During a cardiovascular physiology practical, a student observes that in a healthy volunteer, the most common compensatory response to an acute 20% reduction in blood volume is an increase in which of the following?

## Acute Hemorrhage: Immediate Compensatory Mechanisms **Key Point:** When blood volume acutely decreases, cardiac output must be maintained: $CO = HR \times SV$. Since preload (and thus SV) falls immediately, **heart rate must increase** to preserve perfusion. ### The Baroreceptor Reflex Cascade 1. **Acute blood loss** → ↓ arterial pressure 2. **Baroreceptor firing decreases** → reduced vagal (parasympathetic) tone 3. **Sympathetic outflow increases** → ↑ HR, ↑ contractility, ↑ peripheral vasoconstriction 4. **Net effect:** Tachycardia is the **first and most prominent** compensatory response **Mnemonic:** **SHOCK** = Sympathetic activation → Heart rate ↑, Output maintained, Constriction (vasoconstriction), Kidneys (fluid retention) ### Why Heart Rate Is the Primary Response | Mechanism | Timeline | Magnitude | |-----------|----------|----------| | **↑ Heart rate** | Immediate (seconds) | +20–40 bpm typical | | ↑ Contractility | Immediate (seconds) | Modest (+10–20% SV) | | Vasoconstriction | Immediate–minutes | Maintains BP | | Fluid shifts | Minutes–hours | Restores preload | | RBC production | Hours–days | Restores O₂ capacity | **Clinical Pearl:** A 20% blood volume loss (Class II hemorrhage) typically causes tachycardia (HR >100) with preserved blood pressure initially. If the patient is **not tachycardic**, suspect either (a) severe sympathetic blockade, (b) athletic conditioning (blunted response), or (c) ongoing hemorrhage masking the reflex. **High-Yield:** The baroreceptor reflex is the **fastest** cardiovascular compensation. Sympathetic activation increases HR within 1–2 seconds of pressure drop, long before hormonal responses (RAAS, ADH) take effect. ### Why Stroke Volume Cannot Compensate Alone With acute blood loss: - **Preload ↓** → venous return ↓ → LVEDV ↓ → SV ↓ (Frank-Starling) - Contractility ↑ (from sympathetic stimulation) can only partially offset this - **Result:** SV remains reduced; HR must rise to maintain CO [cite:Guyton & Hall Ch 20]