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The metabolism of aspirin varies somewhat in individuals due to one or more factors that may delay absorption or reduce effectiveness. Aspirin is made of acetylsalicylic acid (ASA) and a small amount of esters and ethers, which are secondary metabolites assisting in absorption by the body's tissues and organs. The effectiveness of aspirin varies according to dosage and according to the general health and medical history of a patient. For instance, high cholesterol in elderly patients somewhat restricts aspirin's effectiveness.
For aspirin to be most effective, the contents of the stomach should be between 2.15 and 4.10 pH acidity. After taking aspirin orally, it begins to dissolve as it enters the stomach's liquids. A half-life cycle for the metabolism of aspirin starts in the stomach approximately 10 minutes after ingestion. By 25 minutes, aspirin blood levels in the bloodstream will drop to 50% of the level it attains as metabolism begins. In this 15 minutes, the aspirin will pass through to the small intestine for accelerated blood absorption and distribution throughout the body's tissues and organs to begin its work.
Towards the end of its time in the stomach, the ASA transforms into salicylic acid due to interaction between the gastric mucosa and an enzyme called aspirin esterase (AE). In some studies, it has been found that AE esterase activity is effected by higher than normal glucose and fatty acids levels in the bloodstream during aspirin's metabolism. The metabolism of aspirin in the small intestine accelerates to distribute it to organs and tissues throughout the body; however, the liver seems to be the primary focus for further metabolizing. Up to 80% of the metabolism of aspirin takes place in the liver, where it undergoes rapid chemical changes in combination with the liver's acids and hepatocytes. The aspirin starts to interfere with prostaglandins throughout the body to reduce pain sensitivities, and to reduce the stickiness of blood platelets that could lead to clots.
Once the metabolism of aspirin is complete, the distribution of the salicylic acid moves beyond blood plasma to fluids throughout the body. Some travels to mix with synovial fluids, which cushion cartilage and bones at joints. At the joints, the aspirin targets prostaglandins to reduce inflammations and pain. It also can be found in the saliva and spinal fluids, as well as the major organs of kidneys, lungs, and the heart. As aspirin readily crosses placental barriers and can be found in breast milk, pregnant and breast-feeding women should ask a health professional before taking aspirin.
Aspirin takes up to 48 hours to be completely excreted from the body via the kidneys. The rate of excretion varies widely due to varying pH levels throughout the body. Health conditions such as diabetes, gastric ulcers, and high cholesterol levels all interfere with or delay aspirin's effectiveness in people of all ages. The elderly, particularly the frail and those who have had prior cardiac events, will also see wide variability in metabolic rates, effectiveness, and excretion rates.