Heat also effects the viscosity of a fluid inside a capillary. An increase in heat decreases the viscosity of the lumenal fluid. A good example of this action can be observed in the human body during exercise. When a human is exercising, there is an increase in the metabolic rate inside the muscles, creating an increase in heat production. The increase in heat is detected by thermoreceptors, a type of sensory receptor located at various points in body. These receptors send a signal to the brain that tells the body to dilate the blood vessels, including capillaries. This creates a visible change in the number of vessels on the skin. This allows for heat transfer via convection to occur.
