Bioavailability refers to the proportion of a substance, such as a drug or nutrient, that enters the bloodstream when it is introduced into the body. This concept is crucial in pharmacology and nutrition, as it helps determine how effectively a substance can exert its effects. Factors influencing bioavailability include the method of administration, the chemical form of the substance, and individual differences in metabolism. For example, when a person takes a medication orally, its bioavailability may be lower than if it were administered intravenously. This is because the digestive system can break down some of the medication before it reaches the bloodstream. Understanding bioavailability is essential for optimizing the effectiveness of treatments and ensuring proper nutrient absorption in the body.