3.1
药代动力学: 概述
药代动力学是一门科学学科，专注于药物在体内的行程，包括四个关键阶段: 吸收、分布、代谢和排泄。第一个阶段，吸收，涉及药物进入血液循环。有几个因素决定了这个过程的范围和速度。例如，肝脏经常在口服药物到达全身循环之前代谢，导致部分吸收。相比之下，静脉内注射(IV)绕过了这种初始代谢，使药物直接进入血液，…
给药后，机体立即开始吸收、分布、代谢和消除的过程。对这些过程的研究称为药代动力学。
吸收是指药物向血液的转移过程。多种因素会影响吸收的速度和程度。
许多口服药物在进入体循环之前通常会经过肝脏代谢，因此这些药物仅能被部分吸收。
相反，通过静脉注射给药的药物可直接进入血液，几乎被完全吸收。
吸收的药物随后被分布到组织中。
消除是指以未改变或代谢后的形式将药物从体内清除。
肾脏排泄仅限于亲水性物质，因为疏水性物质 tends to get reabsorbed。因此，在排泄前将脂溶性药物代谢为水溶性物质是必要的。
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Q1: What are the four main stages of pharmacokinetics?
Pharmacokinetics encompasses four key stages: absorption, distribution, metabolism, and elimination. Absorption involves the drug's transfer into the bloodstream. Distribution spreads the drug to different tissues. Metabolism alters the drug's structure for elimination. Finally, elimination removes the drug from the body in its original or metabolized form.
Q2: Why does oral drug administration result in partial absorption?
Orally administered drugs are often metabolized by the liver before entering systemic circulation, a process called first-pass metabolism. This hepatic metabolism reduces the amount of unchanged drug reaching the bloodstream, resulting in only partial absorption compared to intravenous administration. Understanding factors influencing bioavailability first pass elimination helps explain this phenomenon.
Q3: How does intravenous drug administration differ from oral administration in terms of absorption?
Intravenous administration bypasses hepatic metabolism and delivers the drug directly into the bloodstream, allowing nearly complete absorption. In contrast, oral drugs undergo first-pass metabolism in the liver, resulting in only partial absorption of the administered dose. This route difference significantly impacts drug bioavailability and therapeutic effectiveness.
Q4: Why must lipophilic drugs be metabolized before elimination?
Lipophilic (lipid-soluble) substances tend to be reabsorbed in the kidneys rather than eliminated. Metabolism converts these lipid-soluble drugs into water-soluble substances, preventing reabsorption and enabling renal elimination. This transformation is essential for effective drug clearance from the body and preventing drug accumulation.
Q5: What determines whether a drug undergoes renal elimination?
Renal elimination is primarily limited to hydrophilic (water-soluble) substances. Lipophilic compounds tend to be reabsorbed in the renal tubules, preventing their elimination. Therefore, drugs must be water-soluble or metabolized into water-soluble forms to be effectively eliminated through the kidneys and removed from the body.
Q6: What factors influence the rate and extent of drug absorption?
Multiple factors affect drug absorption, including the route of administration and hepatic metabolism. Oral drugs undergo first-pass hepatic metabolism, reducing absorption, while intravenous drugs bypass this process for nearly complete absorption. Drug properties, gastrointestinal conditions, and membrane transport mechanisms also significantly influence absorption rates and extent.
Q7: How does drug metabolism relate to the distribution and elimination phases?
Following absorption, the drug is distributed to various tissues throughout the body during the distribution phase. Metabolism then alters the drug's chemical structure, making it more suitable for elimination. This sequential process ensures drugs reach target tissues and are subsequently transformed into water-soluble forms the body can effectively excrete through renal or non-renal routes.