## Vascular Resistance Distribution **Key Point:** Arterioles are responsible for approximately 60% of total peripheral resistance in the systemic circulation, making them the primary site of vascular resistance regulation. ### Resistance by Vessel Type | Vessel Type | % of Total Resistance | Diameter | Wall Thickness | Function | |---|---|---|---|---| | Arteries | ~10% | Large | Thick | Pressure reservoir | | **Arterioles** | **~60%** | Small | Muscular | **Primary resistance site** | | Capillaries | ~20% | Smallest | Thin | Exchange | | Venules | ~5% | Small | Thin | Drainage | | Veins | ~5% | Large | Thin | Capacitance | ### Mechanism of Arteriolar Resistance **High-Yield:** Resistance is inversely proportional to the fourth power of radius: $R = \frac{8\eta L}{\pi r^4}$ (Poiseuille's law). Small changes in arteriolar diameter produce massive changes in resistance. 1. **Structural features** — Arterioles have thick, muscular walls rich in smooth muscle 2. **Sympathetic innervation** — Dense autonomic nerve supply allows rapid vasoconstriction/vasodilation 3. **Metabolic responsiveness** — Direct response to local metabolic factors (O₂, CO₂, H⁺, adenosine) 4. **Pressure gradient** — Arterioles experience the largest pressure drop (~60 mmHg) across the vascular tree ### Clinical Pearl Antihypertensive drugs (ACE inhibitors, ARBs, calcium channel blockers, alpha-blockers) primarily target arteriolar smooth muscle to reduce peripheral resistance and lower blood pressure. **Mnemonic:** **RAVE** — Resistance occurs At Vessels that are small and muscular = **Arterioles** are the primary site of Vascular resistance in the systemic circulation.
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