The rate and extent of systemic absorption are influenced by several factors. Key among them are the substance's lipid solubility, molecular size, and ionization state. Lipid-soluble substances typically cross cell membranes more easily than water-soluble ones. Additionally, the route of exposureâwhether inhalation, dermal, or oralâalso impacts absorption. For instance, inhaled substances can rapidly enter the bloodstream through the extensive surface area of the lungs.
