Capillary Blood Flow Regulation: Arterioles and Precapillary Sphincters

Microcirculation Control Mechanisms

Blood flow through systemic capillary beds is regulated at two key sites:

1. Feeder Arterioles (Metarterioles)

• Proximal resistance vessels with smooth muscle.
• Constriction ↑ upstream resistance → ↓ capillary perfusion.
• Dilation ↓ resistance → ↑ flow.

2. Precapillary Sphincters

• Smooth muscle rings at capillary origins.
• Control entry into individual capillaries.
• Closed → blood shunts via thoroughfare channels (metarteriole → venule).

Regulatory Factors

Clinical Relevance

Increased systemic blood pressure triggers myogenic arteriolar constriction and precapillary sphincter closure to protect fragile capillaries from barotrauma and maintain stable perfusion. This autoregulation is impaired in hypertension, diabetes, and sepsis.

The original statement "increased blood pressure causes constriction of feeder arterioles and closing of precapillary sphincters" correctly describes the myogenic autoregulatory response.

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Stimulus	Arterioles	Precapillary Sphincters	Net Effect
↑ Metabolic demand (↓ O₂, ↑ CO₂, H⁺, K⁺, adenosine)	Dilation	Relaxation (open)	↑↑ Capillary flow
Sympathetic activation (α₁)	Constriction	Constriction (close)	↓↓ Capillary flow
Local autoregulation (myogenic)	Constriction to ↑ pressure	—	Flow stabilization
Histamine, bradykinin (inflammation)	Dilation	Relaxation	↑ Flow, ↑ permeability