2. High selective sensitivity in relation to an adequate irritant that allows a receptor to choose certain type of influence among set of others. So, the sensation of a smell can be received at the maintenance of one molecule of substance in 1 м3 air contacting to a mucous membrane of a nose. Buy timbermate excelGoing on and off celexa In reply to cooling there is an excitation холодовых skin receptors, импульсация from them arrives in the centres гипоталамуса. From the thermoregulation centre there are signals to эффекторам, as a result first of all decreases потоотделение, the pose changes, there is a decrease in inflow of blood on periphery by means of narrowing of vessels. At influence of a cold skin vessels, mainly артериолы, are narrowed, therefore the blood most part arrives in vessels of internal areas of a body. In skin blankets the smaller quantity of blood circulates, the skin is cooled, radiation and heat carrying out in environment therefore decreases. At the person in process of blood passage on large arteries of hands and feet its temperature considerably decreases. A cool blue blood, coming back in a body on the vessels located near of arteries, receives the big share of heat given by arterial blood (counterflow heat exchange) that promotes returning of a part of heat to internal areas of a body. At the temperature of air close to zero, such system is not favourable, таккак as a result of an intensive exchange of heat between arterial and a blue blood the temperature of extremities can fall below a freezing point (отморожение).