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Influence of water and blood flow on gas exchange at the gills of rainbow trout, salmo gairdneri Davis, John Christopher

Abstract

Studies were carried out to determine the influence of water and blood flow patterns on gas exchange at rainbow trout gills. A variety of gill water flows and patterns of blood distribution within the gills exist during different physiological states and hence are important to the gas exchange process. An evaluation of techniques for measuring mean expired oxygen tension was carried out to determine a way for accurately measuring ventilation volume (V[sub G]) in trout. Water from opercular and cleithral cannulae had extremely variable P[sub O₂]’s in both free-swimming and restrained trout. Fick principle calculations of V[sub G] based on data from such cannulae could be subject to error. A method for directly measuring ventilation volume in restrained trout by means of a rubber membrane attached to the mouth (oral membrane) to separate inspired and expired water andppermlt collection of the latter is described. Fish (200 g, 8.6 C) fitted with oral membranes had resting mean V[sub G]'S of 37 ± 1.8 ml/min and V[sub G] could be increased seven-fold during struggling or excitement. Increases in V[sub G] were accomplished by a large rise in ventilatory stroke volume and a small increase in ventilation rate. Utilization of oxygen from the water in quiescent fish was 46 ± 1.5% and ranged from 26 ± 64%. The ratio between the volumes of water and blood passing the gill/unit time (ventilation-perfusion ratio) was approximately 5. Area mean differential pressure (buccal pressure - opercular pressure) appeared to be directly related to V[sub G] over the V[sub G] range 40-160 ml/min as this four-fold rise was accompanied by a pressure increase from 0.1 to 0.4 mm Hg. Over this V[sub G] range the calculated resistance of the gill sieve did not change Indicating that changes in water spillage past the gills (anatomical deadspace) did not occur. Artificial perfusion of the gills with water by means of a mouth tube showed that trout could saturate their arterial blood with oxygen and perfusion rates of 85 - 1200 ml/min but could not do so at rates approximating normal resting V[sub G] (45 ml/min). Such perfusion likely provides a poor pattern of water flow over the gills compared to normal irrigation. Infrared photographs of the gills showed a large in vivo increase in the volume of blood in the gill lamellae following adrenaline injection but failed to detect small changes in blood distribution in the gills of uninfected fish. Rainbow trout responded to reductions in gill surface area (blood supply to some gill arches tied off) by elevating ventilation volume and cardiac output. When the blood supply to the pseudobranch was intact the fish could maintain oxygen saturation of arterial blood despite a 50% reduction in gill area by adjusting V[sub G]and possibly Q (cardiac output). Fish with the blood supply to the pseudobranch destroyed had low arterial oxygen tensions and did not increase their V[sub G]to the same extent as those with intact pseudobranchs. The pseudobranch may contain chemoreceptors important in the regulation of arterial PO₂. Circulation times, determined by dye injection, were about 1 minute in quiescent trout (200 g, 10 C). The responses of trout to reduced gill water flow or hypoxia are too rapid to be initiated solely by a venous receptor, at these circulation times, as some authors have suggested, hence the pseudobranch is a logical site for an oxygen receptor. A theoretical analysis showed that about 30% of the V[sub G] was involved in non-respiratory water shunts at the gills during gentle to moderate breathing (V[sub G] = 40 - 120 ml/min) and up to 70% at the highest perfusion rates. Deadspace due to diffusional problems is small ( 2 - 5% of V[sub G]) during gentle-moderate breathing but may rise to 30% of V[sub G] at high perfusion rates. Anatomical deadspace is probably large at high gill water flows and is thus a major contributor to the total water shunt at the gills at such flows. Distribution deadspace, resulting from unequal ventilation and perfusion of the gills, may be an important part of the shunt at low flows but is likely minimal at high flows when vasodilation of the gills occurs. Deadspace problems would not severely limit oxygen extraction over the 40 - 300 ml/min V[sub G] range measured for 200 g trout with oral membranes but may account for declining % utilization of oxygen as V[sub G] increases. The calculated oxygen cost of breathing at this V[sub G]range was only 1.4 - 6.7% of the total VO₂ assuming the efficiency of the system is only 1%. The system is probably better than 1% efficient so the oxygen cost of breathing over this V[sub G] range is very low.

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