Gases Exchange in Animal.

Gases Exchange, Gases exchange in Animals, Gases exchange in Aquatic and terrestrial animals, gases exchange in vertebrates and invertebrates, fish

Gases Exchange in Animals:

Aquatic Animals:

 In some small aquatic animals such as protozoans and large forms, like sponges and hydras, water current bathes the cells, and specialized respiratory structures are absent. In this case, the body surface serves as the respiratory membrane (Fig. 1.1).

The oxygen dissolved in water directly moves into the cells through the plasma membrane and carbon dioxide is released outside. In standing water, organelles like cilia de­velop to maintain a water current, which supplies oxygen into the body and removes carbon dioxide from the body’s surface.

Fig 1.1 Gases exchange in Amoeba.

larger invertebrates possess specialized respi­ratory structures, called ctenidia, located in different positions of the body in different animals, but lie mostly to the anterior part of the body. in annelids, the ctenidia are present in association with Para podia, on both sides of the cephalothorax in crustaceans and in the mantle cavity in molluscs.

Invertebrate Gill:

A ctenidium (gill) is made of a number of lamellae, that are attached to a central axis. Each lamella is composed of a large number of gill filaments, gases exchange occurs. Channels are present in the central axis which carries blood to and from spaces in the lamellae. In crustaceans, the ctenidia (Fig. 1.2) on the sides of the cephalothorax, lie in the branchial chambers formed by the sides of the carapace.

fig 1.2 Respiratory organ in Macrobrachium sp.

The chambers are linked with the exterior, anterior, posterior, and ventral borders in prawn. Scaphognathite’s movements of the second maxillae set a constant back to forward water current in the gill chambers.

In crabs, the carapace ( upper dorsal part of the exoskeleton) is covered along the ventral edge and the water enters through a pore at the bases of the great chela(pincer-like organ), the first pair of walking legs. In mollusks, the gills are stiff in the mantle cavity. In a bivalve, the beating movement of cilia present in the gill filaments causes a current of water to enter the mantle cavity via the tube-like structure called inhalant siphon and es­capes to outside through exhalant si­phon.

In prosobranchs, contraction of muscles in the mantle and nuchal lobes increases the size of the mantle cavity and water enters through the left nuchal lobe or the siphon and goes out through the right siphon due to relaxation of muscles in the mantle.

Vertebrate Gill:

Respiration occurs in adult cyclostomes and fishes by internal gills. Some amphibians do respiration with larval gills which develops in water.

The respiratory organs of cyclostomes, Osteichthyes (Teleostomi),  and Chondrichthyes (Elasmo-branchii) possess some differences, but all have gills located in the lateral walls of the pharynx, hence called pharyngeal gills. During embryonic development, a series of paired outgrowths seem in the lateral walls of the pharynx, ranging from inside to outside. These are gill pouches. The gill pouches finally open to the outside by fine slits, the gill slits or gill clefts. Therefore, each gill pouch is in link with the pharyngeal cavity by an in­ternal and with the outside by an external branchial pore or aperture. Two consecutive gill pouches are separated by a fibrous divider (the original pharyngeal wall), the inter-branchial septum. The membrane lining the anterior and posterior walls of each gill pouch is gathered into a number of plane ridges, the branchial filaments. The filaments are splendidly supplied with blood, where gaseous exchange takes place.

The visceral arches back up the phar­ynx are a series of U-shaped rods. The vis­ceral bar or each half of the arch is stuck in the pharyngeal side of an inter-branchial sep­tum, alternating with the gill pouches resulting, a visceral arch bears the posterior set of filaments of one pouch and the anterior set of the filaments of the next one.

In the Boney Fish class (Osteichthyes (Teleostomi)) the inter­-branchial septa are reduced to thin bars enclosing the visceral arches of the pharyn­geal wall and a duple row of gill filaments develop from each visceral bar. A gill consisting of one set of gill filaments is called hemi-branch or half gill and with a set of two is called holobranch.

In cyclostomes and elasmobranchs, there are numerous pairs of gill pouches (Fig. 1.3). A gill pouch is a biconvex sac, con­taining many highly vascular gill lamel­lae. The gill pouches open into the pharynx. In lampreys, several pairs of gill slits lie, while in hagfishes, they have only one pair of gill slits. In all conditions, the gill slits are parted by partition. The number of paired gill slits varies in different Chondricthyes.

Fig 1.3 A        Chondricthyes Respiratory structure.

Fig 1.3 B        Osteocthyes Respiratory structure.

In teleost’s, the barrier between two-gill slits is minimized to a branchial arch having two sets of gill filaments (Fig. 1.3). Each sides gills are placed in a gill chamber, covered by a bony operculum (L. operculum = lid). A tinny branchiostegal mem­brane exists at the posterior border of the operculum. During respiration, the gill cham­ber becomes firmly closed by pressing the membrane against the body wall. A respiratory current is produced by letting down the floor of the buccal cavity. The water enters the cavity through the open mouth (Fig. 1.4A). With the shutting of the mouth and the oesophagus and raising of the buccal floor and contraction of the pharyn­geal walls, water goes into the gill pouches and passes out through gill slits in cyclostomes and elasmobranch (Fig. 1.3A).

In teleost’s with the opening of the buccal floor and contraction of the pharyngeal walls, the branchiostegal membranes are enforced to open and water from the buccal cavity is driven to the gill chamber via the gill slits and thence to the ex­terior (Fig. 1.3B, 1.4B).

 Fig 1.3 B, 1.4 B Mechanism of gases exchange in Chonrdocthyse
A: Inhalation                             B Exhalation

Afferent and efferent branchial arteries are wedged in the gill arch. which carries deoxygenated blood to the gills and the latter plumbing away oxygenated blood from the gills (Fig. 1.5).

fig 1.5 Diffusion of gases in teleost Gills.

For active respiration, the gills must have a large surface area and so large volume of water  move across the gill lamellae or filaments. Oxygen is less soluble in water and the weight of the water passing through the gill slits is about 0.1 million times the weight of available oxygen. Usually, water moves in one direction across the gills.

The diffusion of gases between the water and the blood or haemolymph in the gills is rather rapid. The diffusion is accelerated by a mechanism termed counter-current exchange in some crustaceans, molluscs and fishes.

In the process, the direction of water flow across the gills is opposite to the flow of blood or fluid (Fig. 1.6). About 90 percent of dissolved oxygen may be removed from water by the counter-current exchange.

Fig i.6 Mechanism of counter current gases exchange.