15A07: Exam Report

Outline the physiological responses to the rapid intravenous administration of 1 litre of 0.9 % saline to a 70 kg euvolaemic person.

25% of candidates passed this question.

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 the kidney.

Most candidates spoke about the  ressure 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.

I2i / 15A07: Outline the physiological responses to the rapid IV administration of 1L 0.9% NaCl to 70kg euvolemic person


N/Saline is an isotonic ECF Replacement fluid

1L NaCl Components

  • 9g NaCl
  • 154mmol Cl
  • 154mmol Na
  • Osmolarity 308 mOsm/L
  • pH 5
  • Water

Immediate Distribution

Immediately distributes throughout ECF compartment.

Isotonic – so exerts no osmotic shift

∴distributes itself according to how the Na will distribute

Between the ECF compartments (ISF & plasma)

There is no initial ↑ ­ in ICF (intrac Na 140mmol/L)

  • ICF: nil
  • ECF: 100% → ISF 75% = 750ml, plasma 25% = 250ml

Initial Haemodynamic Effects

­↑preload => ­↑CO => ­↑BP

Ultimate Haemodynamic Effects

­ ↑intravascular volume 250ml

Excretion of Excess Fluid

  • Pressure Receptors: BP increase not enough to activate Baroreceptor response
  • Osmoreceptors: no change in plasma osmolarity ∴no activation by osmoreceptors
  • Volume Receptors: blood volume increases 250ml

5250/5000 = 5%

below the 10% threshold for atrial stretch Rec mediated inhibition of ADH release,   will not => play a role = no activation

  • Oncotic Pressure

No colloid particles but the ­↑plasma volume (250ml) sufficient to dilute plasma proteins

∴↓oncotic P

­↑GFR &↓Na+ reabsorption in PCT

­↑urinary flow and excretion of XS salt and fluid

KA Glomerulotubular Balance