Outline the physiological responses to the rapid intravenous administration of 1 litre
of 0.9 % saline to a 70 kg euvolaemic person.
Answering this question required the integration of information from areas of cardiovascular,
body fluid and renal physiology which proved difficult for most candidates. Both breadth and
depth was expected so as to score well.
This question is best answered using a time-based approach. For example, upon the rapid
infusion of a litre of normal saline there will be a brief period of hypervolemia, increase in
arterial blood pressure and an associated physiological reflex response to these changes
(e.g. baroreceptors, atrial stretch receptors, etc.). There will also be an associated increase in
renal perfusion and stimulation of intrarenal receptors (e.g. juxtaglomerular apparatus).
Candidates were expected to outline these changes, their effector responses (e.g. autonomic
nervous system reflexes and humoral changes) and their physiological consequences.
A more prolonged redistribution phase of the administered saline then occurs. This saline
redistributes throughout the extracellular fluid space. Candidates were expected to briefly
describe this effect as well as the subsequent management of the sodium and water load by
Most candidates spoke about the pressure effects, and only some compared these with the
volume effects. The effect of redistribution and other effects were not considered by the
majority of the candidates.