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Passive Diffusion

Meaning

Passive Diffusion is a fundamental biophysical process describing the spontaneous movement of a substance, such as a hormone or drug molecule, across a semi-permeable biological membrane without the expenditure of cellular energy. This transport mechanism is driven entirely by the concentration gradient, meaning molecules move naturally from an area of higher concentration to an area of lower concentration. In endocrinology, this is the primary mechanism by which lipid-soluble, unconjugated steroid hormones, such as cortisol and testosterone, enter the salivary glands from the bloodstream to be measured in saliva.