEM OF VERTEBRATES Respiration is the sequence of events that result in exchange of oxygen and carbon dioxide between the environment and organism. External respiration refers to gas exchange between the environment and blood via a respiratory surface. Respiratory organs are of two types: (i) those that have respiratory surface turned out forming an evagination called gills, (ii)those that have respiratory surface turned in forming an invagination called lungs. Our own lungs are a good example of such invagination. The organs of respiration in vertebrates (the gills or lungs and in some cases the skin) help in ventilation/breathing i.e. active movement of the respiratory medium (water or air) across the respiratory surface. Ventilation may be non directional (water/air flows past the respiratory surface in an unpredictable pattern), unidirectional (water or air enters the respiratory surface at one point and exits at another), and tidal (water/air moves in and out from one point only) For the respiratory organs to function efficiently they must have: 1. A provision for renewing the supply of oxygen-containing medium, namely, water or air that comes in contact with respiratory surface and provision for removing carbon dioxide that is released from the respiratory surface. 2. A large surface area which is provided with ample capillary network that has an access to the external environment; 3. A thin and moist membrane surface which facilitates passage of gases. Let us now discuss the structure of gills as respiratory organs of aquatic vertebrates. 110 Unit 4 Respiratory System 4.3 RESPIRATION BY GILLS Gills are the main respiratory organs in fishes and some aquatic amphibians. They are composed of numerous gill filaments or gill lamellae, which are thin walled extensions of the epithelial surface. Each gill contains a vascular network. Blood is brought extremely close to the respiratory surface, thus facilitating ready exchange of gases. 4.3.1 General Gill Structure Gills are enclosed in a gill cavity. This provides protection for the fragile organ and also permits the water to run over the gills in an efficient manner. Gills of fishes consist of several gill arches on either side (Fig.4.1a). The gill arches separate the opercular and the buccal cavities. From each gill arch extend two rows of gill filaments, Fig. 4.1b shows the arrangement of gill filaments in the arches. The tips of the filaments of adjacent arches meet forming a sieve like structure through which the water flows. (a) (b) (c) Fig. 4.1: a) Position of gill arches beneath the operculum on the left side of fish. The operculum has been lifted to show the arch; b) Part of two adjoining gill arches with their filaments. Note that the tips meet to form a sieve like arrangement for flow of water. The water moves through the mouth over the branched gills. Solid arrows show the flow of water; c) Part of a single filament showing the flat lamellae; the flow of water is opposite to the direction in which the blood moves. Fish gills are covered by a thin epidermal membrane which folds repeatedly to form plate like lamellae that look like free flaps (Fig 4.1b). The structure of lamellae increases the surface area for respiration or exchange of gases. The flow of water is opposite to the direction in which the blood moves (Fig 4.1c). The area of gills present in different fishes varies depending on the activity of fishes. Fishes which are more active have larger gill areas. The rate of 111 Block 1 Comparative Anatomy of Vertebrates-I ------------ respiration in fishes is dependent on the amount of dissolved oxygen in water e.g. rate of respiration is more in fishes that live in waters with low oxygen content. Gills are of two types : (i) external gills and (ii) internal gills. (i) External gills (Fig.4.2) develop from the integument covering the outer surfaces of branchial/visceral arches and protrude into surrounding water. They are found in fish and larvae of many vertebrates including lung fishes, amphibians etc. They are usually branched; filamentous structures derived from ecotoderm. The functioning of external gills poses no problem since the gill filaments are in direct contact with water containing dissolved oxygen. The disadvantage of external gills is that they are easily damaged. Fig.4.2: External gills of a salamander larva. Fig. 4.3: Operculum of a bony fish removed to show internal gills. (ii) Internal gills (Fig.4.3) are associated with pharyngeal slits (series of openings in the pharyngeal region between digestive tract and the outside of the body) and pouches (in some vertebrates, diverticula from the gut in the pharyngeal region that never break through to form open passageway to the outside; such diverticula are called pharyngeal pouches). Internal gills are composed of a series of parallel gill lamellae although in some forms they may be filamentous. They may be borne on both sides of the interbranchial septa but in some cases are present on only one side. A series of gill lamellae present only on one side of an interbranchial septum are termed as half-gill or hemibranch. Two hemibranchs join with interbranchial septum to form a complete gill or holobranch (Fig. 4.4).The gills are covered and protected laterally by Fig.4.4: Hemibranch and soft skin folds such as interbrachial septum or by firm operculum. It is holobranch. generally assumed that internal gills are derived from endoderm, although the exact origin is not clear. There are two main types of internal gills in fishes. The first and more primitive types of internal gills are characteristic of elasmobranchs (cartiligenous fishes). In this group the interbranchial septa are exceptionally well developed and extend beyond the hemibranchs (Fig. 4.5a). The interbranchial septa, in addition to separating the gill clefts, serve to protect the gills themselves. The second type of internal gills is found in bony fishes. 112 In these forms, the interbranchial septa are reduced to varying degrees Unit 4 Respiratory System (Fig. 4.5b) from which hemibranchs protrude into a single branchial or extrabranchial chamber located on each side between the operculum and gills. The operculum, protects the gills in the branchial chamber which opens to the outside through a single gill aperture. Opening and closing of operculum bring about gill respiration in these fishes. Fig. 4.5: Types of gill in fishes (a) elasmobranch; (b) teleost. The gills are provided with dense capillary beds in the branchial region. Blood is carried very close to the respiratory surface, thus facilitating ready exchange of gases. In gills of most fishes ventilation or breathing is unidirectional. When internal gills are used in respiration, water containing dissolved oxygen enters through the mouth and passes through the internal gill slits into the gill clefts. As the water passes over the gill lamellae, oxygen is taken from the water and carbon dioxide is released. The water then passes through the external gills slits to the outside. Let us now study respiratory systems of different vertebrates that use gills as their respiratory organs but before we do that try the SAQ given below. SAQ 1 1. Indicate whether the following statements are true or false : (a) External respiration would mean utilization of oxygen for oxidation of nutrients in cells and tissues. T/F (b) One of the criteria for efficient functioning of the respiratory organs is that the respiratory surface should be thin and moist to facilitate exchange of gases.