## Immediate Cardiac Response to Exercise **Key Point:** The **immediate (within seconds) increase in heart rate and contractility during exercise** is mediated by the **sympathetic nervous system** — specifically catecholamines (epinephrine and norepinephrine) acting on **β1-adrenergic receptors** on the heart. ### Timeline of Cardiac Output Regulation | Timeframe | Mechanism | Mediator | Effect | |-----------|-----------|----------|--------| | **Immediate (0–10 sec)** | Sympathetic activation | NE/Epi on β1 receptors | ↑HR, ↑contractility | | **Short-term (seconds–minutes)** | Frank-Starling + sympathetic | Venous return + catecholamines | ↑SV, ↑HR | | **Long-term (hours–days)** | Fluid retention, RBC production | Aldosterone, EPO, ANP | ↑blood volume, ↑O2 capacity | ### β1-Adrenergic Receptor Effects on the Heart 1. **Increased heart rate** (chronotropic effect) — accelerates SA node depolarization 2. **Increased contractility** (inotropic effect) — enhances ventricular force of contraction 3. **Increased conduction velocity** (dromotropic effect) — speeds AV node conduction 4. **Increased lusitropy** — improves diastolic relaxation (allows faster filling) **High-Yield:** These effects are mediated by **cAMP ↑ → PKA activation → phosphorylation of L-type Ca²⁺ channels and ryanodine receptors** → increased intracellular Ca²⁺. ### Why Other Options Are Wrong **Warning:** Hormonal mechanisms (epinephrine from adrenal medulla, ANP) take **minutes to hours** to exert full effect — too slow for immediate exercise response. The **neural (sympathetic) pathway is instantaneous**. [cite:Guyton & Hall Ch 9; Harrison 21e Ch 297]
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