Respiration: an Introduction JJ Cech Jr., University of California, Davis, CA, USA CJ Brauner, University of British Columbia, Vancouver, BC, Canada

GAS EXCHANGE

Respiration: An Introduction JJ Cech Jr., University of California, Davis, CA, USA CJ Brauner, University of British Columbia, Vancouver, BC, Canada

Introduction Tissue Respiration The Environment: Water and Air as Respiratory Media Whole Animal and Techniques in Respiratory Ventilation and Gas-Exchange Organs Physiology Gas Transport and Exchange Further Reading

Glossary Mitochondria Organelles that produce most of the Bohr effect Effect of the proton concentration (pH) aerobic energy required by the cell.

on the oxygen affinity of hemoglobin. P50 The oxygen partial pressure at half-maximal oxygen Carbonic anhydrase A zinc metalloenzyme that saturation of blood or hemoglobin.

reversibly catalyzes the reaction of CO2 and H2O to form Partial pressure The atmospheric pressure exerted + � H and HCO3 . by O2 alone proportional to the total concentration of Diffusion Net movement of a solute from an area of this gas. It is typically measured in either mmHg (torr) higher concentration to an area of lower concentration. or kPa. Equilibrium Pertaining to the situation when all forces Respiratory cascade A model of gas exchange in acting are balanced by others resulting in a stable which gas is viewed as flowing through a series of unchanging system. resistances from the environment to the tissues or vice Haldane effect Proton binding to hemoglobin (as a versa. The model is based on the analogy of water function of oxygenation). flowing down a series of cascades with the difference Hypoxia Low partial pressures of oxygen in external or being that gas flow is driven by differences in partial internal environments. pressure rather than gravity. Interlamellar cell mass (ILCM) A mass made up of Rete Structure consisting of blood vessels arranged to undifferentiated cells and ionocytes, and possibly other facilitate the exchange of heat or oxygen. cell types, filling up a variable part of the space between Root effect A property of fish hemoglobin in which the lamellae of fish gills. protons decrease the maximal oxygen saturation of Lamellae Also known as secondary lamellae, these are hemoglobin. For practical purposes, it is defined as a attached in rows to the gill filaments. They are the reduction of oxygen saturation at atmospheric oxygen primary sites for gas exchange in fish gills. Each lamella tension. is made up of two epithelial layers separated by pillar Ventilation The movement of the respiratory cells. Oxygen is taken up by erythrocytes flowing inside medium (air or water) over the surface of the gas the lamellae from water flowing between the lamellae. exchanger.

Introduction Models: Bioenergetics in Aquaculture Settings and Bioenergetics in Ecosystems), all of which are important Respiration and gas exchange is an essential process to determinants of fitness. maintain an aerobic existence in all vertebrates, including The complex process of respiration in fish is discussed fishes. The uptake of oxygen (O2), along with metabolism in detail in this section. It starts with the environment of organic substrates such as glucose and lipids, is needed where O2 and CO2 move into and out of the animal, to power the biochemical machinery (e.g., in the mito respectively, by simple diffusion. Gases diffuse across a chondria) in cells for body maintenance, as well as for gas-exchange organ which represents the interface other aerobic functions such as growth, movement, repro between the organism and the environment and may duction, and disease resistance (see also Energetic consist of skin, gills, and in some cases an air-breathing

791 792 Gas Exchange | Respiration: An Introduction organ. Ventilation of the respective media (water and in The Environment: Water and Air some cases air if an air-breathing organ is present) in as Respiratory Media conjunction with blood perfusion across the gas-exchange organ, both of which can be altered proportionally Characteristics of the environment can have a profound depending upon the animal’s metabolic state, ensures effect on respiration. Respiratory gas exchange in aquatic sufficient gas exchange to meet the demands of the environments presents different problems when com animal. pared with respiration in air. Water is a dense, viscous The circulatory system provides the conduit through medium, which also has a high heat capacity and 20–30 which the blood is ultimately delivered to the tissues; fold lower oxygen concentration (due to low gas solubi hemoglobin (Hb), maintained within the red blood cell, lity) relative to air. Increases in temperature or salinity plays a vital role in the transport of both O2 and CO2 in all further decrease oxygen solubility in water, and therefore fishes, except icefishes, which represent the only verte oxygen content for a given gas pressure as indicated in brate that lacks Hb. Hemoglobin is a remarkable molecule Figure 1. and is one of the best-understood proteins in terms of how The PO2 and PCO2 of water can vary dramatically changes in the environment of the red blood cell alter the compared to those of atmospheric air. This is because tertiary and quaternary structure of Hb to influence the the gases contained in air do not necessarily exchange nature in which Hb binds and releases ligands such as O2, readily with water. The PO2 of aquatic environments can + CO2, and H ’s in particular. These changes optimize be zero (anoxia), low (hypoxic), normoxic, or high conditions for gas exchange to tissues in general, as well (hyperoxic), depending on the photosynthetic and as specifically to the eye and swimbladder allowing for respiratory rates of the biotic community and on water acute vision and buoyancy control respectively in many circulation characteristics. Some shallow, freshwater habi teleosts. tats may vary between 20 and 40 mmHg PO2 just before Finally, blood reaches the tissues where waste dawn to 200–400 mmHg at mid-day. Thus, water can be products are removed and substrates supplied to less saturated and more saturated with oxygen, even on a cells and mitochondria providing the basics for cellu diel basis. Many fishes, especially those that spend at least lar and mitochondrial respiration, and thus life. The part of their lives in shallow habitats, have evolved struc following describes these various steps in respiration tural and functional abilities to deal with variable water in more detail, providing background information and PO2 values. introducing each article that appears within the Fish in boreal, subpolar, or polar lakes may experi section. ence a seasonal challenge of oxygen availability – a

12 350 8 FW 10 °C

300 10 SW 10 °C 6 250 8 FW 30 °C –1 –1 l l

2 200 2 SW 30 °C 6 µ M 4 ml-O

150 mg-O 4 100 2 50 2

0 0 0 0 20 806040 100 120 % Air saturation

0 50 100 150 200

P O2 (torr or mmHg)

0 5 1015 20 25

P O2 (kPa) Figure 1 The relationship between partial pressure of oxygen (x-axis) and total oxygen content (y-axis) at 10 and 30 �C in either freshwater (FW) or seawater (SW). The three legends for each of the axes represent the different units that are used to describe both partial pressure and content in water. Reproduced from Diaz RJ and Breitburg DL (2009) The hypoxic environment. In: Richards JG, Farrell AP, and Brauner CJ (eds.) Fish Physiology, Volume 27 Hypoxia, pp. 1–23. Academic Press, with permission from Elsevier. Gas Exchange | Respiration: An Introduction 793

gradually deepening hypoxia (lowered PO2,s)duringthe relatively novel finding is discussed in Ventilation and winter as ice cover seals off gas exchange with the atmo Animal Respiration: Plasticity in Gill Morphology. sphere and snow cover, along with shortened While the gills are usually the predominant site for gas photoperiods, attenuate incoming light decreasing exchange in adult fish, this is not the case early in develop photosynthetic production of O2.Extreme wintersmay ment when the total body surface area:volume ratio in larval prolong these conditions leading to a total consumption and juvenile fish is high and gill secondary lamellae in of the remaining dissolved O2 (anoxia) by the lake’s particular have yet to be fully developed. At this point, all biota, leading to a winter fish kill (see also Hypoxia: gas exchange is across the skin. The fundamental principles The Expanding Hypoxic Environment). associated with gas exchange in aquatic media and their Fishes living in high-altitude environments must also implications for larval fishes are discussed in Ventilation cope with low-PO2 water. In this case, it is due to the lower and Animal Respiration: Respiratory Gas Exchange total barometric pressure (and the correspondingly low During Development: Models and Mechanisms. This arti ered partial pressures of the atmospheric gases). Finally, cle lays the foundations for a discussion of when the gills certain types of pollution, including those that introduce become important for gas exchange during development excessive nutrients into waterways (eutrophication), typi whichisdiscussedinVentilation and Animal Respiration: cally lead to wider, diel dissolved-O2 ranges, often Respiratory Gas Exchange During Development: including quite hypoxic PO2, s. Thus, fish living and Respiratory Transitions. Interestingly, the gills may take respiring aquatic media are subjected to large and routine on a more significant role for ionoregulation than for gas changes in O2 availability relative to air-breathing exchange early in development, based upon the time that + animals and many are adapted to these potentially 50% of whole body unidirectional Na uptake and O2 extreme conditions. uptake transitions to the gills. This ontogeny has interesting implications for the evolution of gill function.

Ventilation and Gas-Exchange Organs Gas Transport and Exchange The physical and chemical characteristics of aquatic envir onments, specifically low oxygen solubility of water, Once O2 has diffused across the gill lamellae, about 98% is probably contributed to the evolutionary development of reversibly bound to Hb (oxyhemoglobin) contained within gill structure and function, and to the many mechanisms the red blood cell. The relatively high affinity of Hb for some fishes use to extract oxygen directly from the air. oxygen helps to maintain the partial pressure gradient for Article Ventilation and Animal Respiration:Efficiencyof diffusion, maintaining high rates of oxygen uptake at the Gas Exchange Organs discusses in greater detail the gills. The importance of Hb-O2 affinity (usually character implications of water and air as respiratory media on gas ized by the partial pressure at which 50% of the Hb exchange and how various gas-exchange organs differ in molecules are oxygenated; P50), and the shape of the oxygen terms of efficiency of, and capacity for, gas exchange. The equilibrium curve to O2 uptake and transport are discussed control of gill ventilation is crucial for survival to ensure in Transport and Exchange of Respiratory Gases in the adequate water flow over the gills. While control of Blood:O2 Uptake and Transport: The Optimal P50. Once ventilation and ventilatory responses to hypoxia are within the red blood cell, O2 is bound to Hb which consists discussed in other sections, article Control of of two � and two � globin chains. The structure of Hb, + Respiration: The Ventilatory Response to CO2/H models that describe Hb-O2 binding, and factors that influ discusses the effect of CO2 on ventilation and the role of ence O2 binding in different fish groups are discussed in CO2 in controlling ventilation in fish. Transport and Exchange of Respiratory Gases in the The gill represents the predominant surface for gas Blood: Hemoglobin. exchange, which occurs predominantly across the gill At the gills, any physically dissolved CO2 diffuses lamellae in adult fish, and a great deal of variability in down its partial pressure gradient to be excreted across gill design has evolved among fishes. However, there is the gill lamellae into the water. About 95% of the CO2 – also a great deal of plasticity in gill morphology where transported in the blood exists as HCO3, mostly in the large changes are observed in some fishes during exposure plasma. The dissolved CO2, which moves easily across to hypoxia in particular. In hypoxia, there can be expan gill epithelia, diffuses across the lamellar epithelium into sion of the total lamellar surface area in some fish species, the aquatic environment, which usually has a high absorb such as carp, by as much as sevenfold due to the disap ing capacity (i.e., acts as an infinite sink) for CO2. The pearance of an interlamellar cell mass that exists under corresponding decrease in plasma CO2 creates conditions – + – normoxic conditions. These relatively large and often for HCO3 dehydration (H +HCO3 ! H2CO3! CO2 + rapid changes in gill morphology may be more common H2O), which occurs relatively slowly in the plasma but among fish than previously thought. This exciting, very rapidly in the red blood cell due to high levels of the 794 Gas Exchange | Respiration: An Introduction catalyst carbonic anhydrase. Carbonic anhydrase acceler Blood: Root Effect: Molecular Basis, Evolution of the ates the reaction by up to 25 000 times and fish have many Root Effect and Rete Systems, and the processes through different isoforms, which are discussed in Transport and which the Root effect facilitates O2 delivery to these Exchange of Respiratory Gases in the Blood:Carbonic structures is discussed in Transport and Exchange of Anhydrase in Gas Transport and Exchange. With contin Respiratory Gases in the Blood: Root Effect: Root Effect – – ued HCO3 dehydration, plasma HCO3 is transported into Definition, Functional Role in Oxygen Delivery to the – – + the red blood cell by Cl /HCO3 exchange and H ’s are Eye and Swimbladder. supplied within the red blood cell by Hb, which can act as a buffer, or release H+’s upon Hb oxygenation known as the

Haldane effect. This process continues until the blood Tissue Respiration leaves the gills which is discussed in greater detail in Transport and Exchange of Respiratory Gases in the As blood enters the tissues, O diffuses down its partial Blood: Carbon Dioxide Transport and Excretion. The 2 pressure gradient into every cell in the body. Adenosine microenvironment of the red blood cell is regulated inde triphosphate (ATP) production by oxidative phosphory pendently of that in the blood plasma, which is crucial for lation requires adequate delivery of both oxygen and both O and CO transport. The processes responsible for 2 2 metabolic fuels to cells and is regulated to meet metabolic the regulation of red blood cell pH and volume are demand. The biochemical pathways and fuels used in discussed in Transport and Exchange of Respiratory cellular respiration, along with the influence of the envir Gases in the Blood: Red Blood Cell Function. onment and limits to cellular respiration are discussed At the tissues, the processes discussed above occur in in Tissue Respiration: Cellular Respiration. The ulti reverse. CO produced by the tissues diffuses into the 2 mate destination for O within the body is the blood, where it is hydrated to HCO– and H+. The H+’s 2 3 mitochondrion, an organelle found in most cells of all may be bound by Hb reducing Hb-O affinity by the Bohr 2 eukaryotic organisms. The mitochondrion is the site of effect and reducing Hb-O affinity facilitating O delivery 2 2 ATP production and is responsible for most of the O to the tissues. The Bohr effect is thought to have evolved 2 consumed by fish. Many of the characteristics of mito 3 times independently, and in teleosts was accompanied chondria are similar among eukaryotes as they arose by a reduction in Hb buffer value so that small blood-acid through endosymbiosis prior to the divergence of plants, loads exert relatively large effects relative to those in fungi, and animals. The basic features of mitochondria are other vertebrates. This is discussed in greater detail in discussed in Tissue Respiration: Mitochondrial Transport and Exchange of Respiratory Gases in the Respiration, which provides the background for a discus Blood: Evolution of the Bohr Effect. The relatively large sion of some of the unique specializations that are seen in Bohr effect and low buffer value in teleosts have large fish which is presented in Tissue Respiration: implications for the interaction between O and CO 2 2 Specializations in Mitochondrial Respiration of Fish. exchange, where CO2 production in the tissues facilitates O2 delivery via the Bohr effect, and O2 delivery promotes CO2 removal via the Haldane effect, the reverse processes occurring at the gills. The implications of this are dis Whole Animal and Techniques cussed in Transport and Exchange of Respiratory in Respiratory Physiology Gases in the Blood: Gas Transport and Exchange: Interaction Between O2 and CO2 Exchange, and taking The whole is greater than the sum of its parts, and this into account large disequilibrium states that may exist in certainly applies to respiration and the respiratory system in fish, the benefit to oxygen delivery may be much larger fishes. At each level of biological organization discussed than previously thought. above, respirationinfishiscomplex.However,adding to While the Bohr effect refers to a decrease in Hb-O2 this complexity is that all these processes and reactions affinity with a reduction in pH, the Root effect found must be integrated within the whole animal, the level at in the blood of most teleosts refers to a decrease in the which natural selection operates. Exercise increases the rate O2-carrying capacity of Hb at low pH so that even at at which all these steps in the respiratory system must atmospheric O2 tensions and higher, Hb cannot be fully operate and is often used as a tool to shed light on which oxygenated. The Root effect, together with a vascular if any steps in the respiratory cascade from environment countercurrent system called a rete and the generation to tissues may be rate limiting. Changes in respiration of a localized acidosis, dramatically increases PO2 and during exercise are discussed in Ventilation and Animal facilitates O2 delivery to the poorly vascularized retinas Respiration: The Effect of Exercise on Respiration and and to the swimbladder in most teleosts. The molecular some of the techniques used in the field are described in basis and evolution of the Root effect are discussed in the section Techniques in Whole Animal Respiratory Transport and Exchange of Respiratory Gases in the Physiology. Gas Exchange | Respiration: An Introduction 795

See also: Control of Respiration: The Ventilatory During Development: Models and Mechanisms; + Response to CO2/H . Energetic Models: Bioenergetics Respiratory Gas Exchange During Development: in Aquaculture Settings; Bioenergetics in Ecosystems. Respiratory Transitions; Techniques in Whole Animal Hypoxia: The Expanding Hypoxic Environment. Tissue Respiratory Physiology; The Effect of Exercise on Respiration: Cellular Respiration; Mitochondrial Respiration. Respiration; Specializations in Mitochondrial Respiration of Fish. Transport and Exchange of Respiratory Gases in the Blood: Carbon Dioxide Transport and Excretion; Further Reading Carbonic Anhydrase in Gas Transport and Exchange; Evolution of the Bohr Effect; Gas Transport and Boutilier RG (1990) Vertebrate gas exchange. From environment to cell. In: Advances in Comparative Environmental Physiology, vol. 6, Exchange: Interaction Between O2 and CO2 Exchange; 411pp. New York: Springer Verlag. Hemoglobin; O2 Uptake and Transport: The Optimal P50; Dejours P (1988) Respiration in Water and Air: Adaptations, Regulation, Red Blood Cell Function; Root Effect: Molecular Basis, Evolution, 179pp. Amsterdam: Elsevier. Diaz RJ and Breitburg DL (2009) The hypoxic environment. Evolution of the Root Effect and Rete Systems; Root In: Richards JG, Farrell AP, and Brauner CJ (eds.) Fish Physiology, Effect: Root Effect Definition, Functional Role in Oxygen Vol. 27 Hypoxia, pp.1–23. Academic Press. Delivery to the Eye and Swimbladder. Ventilation and Perry SF and Tufts BL (eds.) (1998) Fish respiration. In: Fish Physiology vol. 17, 356pp. New York: Academic Press. Animal Respiration: Efficiency of Gas Exchange Organs; Randall DJ (1970) Gas exchange in fish. In: Hoar WS and Randall DJ (eds.) Plasticity in Gill Morphology; Respiratory Gas Exchange Fish Physiology, Vol. 4, pp. 253–292. New York: Academic Press.