The transport of oxygen from environment to tissue cells consists of four steps: (1) .... by catecholamine-stimulated release of erythrocytes from the spleen. (Holmgren ... Marinsky et al., 1990), whereas in others Hb multiplicity is unaffected by .... Blood pH is, however, eventually completely recovered when the fish is in hard.
Fish Ecophysiology Edited by J. Cliff Ran.kin and Frank B. Jensen Institute of Biology Odense University Denmark
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CHAPMAN & HALL London· Glasgow· New York· Tokyo· Melbourne· Madras
Chapter six
Environmental perturbations of oxygen transport in teleost fishes: causes, consequences and compensations Frank B. Jensen, Miltko Nikinmaa and Roy E. Weber
6.1 INTRODUCTION Aquatic environments exhibit large spatial and temporal variations In physico-chemical factors that influence the physiology of fishes. Environmental changes In 02 and C02 tensions, temperature, salinity, acidity, and Input of toxic substances can be of both man-made and natural origin. Oxygen-depleted waters, for Instance, occur naturally in habitats with heavy degradation of organic matter. Anthropogenic activity has expanded the number of environments with 'extreme conditions', and has created new environmental problems. Periodic or chronic oxygen depletion resulting from eutrophlcation is now a well-known phenomenon In many freshwater and coastal areas. Combustion of fossil fuels (emitting nitrogen and sulphur oxides) has acidified many freshwater ecosystems. Furthermore industrial activity has Increasingly contaminated waters with toxic substances. In ecophysiologlcal studies it is important to gain insight Into both the acute mechanisms of environmental perturbations and the compern;atory mechanisms that are activated to counteract the internal disturbance. One key physiological process that is extremely sensitive to environmental conditions _ is the transport of gases (02 and C02) between environment and cells. Fish Bcophyslology. lldlted by J. Cllil'Rankln and Frank B. Jensen. Published In 1993 by Chapman & Hall, London. ISBN 0 412 45920 5.
Bnvironmenflll perturbations of oxygen transport
162
The transport of oxygen from environment to tissue cells consists of four steps: (1) ventilation of gills (water convection); (2) diffusion of 02 from water to blood; (3) blood gas transport (convection); and .(4) diffusion of 02 from blood to tissue cells. The steady state flux of oxygen (Mo2) is the same through each step, and at each individual step the transport can be described as a product of a conductance component (highlighted in bold below) and a tension dUference (Dejours, 1975)*: Ventll~tion:
.
Mo 2 = Vw x f3wo 2 x (Pio2 - PEo2) Diffusion across gills: Mo2 = (K x A/I) x Af>o2 (water - capillary blood) Blood 9xyge~ transport: Mo 2 = Qh x f3bo2 x (Pao2 - Pvo2) Diffusion in tissues: Mo 2 = (K' x A '/I') x Af>o2 (capillary blood -tissue)
(6.1) (6.2) (6.3) (6.4)
Environmental changes can affect either tension or conductance components. If one component is reduced. (e.g. tlP in hypoxia), the other component (i.e. conductance) must increase if an unchanged flux of oxygen is to be safeguarded. The present chapter describes how environmental changes affect gas transport, with primary focus on blood oxygen transport and the physiological mechanisms that are available for acclimation/adaptation to environment. Recent reviews related to gas transport in fishes, but with alternative foci and objectives, include: Weber (1982), Randall and Daxboeck (1984), Weber and Jensen (1988), Perry and Wood (1989), Nikinmaa (1990), Jensen (1991).
6.2 HYPOXIA
When a flsh is exposed to hypoxia, the simplest way to avoid the problem ls behavioural, namely escape! Shift from water breathing to air breathing at low water Po2 is a sophisticated version of the 'escape strategy', and is used especially by various tropical species which have developed accessory breathing organs. Often, however, escape ls not feasible (e.g. in a hypoxic lake) and physiological mechanisms must be used to compensate for the 02 shortage. When water Po2 decreases, the internal 02 tensions and the 02 tension differences at each gas transport step also tend to decrease. This ls exemplified in Fig. 6.1 for the 'I - E' (cf. F.quation 6.1) and 'a -v' (~uation 6.3) transport in
•irw Is ventilation volume per unit of time; j3o2 Is the 02 capacitance coefficient (= dCo 2/dPo2); Po2 Is oxygen tension; K is Kroghs diffusion constant; A Is d!ITusion area, I is diffusion distance; Qh Is cardiac output. Indices w. I. B, b. a and v refer to 'water', 'Inspired', 'expired'. 'blood', 'arterial' and 'venous', respectively.
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163
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