Rectal fluid volume

Rectal fluid Volume pH Buffering ability Surface tension Viscosity Composition Luminal pressure from rectal wall... 

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. 

As discussed earlier, the pH of the rectal fluid can have a marked effect on the absorption of drugs from the rectum. Since the rectal fluid has a relatively low buffering capacity and the volume of the rectal fluid is small, it might be expected that the contents of the rectal dosage form largely control the pH of the rectum during administration. On this basis, one may be able to utilize the pH characteristics of the drug and incorporate suitable buffers and other excipients in the... 

Dissolution problems for rectally administered medications because of lower fluid volume in the rectum than in the stomach, duodenum, etc... 

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... 

From a biopharmaceutical viewpoint, a large sized suppository may be advantageous. In principal, a larger volume spreads over a larger intestinal surface. As a result more active substance is released from the base, more active substance dissolves in the rectal fluid and the absorption is faster. This is especially important for substances that are poorly soluble in both fat and water, such as paracetamol. The release of active substance from the base in this case strongly depends on the extent of the interface between fat and rectal fluid available for active substance release. From a technological point of view a large size suppository must be... 

The pharmacist should have anticipated the bio-pharmaceutical consequences of the physico-chemical properties of oxcarbazepine. The drug is classified as a Class II substance for oral application. Logically, lack of adequate solubility is even more evident for the rectal administration as the volume of rectal fluid is limited (see Table 17.1). With an aqueous solubility of approximately 300 mg/L, the solubUity of the substance in the lipophilic base of the suppositories would certainly not be higher than 9.5 mg/mL (being a direct consequence of the value of the log P = 1.5 of oxcarbamazepine). This means that oxcarbazepine is not dissolved in the lipid but dispersed as crystals, which settle from the molten suppository once introduced in the rectal cavity. The amount of rectal liquid is limited and therefore a saturated solution will exist which involves only less than 1 mg dissolved oxcarbamazepine. Low solubility yields a low concentration and hence a low driving force for diffusion to occur. As a consequence, the rate of absorption is relatively low. This slow release may lead to hardly any uptake, due to defecation within several hours after insertion. 

According to Figure 14, the release of drugs from suppository bases is one of the important factors in the rectal absorption of the drugs, since the rectal fluid is small in volume and viscous compared to gastrointestinal fluid [119]. 

Heat exhaustion is a type of heat stress caused by water and/or electrolyte depletion. A victim of heat exhaustion may have a normal or even lower-than-normal oral temperature but will typically have a higher-than-usual rectal temperature (i.e., 99.5°F to 101.3°F). Heat exhaustion is caused by prolonged exertion in a hot environment and a failure to replace the water and/or electrolytes lost through sweating. Heat exhaustion causes the body to become dehydrated, which decreases the volume of circulating blood. The various body parts compete for a smaller volume of blood, which causes circulatory strain. A victim of heat exhaustion should be moved to a cool place and made to lie down. Replacement fluid containing electrolytes should be taken slowly but steadily by mouth. 

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