A substance formed in tissues or blood vessels when there needs to be local or even massive vasoconstriction. The primary precursor is renin, made by the kidneys, and elevated when the blood seems dehydrated or low in volume; the next substance needed for this reaction is a liver protein, angiotensinogen; when both are present in the blood, local factors can then form this pressor substance. Excess production is often implicated in high blood pressure.