The human rectum is 15–20 cm long. It is normally empty and contains 2–3 ml of mucous fluid with pH 7–8.
There are no villi and only a limited surface area of 200–400 cm2 is available for absorption.
Blood and lymph vessels are abundant in the rectal submucosa. Veins from the upper rectum drain into the portal circulation, while veins from the middle and lower rectum drain directly into the inferior vena cava.
However, there are extensive anastomoses among these veins so that precise anatomical differentiation is difficult. It appears that compounds absorbed from the
lower rectum, in contrast to those absorbed from the upper rectum, avoid hepatic first-pass metabolism.
Rectal absorption is generally slower than oral absorption, but for some drugs, rectal absorption exceeds oral absorption presumably due to avoidance of first-pass metabolism after rectal delivery.
This has been reported for morphine, metoclopramide, ergotamine, lidocaine, and propranolol. Human rectal systemic availability of the extensively metabolized drug lidocaine is 65% as compared to 30% after oral administration.
Rectal absorption of drugs from aqueous or alcoholic solutions is generally much faster than from suppositories. Non-surfactant adjuvants, such as salicylates, increase rectal absorption of water-soluble drugs and also of high molecular weight compounds, such as insulin, heparin, and gastrin.