This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Circulation, Subject name: Physiology
Precapillary sphincter relaxation occurs due to: 
Select and explain. 
A. Local chemicals, B. Sympathetic stimulation, C. Circulating catecholamines, D. Capillary filling
A: Local chemicals
Answer: A: Local chemicals 
Explanation: Precapillary sphincters are controlled mainly by the local concentration of tissue metabolites. Precapillary sphincters are considered as pa of the precapillary resistance vessels; they determine the size of the capillary exchange area which is perfused at any given moment in the tissue. In the triple response, relaxation of the precapillary sphincters is due to presence of histamine and/or polypeptides released locally from the damaged skin. Circulating catecholamines may cause constriction of the precapillary resistance vessels and the precapillary sphincters.