The mouth has a very thin epithelial surface and an extensive capillary network. Intact skin is designed as a defense mechanism and is keratinized to decrease permeability. Topically applied medication often have an oil base or a keratin softening agent to facilitate absorption. Based on what you know about other factors affecting absorption, describe how massaging the area of a topical application can enhance absorption. The blood flowing around the intestines does not go directly back to the systemic circulation, but instead it enters the portal circulation.
Portal circulation goes to the liver before returning to the general circulation.
This allows the liver access to a drug before it reaches the systemic circulation and the target tissue. Since the liver is the main site for metabolism, this first-pass effect allows the liver to metabolize much of a drug before it reaches the site of action. First-pass effect applies primarily to medications absorbed via the small intestine. Metoprolol 50-100 mg is a normal oral dose. When the same medication is given IV, the dose is 5-15 mg.