The circulatory pathway of oral drugs excreted via urine includes digestion and absorption, first-pass elimination in the liver, reaching the target organ or target tissue, and reaching the kidneys. 1. Digestion and absorption: after drugs are taken orally into the digestive tract, they are broken down into small molecules that can be absorbed under the action of digestive juices and digestive enzymes, and then enter into the blood circulation through the intestinal wall by means of free diffusion, easy diffusion, active transportation, and cytotoxicity and cytotoxicity, and so on. 2. First-pass elimination by the liver: after absorbed into the blood, the drug firstly reaches the liver, which will metabolize part of the drug, thus losing its pharmacological effect, which is equivalent to the reduction of the drug dosage. 3. Reaching target organs or target tissues: the drug diffuses from the blood to the tissues and organs, exerts pharmacological effects and achieves therapeutic effects. 4. The residual drug reaches the kidneys: it is filtered in the glomerulus into the primary urine, and after concentration and dilution in the renal tubules, it reaches the collecting ducts, and finally passes through the renal pelvis, ureter, bladder and urethra and is discharged out of the body. After several rounds of circulation, all the drugs in the body can be eliminated.