This chapter discusses different aspects of the cardiac anatomy, morphology, and physiology of fish. The fish heart is a four-chambered organ contained within a pericardial sac. Together, the sinus venosus, atrium, ventricle, and either an elastic bulbus arteriosus, or a contractile conus arteriosus raise the potential and kinetic energy of the blood. The principal tissue component of the sinus wall is connective tissue with bounding inner endothelial and outer epicardial linings. The amount of cardiac muscle present in the sinus venosus varies considerably among species. The ventricle shows considerable species variability with respect to its relative mass, gross morphology, histology, and vascularity. The addition of inner fibers, either as a circular arrangement around the sac-like elasmobranch ventricle, or as coils around the vertices of the pyramidal teleost ventricle, is clearly a characteristic of fish that are more active and perhaps provides a mechanical advantage for developing higher blood pressures. The histological structure of the fish coronary artery is characterized by an external parenchyma surrounding a medial layer of vascular smooth muscle; an internal elastic lamina separates the media from the intima, which normally has a single layer of endothelial cells.

1 - The Heart

Publisher Summary This chapter discusses oxygen and carbon dioxide transfer across fish gills. The gills of fish are the major site, though not the only one, for oxygen and carbon dioxide transfer. The skin and fins may also serve in this capacity and many fish have evolved accessory air-breathing organs. These may be the modifications of the skin, buccal, pharyangeal, or gill surface, or they may be the regions of the gut or the swim bladder. In general, the gills of fish are the major pathway for oxygen and carbon dioxide transfer between the environment and the body tissues. The oxygen stores within the body, with the exception of that in the swim bladder, are small. Carbon dioxide stores in the body are large as compared with the rate of production. At resting rates of CO 2 production, it would take the animal several hours to accumulate the equivalent of the body CO 2 stores. Thus, minor changes in the magnitude of the CO 2 stores—for example, related to the acidification of the body tissues—can have a marked effect on CO 2 excretion across the gills.

5 Oxygen and Carbon Dioxide Transfer Across Fish Gills

Cardiac output, blood flow distribution and regional perfusion were determined in free-swimming rainbow trout acclimated to 6, 12 and 18°C, using the indicator dilution and microsphere methods. Cardiac output (ml min−1 kg−1) increased linearly with increasing temperature, while circulation time decreased. Blood flow distribution (% of cardiac output) to the spleen, liver, kidney, gall bladder and gastro-intestinal tract was significantly reduced at 18°C relative to 6°C-acclimated fish. White muscle received the largest fraction of cardiac output, and blood flow distribution to white muscle increased significantly with increasing acclimation temperature. Blood perfusion (ml h−1 g−1) of various organs and red muscle was not influenced by acclimation temperature, while white muscle perfusion increased with increasing temperature. These results demonstrate physiological adaptation of the cardiovascular system of rainbow trout to changes in acclimation temperature.

Temperature-dependence of cardiac output and regional blood flow in rainbow trout, Salmo gairdneri Richardson

SUMMARY To examine cardiorespiratory plasticity, cardiovascular function, oxygen
consumption, oxygen delivery and osmotic balance were measured at velocities
up to critical swimming speed ( U crit ) in seawater-adapted
chinook salmon. We used two groups of fish. The control group had swum
continuously for 4 months at a low intensity (0.5 BL s -1 )
and the other was given a high-intensity training regimen (a
 U crit swim test on alternate days) over the same period of
time. Compared with available data for other salmonid species, the control
group had a higher maximum oxygen consumption
( Ṁ o 2max ; 244μ
mol O 2 min -1 kg -1 ), cardiac output
( Q max ; 65 ml
min -1 kg -1 ) and blood oxygen content
( C a O 2 ; 15 ml O 2 dl -1 ). Exercise
training caused a 50% increase in
 Ṁ o 2max without
changing either U crit or C a O 2 , even
though there were small but significant increases in hematocrit, hemoglobin
concentration and relative ventricular mass. During swimming, however,
exercise-trained fish experienced a smaller decrease in body mass and muscle
moisture, a smaller increase in plasma osmolality, and reduced venous oxygen
stores compared with control fish. Consequently, exercise training apparently
diminished the osmo—respiratory compromise, but improved oxygen
extraction at the tissues. We conclude that the training-induced increase in
 Ṁ o 2max provided
benefits to systems other than the locomotory system, such as osmoregulation,
enabling trained fish to better multitask physiological functions while
swimming. Furthermore, because a good interspecific correlation exists between
 Ṁ o 2max and
arterial oxygen supply
( Ṫ o 2max ;
 r 2 =0.99) among temperate fish species, it is likely that
 C a O 2 and
 Q max are principal loci for
cardiorespiratory evolutionary adaptation but not for intraspecific
cardiorepiratory plasticity as revealed by high intensity exercise
training.

Effects of high intensity exercise training on cardiovascular function, oxygen uptake, internal oxygen transport and osmotic balance in chinook salmon (Oncorhynchus tshawytscha) during critical speed swimming.

Reports of the tuna's extreme swimming capabilities and endothermy have resulted in considerable interest in their metabolism. Progress in tuna energetics has been linked to the development of methods and facilities for their study as well as the newly available data for closely related species and other active teleosts. Metabolic measurements indicate that, although standard metabolic rate (SMR) is elevated in tunas compared with many species, similar SMR and routine metabolic rates are observed in other active fish. The elevated SMR is associated with an increase in aerobic scope, which reduces potential energetic limitations on physiological functions. Consequently, tuna can meet their basal metabolic demands (SMR and locomotion) without compromising recovery from oxygen debt, digestion, growth, or reproduction. Contrary to previous reports, tunas appear to be no more efficient swimmers than other teleosts, at least during steady, sustained swimming in a swim tunnel. High metabolic rates require that all steps in the path from the gills to the mitochondria facilitate high levels of oxygen and metabolite flux. While some differences exist between tunas and other active fishes at the gills, cardiovascular system, and tissue, the most marked difference, other than endothermy, is that the total aerobic capacity of the white muscle tissue mass is greater than that of the red muscle. The white muscle's aerobic capacity facilitates rapid recovery from oxygen debt incurred during burst swimming. Studies comparing the tuna with ectothermic scombrids suggest that endothermy may have evolved following an increase in aerobic poise.

Tuna metabolism and energetics

A spontaneously ventilating, blood-perfused trout preparation was used to examine gas exchange across the gills. The blood flow rate and input oxygen content to the branchial circulation were manipulated to assess the contributions of perfusion and diffusion limitations to oxygen transfer. Increases in the flow rate (Q), or the haematocrit (Hct) were positively correlated with increases in the oxygen uptake across the gills (M g, O g, O2 ). Manipulation of pulse pressure or frequency of the pump, with no changes to Q had no effect on M g, O g, O2 . Addition of adrenaline (1 × 10 −6 M) to the blood also did not effect M g, O g, O2 . Calculations of cardiac output from the Fick principle always yielded values which were overestimates of the actual cardiac output (Q) set by the mechanical pump. The difference between the measured oxygen uptake by the fish from the water (Vg 1, O 1, O2 ) and the amount of oxygen transferred to the blood across the gills (Vg 1, O 1, O2 ) was a reflection of gill tissue metabolism. It is concluded that trout gills, like mammalian lungs, are primarily perfusion limited for oxygen uptake under resting normoxic conditions, but decreases in diffusion limitations come into play under stress conditions, such as environmental hypoxia or exercise. Note:

Charles Daxboecki

Papers

Oxygen Uptake in a Spontaneously Ventilating, Blood-Perfused Trout Preparation