Canonical Question
Fluid Pharmacology
Master answer
Assumptions
- TBW is one-third ECF & two-thirds ICF
- ECF is one-quarter plasma & three-quarters ISF
- The threshold of the volume receptors is 7-10% change in blood volume
- The osmoreceptors are sensitive to a 1-2% change in osmolality.
- Plasma osmolality is normal prior to the transfusion (ie 287-290 mOsm/kg)
Key
- TBW = Total Body Water
- ECF = Extracellular Fluid
- ICF = Intracellular Fluid
- ISF = Interstitial Fluid
1L NS has 9gm NaCl – It is an ECF replacement fluid
| Volume Litre | [Na+] mmol/L | [Cl–] mmol/L | Osmolarity mOsml/L | |
|---|---|---|---|---|
| NS | 1 | 154 | 154 | 308 |
| Plasma | 5 | 140 | 100 | 290 |
Response to 1L Normal Saline Bolus
| INITIAL PHASE | ||
| Brief Hypervolemia | → ↑ arterial BP | → Reflex response (baroceptors, atrial stretch receptors etc) |
| ↑ Renal perfusion → Stimulation of intra renal apparatus (juxtaglomerular apparatus) | ||
| [Na+] | 154/L into 140/L | very negligible rise |
| [Cl–] | 154/L into 100/L | rises 1L NS unlikely to cause hyperchloraemic metabolic acidosis |
| REDISTRIBUTION PHASE (~15 minutes) | ||
| [Na+] similar to ECF → Distributes | ISF (3) | ↑750ml |
| Plasma (1) | ↑250ml | |
| EXCRETION PHASE (~6 hours) | ||
| Plasma osmolality and tonicity mostly unchanged | no osmoreceptor stimulation | |
| Plasma volume ↑ to 5250mls (5% increase) | below sensitivity of volume receptor → no volume receptor stimulation | |
| No protein → oncotic pressure slightly lowered | Fluid moves into ISF (Starling’s hypothesis) | |
| Glomerulo-tubular Imbalance → ↑GFR → ↓ reabsorption of water in proximal tubule → ↑ Urine flow | ||
| Fluid moves back into intravascular compartment till all infused fluid is excreted (~6hrs) | ||
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2015A Q07 | Outline the physiological responses to the rapid intravenous administration of 1 litre of 0.9 % saline to a 70 kg euvolaemic person. | historical_member | — |