Consequently, newborns and the elderly often need smaller, and children larger, doses per pound of body weight. Excretion refers to the processes by which the body eliminates a drug. The kidneys are the major organs of excretion. They are particularly effective in eliminating water-soluble drugs and their metabolites. The kidneys filter drugs from the bloodstream and excrete them into the urine.
Many factors can affect the kidneys' ability to excrete drugs.
A drug or metabolite must be soluble in water and not bound too tightly to plasma proteins. The acidity of the urine affects the rate at which some acidic and alkaline drugs are excreted. The kidneys' ability to excrete drugs also depends on urine flow, blood flow through the kidneys, and the condition of the kidneys. As people age, kidney function decreases. If the kidneys aren't functioning normally, a doctor may adjust the dosage of a drug that's eliminated primarily through the kidneys.