Rectal veins, superior

Blood from the haemorrhoidal venous plexus passes via the azygous superior rectal vein into the inferior mesenteric vein and thereafter into the portal vein. By contrast, the paired middle rectal vein and inferior rectal vein discharge their blood via the iliac vein into the inferior vena cava. In portal hypertension, anorectal varices are found in the region of the rectum, the anal canal and the external anal region. Haemorrhoids are distended and dislocated cavernous bodies in the rectum, which have no connection to the portal venous system. Although haemorrhoids and anorectal varices are two different clinical pictures, it is quite possible for them to occur simultaneously. The frequency of anorectal varices (40-80%) is dependent upon the extent and duration of portal hypertension. The bleeding tendency is low (7-14%). However, there have also been reports of massive haemorrhages. (21,45,55,66,83,105,156) (s. tab. 14.10)... 

Anorectal varices originate from the superior rectal vein, which is connected to the portal vein system. This portacaval anastomosis drains the blood into the iliac vein. Occasionally, massive bleeding may occur from rectal varices. Endoscopy is required for diagnosis and sclerotherapy. (70) (s. p. 256)... 

Figure 9.52 shows the blood supply to the rectal area. The main artery to the rectum is the superior rectal (haemorrhoidal) artery. Veins of the inferior part of the submucous plexus become the rectal veins, which drain to the internal pudendal veins. Dmg absorption takes place through this venous network. Superior haemorrhoidal veins connect with the portal vein and thus transport dmgs absorbed in the upper part of the rectal cavity... 

Figure 9.52 Blood supply to the rectum and anus. The significance of the location of the superior and the inferior rectal veins is discussed in the text.

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... 

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