The increase in the internal diameter of blood vessels, especially arterioles or capillaries. The vasodilation of arterioles is mediated by the relaxation of smooth muscle fibres of the arteriole walls due primarily to autoregulation following local changes in the tissue environment, and indirectly also to the influence of the autonomic nervous system. Low oxygen concentrations and high carbon dioxide concentrations in a tissue cause local relaxation of systemic capillary smooth muscle, thereby widening local capillaries and increasing local blood supply (the opposite applies in the lungs, where low O₂ causes vasoconstriction). Inhibition of sympathetic nervous stimulation by parasympathetic fibres also causes vasodilation, as do the parasympathetic neurotransmitters acetylcholine, substance P, and vasoactive intestinal polypeptide. See blood pressure.