Water versus Air Breathing

Air-breathing organisms live in an atmosphere that exerts a pressure of 760 mmHg at sea level and contains 20.95% O2 but only 0.03% CO2. Water-breathing animals like fish, clams, and crustaceans, however, extract O2 from a medium that, compared with air, has dissolved in it only 3% as much O2 (per volume), is 100-fold more viscous, and through which O2 diffuses 8000-fold slower. The concentration of O2 in water varies inversely with temperature: cold water contains more O2 than warm water. As water temperature increases, the metabolic rate of ectotherms, like fish, crayfish, or insects, also increases, yet the amount of O2 available to support cellular metabolism declines. Anthropomorphic warming of our lakes and rivers causes mortality of both aquatic invertebrates and vertebrates because many of them asphyxiate.

Even when water is saturated with O2, its high viscosity requires many aquatic organisms to expend considerable energy on respiration; fish use about 20% of their energy intake on respiratory movements whereas mammals use 1-2%. However, extraction efficiency, the amount of O2 removed during respiration compared with the amount available, is high for aquatic animals; fish extract 80% of O2 from water that passes over their gills, but human lungs have an extraction efficiency of about 20%. The release of CO2 from gills is rapid because this gas is 20-30 times more soluble than O2 in water. Because of the rapid diffusion of CO2 in water, ventilation rate is controlled by the amount of O2 in blood of fish. In air-breathing vertebrates, CO2 concentration in blood is the primary signal for control of frequency of ventilation.

Water-breathing organisms typically possess gills, gas-permeable evaginated outgrowths of the body wall, that vary in complexity from simple external structures of polychaete worms and some mollusks to complex gills enclosed in chambers as are found in bony fishes and crustaceans. Fish draw water into their mouths and force it over rows of gill filaments, each of which bears a series of folds called secondary lamellae on the upper and lower surface, the primary sites of gas exchange (Figure 11). Water exits through the operculum on the lateral side of the head. Hence, water flows unidirectionally across gill lamellae with blood flowing in the opposite direction, a counter-current system that maximizes extraction efficiency of O2. Tunas, mackerel, and dophin fish swim continuously with their mouth open, forcing water past their gills - a system called ram ventilation. Nearly 370 species of fishes have functional lungs in addition to gills; these dual breathers can come onto land to forage when the environment is moist, but then must return to the water. The climbing perch of India obtains about half of its O2 from air.

Air breathing provides its own set of constraints, most notably desiccation since maintenance of air-exchange membranes requires that air become saturated with

Figure 11 Portions of gas-exchange organs from fish, reptiles or mammals, and birds. Shown are the exagenated (outwardly displayed) tissue of gills, the dead end alveoliar sacs that are the location of gas exchange in reptilian and mammalian lungs, and parabronchi, through which gas flows (unidirectionally) as gas exchange occurs in avian lungs. Pink represents vascularized tissue whereas white (or background) represents the medium with which gas exchange occurs - water for fish and inhaled air for reptiles, mammals, and birds (for the latter, arrows indicate direction of flow).

Figure 11 Portions of gas-exchange organs from fish, reptiles or mammals, and birds. Shown are the exagenated (outwardly displayed) tissue of gills, the dead end alveoliar sacs that are the location of gas exchange in reptilian and mammalian lungs, and parabronchi, through which gas flows (unidirectionally) as gas exchange occurs in avian lungs. Pink represents vascularized tissue whereas white (or background) represents the medium with which gas exchange occurs - water for fish and inhaled air for reptiles, mammals, and birds (for the latter, arrows indicate direction of flow).

water vapor. Consequently, only a few groups of animals have been successful living in a terrestrial environment, notably mollusks, arthropods, and vertebrates. Insects and some arachnids employ a system of dendritically branching tubes, called trachea, that directly delivers O2 to their cells, and therefore their 'blood' (hemolymph) plays no role in gas exchange. Valves, called spiracles, located on the lateral body wall open and close to control movement of air in and out of the body. The smallest trachea, called tracheoles, terminate in juxtaposition to each cell, making diffusion of O2 rapid; tracheoles can supply 10 times more O2 per gram tissue than can blood capillaries of vertebrates. Originally it was thought that diffusion was entirely responsible for movement of air in and out of the tracheal system, but it is now known that insects can convectively transport air by pumping their abdomen and by contracting their trachea. Most tetrapods (amphibians and their decedents) primarily employ lungs for gas exchange, which we discuss primarily in terms of mammals and birds.

Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment