The skin. Some say the biggest organ of the body is the skin. So when you spread stuff on your skin, what happens?
First of all, the outer layer, called the epidermis, is made to be a barrier between your body insides, and the outside world. It’s pretty good at that! It is made up of layers of dead cells containing a protein called keratin, which are held together by a lipid substance (both to provide a waterproof barrier). These dead cells are the result of new skin cells constantly being produced and getting pushed to the surface, where they eventually slough off. So your skin is constantly renewing itself.
For the chemical constituents of herbs to get into your skin, they must have a molecular weight of less than 500. I just went looking to see what measurement molecular weight is and it’s complicated. All I need to know is that most herbal constituents fall into the less than 500 range. That may be all you need to know, too. Unless you want to dive deeper into molecular weights. Be my guest.
Another thing to know is that carrier oils help the herbs get past the epidermis because the lipid substance between the cells likes lipids. The carrier oils have a molecular weight of more than 500, so they are not absorbed. But they help the herbs be absorbed. Water is not a good carrier for herbal skin products. (Pour some water on your skin. You’ll note that it doesn’t get absorbed.)
Another way herbs are absorbed by the skin are via hair shafts. This is called the shunt route.
Other factors affecting absorption are skin temperature – warmer is better, hydration of the top layer of the epidermis – wetter is better, the actual lipid solubility of the herb itself, viscosity (thickness) of the carrier or herb – low is best, and the site where you are applying the herb – thin skin is best! Also, if your skin is damaged, the protective barrier has been broken and herbs can get into the bloodstream that way. And covering the skin helps, too.
The skin is part of what is called the integumentary system. The anatomy of the skin is fascinating, but that’s for another blog post (oh, look, here’s one!). Or you can do what I’ve done and buy an anatomy coloring book. It’s a great way to learn!
(The main reference for this post was a lesson written by Tammi Sweet, part of Rosemary Gladstar’s Herbal Course. She has much more detail in her lesson than I’ve summarized here.)