3.4
药物吸收：影响胃肠吸收的因素
口服药物吸收的过程可能受到几个因素的影响。弱酸性药物由于其非电离状态而更容易从胃中吸收。然而，在上肠部位，药物往往电离，因此吸收效率可能较低。有趣的是，尽管胃对药物吸收似乎有优势，但其粘液层可能阻碍了扩散。它的表面积也比肠道小，进一步减缓了吸收速度。
相比之下，肠绒毛提供了更大的吸收表面积，再加上高…
口服药物的吸收受到多种因素的影响。
弱酸性药物在胃中更易被吸收，因为它们在胃中主要以非离子化形式存在。
相比之下，离子化药物在上肠的吸收可能较低。
实际上，胃的黏液层使得物质扩散较为困难。与肠道相比，胃的表面积较小，进一步降低了胃内吸收的速率。
肠绒毛提供了更大的表面积，加之血流量较高，从而导致药物吸收增加。
如果食物存在于胃中，药物可能会与食物成分形成复合物，导致吸收不良。
食物还会延缓胃排空，即药物进入小肠的过程，从而减慢肠道对药物的吸收。
腹泻期间肠道内容物的快速移动会进一步减缓药物吸收。
此外，肠道上皮含有P-糖蛋白转运体，常将药物泵回肠腔，从而减少药物在血液中的吸收。
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Q1: Why is the small intestine a better site for drug absorption than the stomach?
The small intestine provides superior drug absorption due to its larger surface area created by intestinal villi, combined with higher blood flow. In contrast, the stomach has a smaller surface area and a thick mucous layer that hinders diffusion. These anatomical and physiological features make the intestine the primary site for drug absorption.
Q2: How does food in the stomach affect oral drug absorption?
Food interferes with drug absorption through two mechanisms: drugs form complexes with food constituents, reducing their bioavailability, and food delays gastric emptying, slowing the movement of drugs into the small intestine. This delayed transit further reduces the rate of intestinal absorption, ultimately decreasing overall drug absorption.
Q3: What role do P-glycoprotein transporters play in drug absorption?
P-glycoprotein transporters in the gut epithelium actively pump drugs back into the gut lumen, reducing their absorption into the bloodstream. This efflux mechanism acts as a barrier to drug absorption, limiting the amount of drug that enters systemic circulation from the gastrointestinal tract.
Q4: Why are weakly acidic drugs absorbed more readily from the stomach?
Weakly acidic drugs are predominantly nonionized in the stomach's acidic environment, allowing them to cross the stomach lining more easily through passive diffusion. In contrast, drugs in the upper intestine are often ionized, making them less able to penetrate the intestinal epithelium, resulting in lower absorption rates.
Q5: How does diarrhea impact drug absorption from the gastrointestinal tract?
Diarrhea accelerates the movement of gut contents through the gastrointestinal tract, reducing the time available for drug absorption. This rapid transit limits drug contact with the intestinal epithelium, resulting in decreased absorption and lower drug bioavailability in the bloodstream overall.
Q6: What is the relationship between surface area and drug absorption in the GI tract?
Surface area directly influences absorption rates: the intestine's larger surface area, enhanced by villi, supports greater drug absorption compared to the stomach's smaller surface. Combined with higher intestinal blood flow, this increased surface area facilitates more efficient drug uptake into systemic circulation.
Q7: How do ionization states of drugs affect their absorption at different GI sites?
Drug ionization state determines absorption efficiency at specific GI locations. Nonionized drugs, like weakly acidic drugs in the acidic stomach, cross membranes readily. Ionized drugs in the upper intestine have reduced membrane permeability, though the intestine's superior surface area and blood flow often compensate for lower ionization-based absorption.