Precapillary sphincter relaxation occurs due to:

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.