E4 Transportation System

Topic

The oral transport system describes how substances—such as ions, acids, water, and drugs—move through dental tissues and the tissues surrounding the tooth. This system operates via three fundamental mechanisms: diffusion, convection, and active cellular transport.

Diffusion moves substances within the oral transport system from areas of higher concentration to areas of lower concentration; this process slows down as the density of the tissue the substance must traverse increases—such as when bacteria and their acids diffuse through dense connective tissue or when an erosive acid makes its way through dental enamel.

Convection, the second mechanism, occurs primarily in the dentin. Dentin is a highly permeable tissue due to the vast number of dentinal tubules running through it—approximately 45,000 tubules per square millimeter of surface area, each with a diameter of merely one to three micrometers—which act as transport channels connecting the dental pulp to the enamel.

Dentin fluid circulates through these tubules, driven by a pressure gradient between the dental pulp—where positive pressure ranges from ten to fifteen centimeters of water column—and the oral cavity, where pressure is zero or even negative in the presence of an external stimulus. The greater the pressure difference between the dental pulp and the oral cavity, the more intense the flow of dentin fluid through the tubules.

This flow of dentin fluid activates the mechanoreceptors of A-delta nerve fibers located in the dental pulp—the same mechanism responsible for the sharp pain characteristic of dentin hypersensitivity. The third mechanism, active cellular transport, moves substances within the oral transport system across the cell membranes of dental and periodontal tissues by expending cellular energy—unlike diffusion and convection, which do not require the cell to expend energy for substance movement.

Within this same oral transport system, salivation—the movement of water and other components into the oral cavity as saliva—is controlled by the autonomic nervous system and the secretion of glandular acini; these are the same two factors that determine the basal salivary flow associated with xerostomia.

At rest, this basal salivary flow remains between approximately 0.1 and 0.4 milliliters per minute, but it can rise to nearly 7 milliliters per minute when salivation is stimulated, such as during chewing. Thus, diffusion, convection, and active cellular transport—along with the salivation just described—are the mechanisms that collectively explain how substances move through the dental and periodontal tissues that constitute the oral transport system.

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