The opening and closing of precapillary sphincters is controlled by which nervous system?

• A. Sympathetic nervous system
• B. Parasympathetic nervous system
• C. Endocrine system
• D. Local metabolic factors

Answer: (A)

Opening and closing of precapillary sphincters are governed by the vascular system’s nervous control, with the sympathetic nerves playing the key role in setting the baseline tone. When sympathetic nerves fire, they release norepinephrine that acts on smooth muscle in the arterioles and precapillary sphincters to cause constriction, reducing capillary blood flow and redirecting perfusion as needed.

But the system is not just a simple on/off switch. Local metabolic factors—like increased CO2, decreased O2, lower pH, and metabolites such as adenosine—signal tissues that more blood is needed. These local signals can counteract sympathetic constriction and open the sphincters to boost capillary flow during activity. So the sympathetic system provides the broad, rapid control, while tissue metabolism finely tunes the actual opening of the capillary entrances.

Parasympathetic innervation of most systemic vessels is minimal in this context, and while hormones can influence vascular tone, the rapid, wobble-free control of these sphincters hinges on sympathetic nerve activity and local metabolic cues.