What does the absorption rate of body tissues depend on? For example, at what rates would the rectum, vagina, mouth, and skin be able to absorb a certain chemical? Does it depend on the size of the molecule? If it is water-soluble does that make it easier to absorb? In fact, what makes something good for circulating into the blood, and passing into other body-tissues, like through the blood-brain barrier? Hope I was clear. Thank you!