The vascular tone increases as the arterial size decreases. The tone at any time reflects the effects of excitory and inhibitory pathways. Neurotransmitters, physical forces like shear stress, intravascular pressure, metabolites and milleu of the surrounding tissue can all affect the vascular tone. Arterial segments where the endothelium is absent can also respond to stretch-the so-called intrinsic myogenic tone. The endothelium is an important modulator of tone. It can produce, release, activate many vasoactive factors like NO (nitric oxide), prostacyclin and thromboxane. Cholinergic stimulation releases an Endothelium derived relaxation factor (EDRF) shown to be NO. Endothelial denudation abolishes the vasodilatory effect of acetylcholine.
There are chemoreceptors in the aortic and carotid bodies which sense a rise in PCO2 (hypercapnia) and fall in O2 saturation (hypoxia) and changes in pH. Appropriate stimuli are sent to the medullary centers. Vasomotor control is also exercised by higher centres in the cerebral cortex and hypothalamus which send impulses to the medullary centres. Local tissue mechanisms also affect local vasoconstriction and vasodilatation.