Year

1994

Degree Name

Doctor of Philosophy

Department

Department of Biomedical Science

Abstract

Although it is known that body-fluid distribution is dependent on posture, environment, and exercise, previous research has concentrated on intravascular fluid shifts, describing redistributions through changes in venous haematocrit and haemoglobin concentration. It was the purpose of this investigation to examine the distribution of fluid throughout the body during postural, thermal, and exercise stress, using the dilution of radionuclides.

Body-fluid distribution was measured in eight physically active males, using the simultaneous dilution of 450 μCi of tritiated water, 20 μCi of radiobromine, 2 μCi of radioiodinated fibrinogen, and 8 μCi of radiochromated erythrocytes, to measure total body water, extracellular fluid, plasma (PV), and erythrocyte (RCV) volumes, respectively.

During 30 min of upright rest following sitting, intravascular volume (BV) decreased by 406 ml (± 89; mean ± SEM) , causing a concurrent expansion of interstitial volume (655 ± 1 6 5 ml); B V tended to increase (89 ± 82 ml) during supine rest, this time depleting the interstitium (-86 ± 145 ml). Changes in BV were largely accounted for by changes in P V (-233 ± 64 and +52 ± 70 ml, respectively), probably mediated by changes in capillary hydrostatic pressure.

BV decreased during 30 min of seated cool exposure (-302 ± 76 ml; 14°C), and increased during heat exposure (124 ± 150 ml; 36°C), although considerable intersubject variation occurred in the heat. Fluid shifts were again accounted for by changes in PV (-205 ± 60 and +108 ± 123 ml, respectively), attributed to changes in venomotor tone.

BV decreased during 10 min of cycle exercise, by 287 (± 60), 114 (± 86), and 470 (± 192) ml in cool, control, and hot environments, respectively. B V subsequently recovered in cool and control as exercise progressed, but remained depleted throughout 50 min in the heat, reflecting increased peripheral blood flow and general dehydration; 861 (± 100) ml of fluid were lost from the body, drawn proportionately from the intra- and extracellular compartments. In all three environments changes in B V were largely accounted for by changes in PV, although R C V also decreased in each case, by a mean of 88 (± 13) ml at minute 20.

Under all circumstances, changes in BV were considered important in the regulation of cardiovascular strain, while changes in the dynamics of blood flow apparently distorted the relationship between whole-body and venous haematocrits. Thus, it was considered that changes in venous haematocrit did not accurately reflect intravascular fluid shifts during cycle exercise.

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