Rectal absorption surface area

Intestinal surface area and total blood flow to the GIT are smaller than in adults and may influence the efficiency of absorption. With regard to the use of rectal suppositories, one must keep in mind that the completeness of absorption will be a function of retention time in the rectum. Since bowel movements in... 

The sublingual surface area is relatively small but has a rich blood supply. The major advantage of this route is avoidance of intestinal destruction and hepatic first pass metabolism. However, absorption can be highly variable critical factors are the residence time of the drug in the mouth and saliva flow. Premature swallowing or excessive saliva production preclude efficient absorption. Nitroglycerin, nifedipine, propranolol, and buprenorphine are all available as sublingual preparations. Rectal... 

The distal portion of the large intestine is the rectum. Rectal absorptive capacity is considerably less than that of the upper GI tract owing to a limited surface area, a result of the absence of microvilli. Also, the blood supply to colon and rectum is less than that to the small intestine. The rectal artery branching off the inferior mesenteric artery of the... 

The vagina offers a relatively large surface area (approximately 60 cm2) for drag absorption. However, it is much smaller than that offered by the nasal (150 cm3), rectal (200-400 m2), pulmonary (75-700 m2) and intestinal (200 m2) routes. 

Less absorption rectally than orally because the absorbing surface area of the rectum is smaller... 

The surface area available for absorption in the buccal mucosa is much smaller than the gastrointestinal, nasal, rectal, and vaginal mucosae. The buccal mucosa is continuously bathed by saliva, and the secreted saliva lowers the drug concentration at the absorbing membrane. These two factors, along with the permeability... 

Absorption from the rectum depends on various physiological factors such as surface area, blood supply, pH, fluid volume, and possible metabolism by microorganisms in the rectum. The rectum is perfused by the inferior and middle rectal arteries, whereas the superior, the middle, and the inferior rectal veins drain the rectum. The latter two are directly connected to the systemic circulation the superior rectal vein drains into the portal system. Drugs absorbed from the lower rectum are carried directly into the systemic circulation, whereas drugs absorbed from the upper rectum are subjected to hepatic first-pass effect. Therefore, a high-clearance drug should be more bioavailable after rectal than oral administration. The volume of fluid in the rectum, the pH of that fluid, and the presence of stool in the rectal vault may affect drug absorption. Because the fluid volume is... 

---