Rectum physiology

Anatomy and Physiology of the Rectum and Its Role in Drug Absorption... 

Many physiological aspects affect drug absorption from the rectum (Table 7.1). Influential factors include the pH of the rectal contents, state of the mucus layer, volume and viscosity of rectal fluid, luminal pressure from the rectal wall on the dosage form, enzymatic and microbacterial degradation by rectal epithelium, presence of stools, and venous drainage differences within the rectosigmoid regions. 

Orally administered drugs are mainly absorbed in the small intestine (duodenum, jejunum, and ileum) and in the large intestine (colon) however, other regions, such as buccal cavity, stomach, and rectum, also can be considered potential sites for drug absorption. The various anatomical and physiological characteristics of each segment are briefly described in Table 1. 

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... 

Physiological.—Hydrogen sulfid is produced in the intestine by the decomposition of albuminous substances or of tanrochloric acid it also occurs sometimes in abscesses, and in the urine in tuberculosis, variola, and cancer of the bladder. It may also reach the bladder by diffusion from the rectum. 

Yoshioka K, Matsui Y, Yamada O, et al. (1991) Physiologic and anatomic assessment of patients with rectocele. Dis Colon Rectum 34 704-708... 

The rectum is the lowest part of the large intestine. It is 15-20 cm long with a diameter of about 5 cm. In the rectum 1-5 mL of viscous fluid with a pH between 6.4 and 7.4 and a small buffer capacity is present. The temperature (under physiological conditions) is 36.2—37.6 °C. The rectum is a flat tube, because of the pressure of the bowels. After administration of a fluid into the rectum the Uquid will spread due to this pressure. Larger volumes (enemas) are spread into the colon as well. A defecation reflex will occur when volumes exceeding 100 mL are applied, which limits the volume of enemas. 

The rectal cavity has the potential of convenient access and easy administration of suppositories or gels, although patient acceptance of rectal delivery is low in some cultures. It has a limited surface area (de Boer et al, 1992) and relatively high proteolytic activity (Lee et al, 1987), but with respect to administration of insulin the rectal route offers the particular physiological advantage of potential delivery, via the upper rectal veins, into the portal system. This would mimic the natural secretion of insulin and result in reduced peripheral hyperinsulinemia. However, nearly two-thirds of the insulin absorbed firom the rectum reaches the general circulation via the lymphatic pathway (Caldwell et al, 1982). 

Extrinsic and intrinsic nervous systems as well as local and circulating gastrointestinal hormones contribute to local and centrally mediated reflex mechanisms that regulate GI function, ingestion, secretion, mixing and propulsion, and absorptimi of food and defecation reflexes that force the fecal material into the rectum by intense mass movements (detailed description can be found in Guyton and HaU Text Book of Medical Physiology Hall 2011). 

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