Fluid movement inside the dentinal tubules may be away from or towards the pulp. Dentine contains many thousands of microscopic tubular structures that radiate outwards from the pulp; these dentinal tubules are typically 0.5–2 micrometres in diameter. Changes in the flow of the plasma-like biological fluid present in the dentinal tubules can trigger mechanoreceptors present on nerves located at the pulpal aspect, thereby eliciting a pain response. This hydrodynamic flow can be increased by cold, (air pressure), drying, sugar, sour (dehydrating chemicals), or forces acting on to the tooth. Hot or cold food or drinks, and physical pressure are typical triggers in those individuals with teeth sensitivity. Movement of dentinal fluid away from the pulp can be caused by triggers such as cold and drying and movement towards the pulp can be caused by heat. Research has shown that triggers causing dentinal fluid to move away from the pulp elicit more of a painful response.