To blush is to display redness in one's face; the term is usually used when the redness is a result of an emotional response, which could reflect embarrassment, shame, or modesty. Blushing can also be associated with being in love. Blushing is generally distinguished, despite a close physiological relation, from flushing, which is more intensive and extends over more of the body, and seldom has a mental source.
If redness persists for abnormal amounts of time after blushing, then it may be considered an early sign of rosacea.
A medical condition known as Idiopathic craniofacial erythema exists, in which the sufferer blushes strongly with little or no provocation. This is related to social phobia.
Erythrophobia (literally "fear of redness") refers to pathological blushing.
Along with this basal tone, all resistance vessels in the skin receive a tonic outflow from sympathetic vasoconstrictor fibers. This tonic outflow is inversely associated with body temperature. Vasodilation therefore occurs passively in resistance vessels the (alpha-adrenergic) vasoconstrictor tone decreases. Furthermore, an active neurogenic vasodilation must be assumed in the human skin. However, it is not clear if this vasodilation is mediated by specific vasodilator nerve fibers or if neuro-humoral effects are involved that are associated with the sympathetic cholinergic activation of sweat glands (Rowell. 1993; Lembeck & Holzbauer. 1988). Although some experiments lead to the conclusion that sympathetic outflow is involved in facial vasodilation (Drummond & Lance. 1987), 40 years of research have not clarified the mechanism behind active vasodilation.
Cutaneous veins also are richly innervated with sympathetic vasoconstrictor fibers. The effect of activation of the outflow of these fibers is reduced by local cooling. In addition, cutaneous veins are temporarily reactive to various other stimuli. Each of the following can cause remarkably intense venoconstriction without obvious value to the organism: emotional stimuli (e.g. startle, apprehension, discomfort), hyperventilation, deep inspiration, and the Valsalva maneuver (Rowell. 1974).
In resistance vessels, increased pressure mainly increases flow per time unit. In the cutaneous venous plexus with its generally slow flow rate, pressure mainly influences volume. Because of its enormous volume variability and its large potential capacity the venous plexus is believed to determine skin color (Rowell. 1993; Mellander, Andersson, Afzelius, & Hellstrand. 1982; Rowell. 1974). Since pronounced blushing is also characterized by a deep reddening of the skin, vasodilation of the venous plexus is probably the physical mechanism underlying it. But why is emotional blushing only visible or apparent in a specific area called the blush region? That area is restricted to the face, ears, neck, and in some rare cases the upper body. Two main hypotheses to explain this regional restriction have been proposed. One is that vasodilation takes place throughout the entire skin of the body but is only visible in the blush region due to special anatomical structure of that region. The second is that a specific form of vasodilation takes place exclusively in the blush region. It is likely, that a combination of these two factors accounts for blushing.
Evidence for special vasodilation mechanisms was reported by Mellander and his colleagues (Mellander, Andersson, Afzelius, & Hellstrand. 1982). They studied buccal segments of the human facial veins in vitro. Unlike veins from other areas of the skin, facial veins responded with an active myogenic contraction to passive stretch and were therefore able to develop an intrinsic basal tone. Additionally Mellander et al. showed that the veins in this specific area were also supplied with beta-adrenoceptors in addition to the common alpha-adrenoceptors. These beta-adrenoceptors could exert a dilator mechanism on the above-described basal tone of the facial cutaneous venous plexus. Mellander and his colleagues propose that this mechanism is involved in emotional blushing. Drummond has partially confirmed this effect by pharmacological blocking experiments (Drummond. 1997). In a number of trials, he blocked both alpha-adrenergic receptors (with phentolamine) and beta-adrenergic receptors (with propranolol introduced transcutaneously by iontophoresis). Blushing was measured at the forehead using a dual channel laser Doppler flowmeter. Subjects were undergraduate students divided into frequent and infrequent blushers according to self-report. Their mean age was 22.9 years, which is especially favorable for assessing blushing, since young subjects are more likely to blush and blush more intensively. The subjects underwent several procedures, one of which was designed to produce blushing. Alpha-adrenergic blockade with phentolamine had no influence on the amount of blushing in frequent or in infrequent blushers, indicating that release of sympathetic vasoconstrictor tone does not substantially influence blushing. This result was expected since vasoconstrictor tone in the facial area is known to be generally low (van der Meer. 1985). Beta-adrenergic blockade with propranolol on the other hand decreased blushing in both frequent and infrequent blushers. However, despite complete blockade, blood flow still increased substantially during the embarrassment and blushing inducing procedure. Additional vasodilator mechanisms must therefore be involved. So far, no specific mechanism has been suggested.