Anoxemia as the cause of death in shock

Pain: evolutionary background and primary stimulus

The evolution of aerobic organisms required the development of oxygen sensors so the organism could avoid anoxic environments. In man these oxygen sensors are known as pain receptors. The skin pain receptor includes the epidermis and the dermal papilla with its capillary and terminal nerve. A pain stimulus such as a pin prick occludes blood flow into the papilla. The epidermal cells consume enough residual oxygen to produce a fall in oxygen tension at the nerve ending sufficient to cause the nerve to fire. The maximum time required for the oxygen drop is 0.04 seconds.