Non-Oral Extravascular Drug Absorption Routes

Lipophilic drugs are better absorbed sublingually and buccally because they are stable at salivary pH 6 and exhibit minimal binding to the oral mucosa, allowing efficient passive diffusion across mucosal membranes. These routes bypass first-pass hepatic metabolism, enabling rapid systemic delivery.

Sublingual nitroglycerin is absorbed from the mouth's venous system and drains directly into the superior vena cava, bypassing first-pass hepatic metabolism. This direct route to systemic circulation allows rapid delivery to the failing heart, improving blood flow efficiently and providing immediate therapeutic benefit.

Intramuscular injection absorption rates depend on blood supply at the injection site. The arms, with rich vascular networks, yield faster absorption than the thighs, while the buttocks register the slowest absorption due to lower blood perfusion and reduced factors influencing drug absorption anatomical parameters.

Topical absorption is slow on the foot and palm because these areas have a thick stratum corneum, the skin's outermost layer, which creates a significant barrier to drug diffusion. This dense layer limits passive diffusion of drugs across the skin barrier.

Occlusive dressings accelerate drug absorption on burned or injured skin by maintaining skin moisture and preventing water loss. Damaged skin has a compromised stratum corneum, and the moist environment created by occlusive dressings further enhances passive diffusion of medications into systemic circulation.

Gaseous and aerosol drugs are absorbed rapidly via inhalation because the lungs possess an extensive surface area and rich vascular network. This large absorptive surface combined with high blood perfusion enables quick drug uptake, making inhalation ideal for emergency treatments like bronchodilators.

Non-oral extravascular drug absorption is controlled by the drug's physicochemical properties, such as lipophilicity and pH stability, and the patient's anatomical and pathophysiological state. These factors collectively determine whether drugs undergo passive diffusion effectively across different tissue barriers to reach systemic circulation.