Expansion
Baroreflex tachycardia and vasoconstriction, then fluid shift and hormonal retention
Expansion
Immediate, within seconds
- Baroreceptor unloading gives tachycardia, increased contractility, arteriolar constriction and venoconstriction
- Flow is redirected away from skin, gut and kidney towards heart and brain
Minutes to hours
- Transcapillary refill: reduced capillary hydrostatic pressure draws interstitial fluid into the circulation, restoring volume but diluting the haemoglobin
- Renin-angiotensin-aldosterone activation, antidiuretic hormone release, catecholamine surge
Days
- Plasma protein replacement by the liver
- Erythropoietin driven red cell regeneration, taking weeks
Classes of haemorrhage in an adult:
| Class | Loss | Key sign |
|---|---|---|
| I | up to 15% | Minimal change |
| II | 15 to 30% | Tachycardia, narrowed pulse pressure |
| III | 30 to 40% | Hypotension, confusion |
| IV | over 40% | Profound hypotension, anuria |
Note that haemoglobin is initially normal, since whole blood is lost; it falls only as dilution occurs.