What is meant by therapeutic effect of heat?

When tissue temperature is increased using diathermy, it results in hyperemia-an increase in blood flow through that part due to dilation of arteries and reduction in hydrodynamic impedance. Since pain is due to an oxygen deficiency in the tissues and an accumulation of pain eliciting (inducing) substances, the increased blood flow results in increased oxygenation and removal of these substances. Thus pain is reduced.