Bioavailability refers to the proportion of a nutrient or drug that enters the bloodstream when it is introduced into the body. It is a crucial factor in determining how effectively a substance can exert its intended effects. For example, when a person takes a medication, its bioavailability influences how much of the drug is available to act on the body’s systems. Several factors can affect bioavailability, including the method of administration (such as oral or intravenous), the chemical form of the substance, and individual differences in metabolism. Understanding bioavailability is essential in fields like pharmacology and nutrition, as it helps in designing effective treatments and dietary recommendations.