RESPIRATORY SYSTEMS

            Oxygen is used as the terminal acceptor of the electron transport system which is found in mitochondria.  This means oxygen must diffuse into all cells, even in large multicellular organisms.  In addition, carbon dioxide is produced during cellular respiration by decarboxylation reactions which are the exact reverse of carboxylation reactions occurring in photosynthesis.  This gas must be removed because if it accumulates it will change the internal environment of the organism which must remain relatively stable for proper function.  Carbon dioxide combines with water to form carbonic acid which will change the pH of blood and tissue fluids interfering with enzyme reactions and causing a multitude of difficulties.  Consequently, highly developed structures have evolved for gas exchange.

            Oxygen cannot diffuse directly to the mitochondria, it must become dissolved in the fluid environment of the cell.  Regardless of the size or living conditions of the animal, all gas exchange requires the gas to go into solution.  Because the volume of a cell increases as a cube function and the surface area as a square function, the size of cells is limited.  This is necessary as eventually not enough surface area would be available for exchange between the cell and its environment.  By becoming multicellular, surface area is increased.  With division of labour, some cells become gas exchange surfaces.  As the organism increases in size, more respiratory surface is needed.  This has resulted in the development of infolded (invaginated) and outfolded (evaginated) respiratory organs.  In general, lungs have developed as invaginations and gills as evaginations.

            Because oxygen is about 20 times less soluble in water than air (210 cm³ of O₂/litre of air, 4 to ll cm³ of O₂/litre of water) aquatic animals have developed a wide variety of mechanisms for extracting it from water.  Most of these depend on keeping the concentration of oxygen as high as possible on the outside of the respiratory membrane and as low as possible on the inside, so inward diffusion occurs as rapidly as possible.  Respiratory pigments combine with oxygen so that the concentration is kept low in the surrounding fluid.  The flow of oxygen-rich water is often in the opposite direction of oxygen-poor blood to keep the concentration gradient as high as possible.  To facilitate diffusion, respiratory membranes are very thin and, consequently, fragile.  Gills are often protected by a covering which sometimes becomes involved in pumping water across them.  Terrestrial animals also require oxygen to be dissolved in water in order to be absorbed, and organisms with lungs, therefore, must keep them moist.  Because lungs are the result of invagination, they are internal and well protected.

            Perch Respiratory System

            Remove the right operculum  of the perch to expose the gills (Figure 14).  The gills are attached to each of the four pairs of branchial arches (Figure 15). 

                                   

Figure 14:  Picture of the gills of the perch.

            On the anterior surface of the arches are the gill rakers.  Each gill has two sets of filaments, one extending anteriorly, and the other posteriorly.  Gills with double sets of filaments like these are known as holobranchs (Figure 15).  Each gill filament is made of many small thin walled folds called lamellae.  The lamellae contain the gill capillaries and exchange of oxygen and carbon dioxide between water and blood occurs across the thin walls of the lamellae.  In fish, water is constantly being passed over the gills.  Water is drawn into the mouth and then into the pharynx while the opercula are closed.  Valves in the mouth close, the oral cavity contracts and water is forced past the gill rakers, through the gill slits, over the gill filaments and out behind the posterior free ends of the opercula which are now open. As water passes over the gill filaments the exchange of oxygen and carbon dioxide occurs (Figure 15).  The arrangement  of gill capillaries within the lamellae is such that the blood flow is opposite to the flow of water over the gills.  This is an example of a counter current system.  It is an extremely efficient way to maximize movement of oxygen into the blood.

            In addition to their respiratory function, gills also perform important excretory functions.  Without exception, all fish primarily excrete their nitrogenous wastes via the gills rather than via kidneys.  Marine fish also excrete excess salts via the gills.  Freshwater fish absorb needed salts via the gills.

Figure 15: Structure of a fish gill.