Các con đường intercellular bây giờ là chấp nhận như là con đường chủ yếu cho sự hấp thụ. Nhớ lại rằng các tỷ lệ thâm nhập thường tương quan với hệ số phân vùng. Trong thực tế, đây là một con đường rất quanh co, và h (độ dày da) trong pháp luật khuếch tán đầu tiên của Fick thực sự là 10 × khoảng cách đo | ROUTES OF ABSORPTION 93 The intercellular pathway is now accepted as the major pathway for absorption. Recall that the rate of penetration is often correlated with the partition coefficient. In fact this is a very tortuous pathway and the h skin thickness in Fick s first law of diffusion is really 10 X the measured distance. By placing a solvent . ether acetone on the surface or tape stripping the surface the stratum corneum SC is removed and absorption can be significantly increased by removing this outer barrier. This may not be the case for very lipophilic chemical. This is because the viable epidermis and dermis are regarded as aqueous layers compared to the SC. Note that the more lipophilic the drug the more likely it will form a depot in the SC and be slowly absorbed over time and thus have a prolonged half-life. The transcellular pathway has been discredited as a major pathway although some polar substances can penetrate the outer surface of the protein filaments of hydrated stratum corneum. The transfollicularpathway is really an invagination of the epidermis into the dermis and the chemical still has to penetrate the epidermis to be absorbed into the blood stream. This is also a regarded as minor route. Sweat pores are not lined with the stratum corneum layer but the holes are small and this route is still considered a minor route for chemical absorption. In general the epidermal surface is 100 to 1000 times the surface area of skin appendages and it is likely that only very small and or polar molecules penetrate the skin via these appendages. Variations in areas of the body cause appreciable differences in penetration of toxicants. The rate of penetration is in the following order Scrotal Forehead Axilla Scalp Back Abdomen Palm and plantar. The palmar and plantar regions are highly cornified and are 100 to 400 times thicker than other regions of the body. Note that there are differences in blood flow and to a lesser extent hair density that may .
