Stimulus is a change that takes place internal or external to our body. The external changes are detected by sensory receptors in the sensory organs. The receptors get information from the stimulus. A reflexive reaction is expected after a stimulus is provided to the body. A stimuli transduction (through transducers) happens in which a signal for the nervous system is generated in terms of a small potential, which is more than a threshold.
The main function of the stimuli is to alert the body of a condition and inform it to take a reflexive reaction.
Blood pressure and heart beat rate are continuously monitored by stretch receptors in carotid arteries in the heart. These receptors detect stretching of the arteries and generate potential on the nervous system. Then the central nervous system responds accordingly.
Sensory organs respond to stimuli given through touch, taste, vision etc.
Thermoreceptors are the sensors in the skin which receive the external stimuli of temperature changes. The play part in initiation of the regulation of flow of liquids from sweat glands to the outer skin.