The Importance of Surface Area in Drug Absorption

As aspirin moves down the intestines, it's getting more ionized. So a weak acid would be absorbed better in the ilium. A polar, non-polar, more lipophilic can get through. And again, if it's water soluble and small enough, it'll go through aquaporins. If it's too big, it's not going to get through. Usually, large molecules need to be taken in through IV or other mechanisms.