Widening of the blood vessels and therefore blood flow and blood pressure is regulated, amongst other things, by a molecule called bradykinin. Bradykinin binds to the bradykinin receptor (a protein encoded by the BDKRB2 gene). Different variants of this gene lead to lower or higher expression of this receptor and are associated with hypertension risk and also with endurance athletes.
Source: 1000 Genome Project. Global averages for both sexes