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The effects of large changes in hemoglobin concentration were studied in rainbow trout in fresh water between 8 and 14 C. Anemia was produced by injecting phenylhydrazine hydrochloride or by replacing blood with either saline or plasma.No significant changes were observed in the rate of oxygen consumption, arterial or venous oxygen tensions, ventilation volume, inspired and expired water oxygen tensions, or dorsal aortic blood pressure. The primary compensatory mechanism invoked was an increase in the cardiac output, which was accomplished almost entirely by increases in stroke volume. Although the viscosity of the blood was reduced, there must also be large changes in the peripheral resistance to blood flow, since greatly increased cardiac output was achieved without significant increase in blood pressure.The change in blood oxygen capacity and increase in cardiac output caused significant lowering of the ventilation–perfusion ratio, but the capacity–rate ratio of water to blood varied only a little. A small rise occurred at low hematocrit values, due to small changes in a number of parameters.The experiments illustrate what happens when blood oxygen capacity is reduced, but do not elucidate the mechanism for control of stroke output of the heart. They also indicate that a species' hemoglobin level is maintained at a level that allows cardiac output to vary over an optimal range of its efficiency curve.

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A sensitive pH-stat assay was used to determine the distribution in aquatic and terrestrial decapod crustaceans, correlation to osmoregulation, and selected kinetic properties of carbonic anhydrase. In all species the enzyme was concentrated in the gills, and specifically the posterior three or four gill pairs. Activity was highest in the gills of osmoregulating species, and the enzyme could be localized in the areas of the gill lamellae which contained a high density of salt absorbing cells. The enzyme from the blue crab, Cullinectes supidus, exhibits a number of characteristics remarkably similar to the Na+/K+ ATPase from that species. Both enzymes show the same inter-and intrabrachial distribution and the same reactions to altered environmental salinity. Branchial CA from both species was inhibited by high concentrations of NaCl and increasing pH. The enzyme showed a temperature optimum of 25°C. On the basis of these initial results, it appears that crustacean gill carbonic anhydrase plays an important role in the blood osmoregulatory process.