Isovolemic hemodilution compensation

The body responds to acute blood loss with four basic compensatory mechanisms. Knowledge of these mechanisms allows physicians to adequately assess for the need for blood transfusion.

1. The first response is increased cardiac output. Stroke volume increases due to decreased systemic vascular resistance (SVR). SVR decreases due to the diminished viscosity of the blood.
2. As cardiac output increases, the distribution of blood flow changes. Blood flow is increased to vital organs, or organs that have higher oxygen extraction ratios. This (blood flow redistribution) is the primary mechanism for cardiac compensation to anemia. This is aided by an increase in oxygen extraction from hemoglobin. The heart itself has limited ability to increases its oxygen extraction, but compensates due to its increased coronary blood flow.
3. The oxygen-hemoglobin dissociation curve adjusts during periods of anemia. Anemia causes an increase in 2,3-DPG, which shifts this curve to the right. This decreases the affinity of hemoglobin for oxygen, which facilitates oxygen extraction by tissues.
4. During acute surgical blood loss, the adrenergic nervous system is stimulated, which leads to vasoconstriction and tachycardia.