<prompt> Which of the following act to cause local VasodilatorsO2 tensionPHCO2 tensionCO2 tension  <response>: CO2 tension 500-1-K.S 597-GN 563-Ganong 23rdLocal vasodilatorsVasodilators of metabolic originVasodilators of endothelial origin The vasodilators of metabolic origin are carbon dioxide, lactate, hydrogen ions and adenosine, Histamine The metabolic changes that produce vasodilation include in most tissues decrease in O2 tension and pH These changes causes relaxation of the arterioles and precapillary sphincters. Increase in CO2 tension and osmolality also dilate the vessels. The direct dilator action of CO2 is most pronounced in the skin and brain Hyperthermia exerts direct vasodilator effect K is another substance that accumulates locally has demonstrated dilator activity Nitric oxide NO is an endothelium derived factor EDRF. It is synthesized from Arginine NO is a vasodilator, deficiency of this leads to hypertension. The other functions of this are penile erection with vasodilatation and engorgement of corpora cavernosa, activation of macrophages in brain, destruction of cancer cells and relaxation of smooth muscles of GIT.Local vasoconstrictors - The local vasoconstrictor substances are of endothelial origin are known as endothelins ET. The production of endothelins is because of the stretching of blood vessels. The endothelins act by activating phospholipase, which in turn activates the prostacyclin and thromboxane A2. These two substances cause constriction of blood vessels and increase in blood pressure. 