How does drug distribution occur in the body?

Drug distribution in the body primarily occurs via circulation in the bloodstream to tissues and organs. Once a drug enters the systemic circulation, it is transported throughout the body, allowing it to reach various sites of action. This process involves the movement of the drug through the blood vessels, where it can diffuse into surrounding tissues and organs based on factors such as blood flow, tissue permeability, and the drug's chemical properties.

This mechanism is crucial because it determines how effectively a drug can exert its therapeutic effect and how quickly it can reach areas that may need treatment. The distribution is influenced by factors such as the drug's solubility, molecular size, and the presence of barriers like the blood-brain barrier or placental barrier, which may restrict the passage of certain substances.

In contrast, other options describe processes that are not primary mechanisms of drug distribution. For instance, while intravenous injections deliver drugs directly into circulation, they are just one method of administration and do not encompass the entirety of drug distribution pathways. Absorption in the stomach pertains more to drug entry into the bloodstream rather than distribution, and diffusion across cell membranes into muscle is a more localized form of action rather than a general distribution method.