Year: 1994
Type: dissertation
Abstract: The optimal haematocrit (Hct) hypothesis suggests that vertebrate Hct is set to maximize the rate of oxygen transport in arterial blood (To2).Since T o 2 is equal to the product of the oxygen content of arterial blood (Cag2) and cardiac output (Q), this implies that Hct is adjusted to maximize blood oxygen carrying capacity without compromising blood flow and cardiac work through elevated blood viscosity (q).Thus, the Hct value which coincides with maximum T o 2 (ToZmax) represents the optimal Hct (Hctopt).I tested this hypothesis in rainbow trout (Oncorhynchus mykiss) by altering Hct between extreme states of anaemia and polycythemia (Hct = 8 -55%).I predicted that the effects of q on cardiac work would be greatest at peak aerobic exercise levels when Q would be maximum (emax).Moreover, I predicted that ToZmax and maximal oxygen uptake (Vo2,,,) would peak at Hctopt.Therefore, my experimental approach was to challenge fish to swim to their critical swimming velocity (Ucrit) in a swim-tunnel respirometer while oxygen uptake (Vo2) and other cardiovascular variables were measured.Furthermore, since blood viscosity is higher at lower temperatures, experiments were performed at both 5 O C and 13 O C .The mean normal Hct (normocythemia) for rainbow trout at rest was 27 -30%.Cao2 was linearly related to Hct across the experimental range from 8 -55%.Consistent with the Hctop, hypothesis, the decreased Cag2 in anaemic fish (Hct < 21%) caused a significant reduction in Uc,it and V02max.As expected, there was an exponential relationship between q, measured in vitro, and the experimental Hct, and Qmax was significantly lower in fish with the highest Hct values.Contrary to the Hctopl hypothesis, ToZmax was not compromised with polycythemia (Hct > 33%).Despite the effects of q , ToZmax increased with Hct, up to Hct = 55%, at both 5 "C and 13 "C, and moreover, polycythemia produced significant, albeit small, increases in Ucrit Futhermore, peak V 02max occurred at an Hct (42%) well above normal Hct (normocythemic) values.It appears that both Ucrit and VoZmax in normocythemic fish are limited by the capacity for internal convection of 0 2 .I suggest that while the lower limit for normocythemia is set by C q 2 , the regulation of the upper limit for normocythemia involves more than q effects on cardiac work.One factor which may set upper Hct is the Hct-dependent decrease in arterial oxygen tension ( P w 2 ) observed at Ucrit.This decreased Pao2 (indicative of diffusion limitations to O2 transfer at the gills) did not cause decreases in C w 2 at most Hct values, but the peak for VoZmax may coincide with an Hct at which this arterial hypoxaemia begins.Other potential limiting factors for upper Hct were revealed in experiments with exercise-trained chinook salmon (0. tshawytscha).Small (<20%) Hct increases in trained, compared with control, fish were not associated with decreased Qmax.Trained fish tended to have higher Vo2,,, but not Ucrit, values.Evidence is presented which indicates that Hct is important in determining Vo2 for metabolic functions other than contraction of locomotory muscles (e.g., osmoregulation and growth).Thus, although Hct is a determinant of V 02, it is not optimized for swimming performance.This may be related to the high cost of swimming for these species; i.e., V 02 (cost) increases faster than Ucrit (benefit).This thesis is dedicated to my children, Nial and Mary, with my love, and to the memory of Arthur and Phyllis Gallaugher, my parents, and of my grandfather, P. J. Bain.We shall not cease from exploration And the end of all our exploring Will be to arrive where we started And know the place for the first time.
Source: Summit (Simon Fraser University)
Author: P. Gallaugher
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2311190872