Which nephron process is active, saturable, and can be competitively inhibited by another drug using the same transporter?
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Clinical Pharmacology · Pharmacokinetics
Excretion
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Excretion is how the body physically removes drugs and their metabolites, mostly through the kidneys and, to a lesser extent, the liver/bile into feces. It's the final act of pharmacokinetics, and it determines how long a drug's effects last and how often a dose must be repeated. Clearance is the number that quantifies this process, and when the kidneys or liver aren't working well, clearance drops, drugs accumulate, and doses must be adjusted to avoid toxicity.
The college version
Excretion is the irreversible removal of a drug (unchanged) or its metabolites from the body. Metabolism chemically transforms a drug; excretion physically removes it. The two work together — metabolism typically makes drugs more water-soluble, easing excretion.
Renal Excretion
The kidneys are the primary excretory organ for most drugs; renal excretion is the net result of three processes along the nephron.
Glomerular filtration: Blood arrives at the glomerulus under pressure, and small, unbound (free, not protein-bound) drug molecules are filtered into the tubule. Protein-bound drug and large molecules stay in the blood, filtered poorly until they dissociate.
Tubular secretion: Active transporters in the proximal tubule pump certain drugs from blood directly into tubular fluid, even against a concentration gradient. This active, energy-requiring process is saturable — transporters can be overwhelmed, and two drugs sharing one can compete, raising blood levels of either (a classic drug-interaction mechanism).
Tubular reabsorption: As filtered fluid moves through the tubule, water is reabsorbed, concentrating the remaining drug. Lipid-soluble, non-ionized molecules diffuse back into blood, escaping excretion; ionized, polar molecules stay trapped in urine and are excreted.
Urine pH matters because it shifts how much drug exists in ionized versus non-ionized form. Weak acids ionize more (and are trapped/excreted faster) in alkaline urine; weak bases ionize more in acidic urine. Clinically, alkalinizing urine (e.g., with sodium bicarbonate) speeds elimination of certain weak-acid overdoses by trapping the drug and preventing reabsorption.
Biliary and Fecal Excretion
The liver excretes drugs and metabolites into bile, which drains into the intestine and leaves as feces. This route matters especially for larger, more polar metabolites, particularly glucuronide conjugates from Phase II metabolism.
A key phenomenon is enterohepatic recirculation: gut bacteria can cleave the conjugate (via beta-glucuronidase), regenerating active parent drug, which is reabsorbed into the bloodstream instead of being excreted in stool. This loop can significantly prolong a drug's duration of action and half-life, since the drug must circulate through the gut repeatedly before elimination completes.
Minor Excretory Routes
Small amounts of some drugs leave via the lungs (volatile anesthetics, alcohol — why breathalyzers work), sweat, and saliva. Excretion into breast milk is clinically important despite small quantities, since nursing infants can be exposed to maternal drugs, a factor weighed when prescribing to breastfeeding patients.
Clearance and Renal Impairment
Clearance is the theoretical volume of blood (or plasma) fully cleared of drug per unit time, expressed in mL/min or L/hr. It best describes overall elimination efficiency; total body clearance sums clearance from all organs (renal + hepatic + other routes).
Clearance matters clinically because it determines the maintenance dose needed for a steady concentration: high clearance means faster removal and more frequent/larger doses, while low clearance means the drug lingers and doses must be smaller or spaced further apart.
When kidney function declines (age, chronic kidney disease, acute injury), glomerular filtration rate falls and renally-cleared drugs are excreted less efficiently. Without dose adjustment, the drug accumulates each dose, raising toxicity risk — so clinicians estimate renal function (creatinine clearance, eGFR) before dosing renally-eliminated drugs and often reduce dose or extend the interval in impaired renal function.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of your body like a house that needs to take out the trash. Your kidneys are the main trash chute — they filter your blood constantly, and most drug "garbage" goes out through your pee. Some garbage also goes out a side door: through your liver into bile, then down into your poop.
But here's a weird trick some trash can pull: right when it's about to go out in your poop, tiny gut bacteria "unlock" it, and it sneaks back into your bloodstream instead of leaving the house. That's like putting a trash bag on the curb and someone opening it up and bringing half of it back inside — it takes longer to actually get rid of.
And if your trash chute (kidneys) gets clogged or weak — like from being sick or getting older — the trash doesn't leave fast enough, and it starts piling up in the house. That's why doctors give smaller doses, or space them further apart, to people whose kidneys aren't working well: so the "trash" doesn't build up to dangerous levels.
Check yourself
2 review questions from the chapter. Try each one, then open the answer.
A patient on a long-term medication develops acute kidney injury, but the prescriber does not adjust the dose. What is likely to happen to the drug's blood levels over the next several doses, and why?
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Levels will rise (accumulate) with each dose.
If the kidneys are hurt, they can't filter and remove the drug as fast as before, so each new dose adds on top of drug that hasn't left yet, like adding water to a bathtub whose drain is clogged — the level keeps climbing toward toxic.
A drug undergoes extensive Phase II glucuronidation and biliary excretion, but its measured half-life is much longer than expected from its metabolism rate alone. What elimination phenomenon could explain this, and what is the underlying mechanism?
Show answer
Enterohepatic recirculation.
The glucuronide metabolite reaches the gut to be pooped out, but gut bacteria break the conjugate apart and free the original drug, which gets reabsorbed back into the blood instead of leaving the body — so it takes several extra loops before it's truly gone, stretching out the half-life.
Quick check
3 questions here. Answers stay hidden until you check.
A patient has taken an overdose of a weak acid drug. Which intervention would most speed up its renal elimination?
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What best describes clearance?
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