The length of the human rectum ranges from 15-20 cm. It is naturally empty and contains 2-3 ml of 7 to 8 mL mucosal fluid and does not contain villi and has a limited surface absorption area of about 200-400 cm 2. The subcutaneous mucosa is rich in blood vessels and lymphocytes. The discharge of the vein from the upper rectum to the portal circulation while the vein is discharged from the rectum, middle and lower directly in the inferior vena cava and there are many anastomoses that extend between these veins, making it difficult to distinguish the exact anatomy. The compounds that are absorbed in the lower rectum appear to avoid the metabolism of the first hepatic pathway, unlike the compounds absorbed in the upper rectum. The rectal absorption is usually slower than oral absorption, but rectal absorption is assumed to exceed oral absorption due to avoidance of the first hepatic passage after rectal delivery for certain drugs such as morphine, mitochloropamide, argotamine, lidocaine and propranolol. Where systemic bioavailability in humans for the lidocaine is 65%, compared to 30% after oral administration.
Usually, rectal absorption of drugs with aqueous or alcoholic solutions is greater than suppositories. Non-active substances on the surface such as salicylates, rectal absorption of water-soluble drugs, and high molecular weight compounds such as insulin, heparin, and gastrin are also lacking.
Usually, rectal absorption of drugs with aqueous or alcoholic solutions is greater than suppositories. Non-active substances on the surface such as salicylates, rectal absorption of water-soluble drugs, and high molecular weight compounds such as insulin, heparin, and gastrin are also lacking.
