The blood supply to the colon and upper rectum derives from the superior and inferior mesenteric arteries, and venous return is via the superior and inferior mesenteric veins (Figure 7.1). These join the splenic vein as part of the portal venous system to the liver. Thus any drugs absorbed from the colon and upper rectum are subjected to first pass elimination by the liver. Measurement of blood flow through the colon is difficult and reported values range from 8 to 75 ml. min-1 4. Blood flow through the colon is considerably less than that to the small intestine, and the proximal colon receives a greater share of the blood flow than the more distal part5.
The anal canal connects with the rectum at the point where it passes through a muscular pelvic diaphragm. The upper region of the anal canal has 5 to 10 vertical folds in the mucous membrane lining, called the anal, or rectal, columns; each column contains a small artery and vein. These are the terminal portions of the blood vessels that furnish the rectal and anal areas; they are susceptible to enlargement, commonly known as haemorrhoids. The mucous membrane of the upper portion of the rectum is similar to that in the rest of the large intestine; it contains mucus-producing and absorptive cells. Drugs absorbed from the lower rectum and anal canal are transported via these haemorrhoidal plexuses and internal iliac veins to the vena cava, and thus have the advantage of avoiding first pass elimination. However, not all rectally absorbed drug passes through this route, as the veins in this region are heavily anastomosed, causing a fraction of the blood flow to return via the hepatic portal vein to the liver. In cases of portal hypertension, this effect is reduced and a larger proportion of a rectally administered drug can avoid first pass metabolism.
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