Volume of distribution: why Vd can be bigger than the body
Volume of distribution is the amount of drug in the body divided by the plasma concentration. It is an apparent volume, so it can be far larger than the body itself.
Chapters (12)
Calculate volume of distribution (Vd = dose / plasma concentration) and predict how plasma protein binding and lipophilicity (tissue binding) raise or lower it, using 3 L, 14 L and 42 L as reference compartments.
The short version
- Volume of distribution is the amount of drug in the body divided by the plasma concentration; after an IV dose, dose / concentration extrapolated to time zero.
- It is an apparent volume, not a real one, and it can far exceed total body volume.
- Protein binding in plasma lowers Vd; tissue binding and lipid solubility raise it.
- Compare Vd with about 3 L (plasma), 14 L (extracellular fluid) and 42 L (total body water) to see where a drug sits.
- Loading dose = target concentration × Vd, so a huge Vd needs a huge dose.
The puzzle: 350 mg and a 700 L volume of distribution
A 70 kg man is given 350 mg of a drug into a vein. Later, his plasma concentration is 0.5 mg/L. Work that back and the drug seems to be dissolved in 700 L. His whole body holds only about 42 L of water. How?[1][3][5]
The answer is that volume of distribution (Vd) is not a real volume. It is a ratio, and it tells you how widely a drug has spread out of the plasma.
Reference compartments: 3 L, 14 L and 42 L
Think of the body as a set of nested compartments. In a 70 kg adult these are rounded teaching values.[5][6]
| Compartment | Volume | What it includes |
|---|---|---|
| Plasma | ≈ 3 L | The blood fluid we actually sample |
| Extracellular fluid | ≈ 14 L | Includes the plasma |
| Total body water | ≈ 42 L (about 0.6 L/kg) | Includes the cells too |
In practice we sample the plasma, so the standard Vd calculation starts from a plasma concentration.[3]
Vd = dose / concentration: the volume of distribution formula
Volume of distribution is the amount of drug in the body divided by the plasma concentration. After an intravenous dose, that is the dose divided by the concentration extrapolated back to time zero.[1][3]
C₀ = plasma concentration extrapolated to time zero after an IV dose; Vd is in litres, or litres per kilogram
- 350 mg / 0.5 mg/L = 700 L
- 700 L is far above total body water (≈ 42 L)
- Most of the drug has left the plasma and gone into tissue, so the plasma looks almost empty
Vd is an apparent volume, not a real one, and it can be far larger than the body.[2][3] The more drug that leaves the plasma, the lower the plasma concentration and the larger the Vd.[2]
Protein binding and tissue binding: two drugs, one dose
Compare two drugs, each given as a 200 mg dose.
| Drug | Where it goes | Plasma concentration | Vd |
|---|---|---|---|
| Drug X | Tightly bound to plasma proteins, stays in the blood | 25 mg/L | 200 / 25 = 8 L |
| Drug Y | Lipid soluble, slips into fat and tissue | 0.4 mg/L | 200 / 0.4 = 500 L |
Warfarin is the classic highly protein-bound example, with a Vd of only 8–10 L.[2][8] Chloroquine goes further than Drug Y, into the thousands of litres.[3][8]
Only unbound drug can diffuse out of the plasma into tissue, so bound drug mostly stays behind in the blood.[2] The rule: protein binding in plasma lowers Vd; tissue binding and lipid solubility raise it.[2]
Water-soluble drugs and patient factors that change Vd
Water-soluble drugs cross cell membranes poorly, so they tend to stay in extracellular fluid and have small volumes.[3]
The patient matters too:
Reading Vd as a ladder, and why it sets the loading dose
- About 3 L means the drug is confined to plasma.
- About 14 L means extracellular fluid.
- About 42 L means all body water.
Above 42 L, water alone cannot explain it. The drug must be held in tissue.[2][3]
For non-IV routes this is also divided by bioavailability; the full calculation comes in a later lesson
A huge volume of distribution means a huge dose to fill it.[4]
Practice question
Original · USMLE styleA 28-year-old man weighing 70 kg receives a single intravenous bolus of 240 mg of a drug. After distribution is complete, the plasma concentration extrapolated to the time of injection is 16 mg/L.
Common questions
What is volume of distribution?
It is the amount of drug in the body divided by the plasma concentration. After an intravenous dose, it is the dose divided by the plasma concentration extrapolated back to time zero.
Why can volume of distribution be larger than body volume?
Vd is an apparent volume, not a real one. When a drug is pulled out of the plasma into tissue, the plasma concentration is low, so the calculated volume becomes very large.
How does protein binding affect volume of distribution?
Protein binding in plasma lowers Vd. Only unbound drug can diffuse into tissue, so bound drug mostly stays in the blood and the plasma concentration stays high. Warfarin, at 8–10 L, is the classic example.
How do lipid solubility and tissue binding affect Vd?
They raise it. A lipid-soluble drug slips into fat and tissue, so little stays in the plasma. Chloroquine goes into the thousands of litres.
What units does volume of distribution use?
Litres for a given patient, or litres per kilogram.
Transcript
Read the full narration, by chapter680 words
0:00The puzzle: 350 mg and a 700 L "volume"
A 70-kilogram man is given 350 milligrams of a drug into a vein. Later, his plasma concentration is half a milligram per litre. Work that back and the drug seems to be dissolved in 700 litres. His whole body holds about 42 litres of water. How? Today: calculate the volume of distribution, and predict how protein binding and fat solubility change it. Think of the body as a set of compartments.
0:27Reference compartments: 3 L, 14 L, 42 L
In a 70-kilogram adult, plasma holds about three litres. Extracellular fluid, which includes the plasma, holds about 14. Total body water, which includes the cells too, is about 42.
0:44Vd = amount / plasma concentration, units
But we only ever sample the plasma. So the volume of distribution is the amount of drug in the body, divided by the plasma concentration. After an intravenous dose, that is the dose divided by the concentration extrapolated back to time zero. It is measured in litres, or in litres per kilogram. It is an apparent volume,
1:06An apparent volume, not a real one
not a real one, and it can be far larger than the body. Back to our man. 350 milligrams divided by half a milligram per litre is 700 litres. The drug is not sitting in 700 litres of anything. Most of it has left the plasma and gone into tissue, so the plasma looks almost empty. The more drug that leaves the plasma, the lower the plasma concentration, and the larger the volume of distribution.
1:36Drug X: stays in the blood (8 L)
Compare two drugs, each given as a 200 milligram dose. Drug X is tightly bound to plasma proteins, so it stays in the blood. The concentration is 25 milligrams per litre, so 200 over 25 gives eight litres. Warfarin is the classic example, with a volume of only eight to 10 litres. Drug Y is lipid soluble and slips into fat
2:00Drug Y: pulled into tissue (500 L)
and tissue. The concentration is only 0.4 milligrams per litre, so 200 over 0.4 gives 500 litres. Chloroquine goes further still, into the thousands of litres.
2:17Protein binding and water-soluble drugs
Protein binding in plasma lowers the volume. Tissue binding and lipid solubility raise it. Only unbound drug can leave the plasma, so bound drug stays behind in the blood. Water-soluble drugs cross cell membranes poorly, so they tend to stay in extracellular fluid and have small volumes.
2:37Patient factors: edema, ascites, age
The patient matters too. Edema, ascites and effusions add water, which enlarges the volume of water-soluble drugs. Older people have less body water, which shrinks the volume of water-soluble drugs.
2:52The ladder and the loading dose
Use the numbers as a ladder. About three litres means confined to plasma. About 14 means extracellular fluid. About 42 means all body water. Above 42 litres, water alone cannot explain it. The drug must be held in tissue. This matters because the loading dose equals the target concentration times the volume of distribution. A huge volume means a huge dose to fill it.
3:22Practice question
Your turn. A 28-year-old man weighing 70 kilograms receives a single intravenous bolus of 240 milligrams of a drug. After distribution is complete, the plasma concentration extrapolated to the time of injection is 16 milligrams per litre. Which of the following is closest to the volume of distribution, and what does it suggest? A, 0.07 litres, smaller than the plasma volume. B, three litres, confined to plasma. C, 15 litres, mainly extracellular fluid. D, 42 litres, distributed through total body water. E, 3840 litres, sequestered in tissue. Pause here and decide.
4:14Answer and explanation
A is out: that is the concentration divided by the dose, the formula upside down. B is out: a three-litre volume would mean a plasma concentration of 80 milligrams per litre. D is out: 42 litres would give a concentration of only about 5.7. E is out: that is the dose multiplied by the concentration, not divided. So the answer is C. 240 divided by 16 is 15 litres, close to extracellular fluid.
4:48Recap and next lesson
Three things to keep. One: volume of distribution is dose divided by plasma concentration, an apparent volume that can exceed the body. Two: protein binding in plasma shrinks it, while tissue binding and fat solubility enlarge it. Three: compare it with three, 14 and 42 litres to see where the drug sits. Next, clearance and half-life.
Sources
Numbers in the text, e.g. [1], point here- MSD Manual Professional Edition. Overview of Pharmacokinetics. msdmanuals.com/professional/clinical-pharmacology/pharmacokinetics/overview-of-pharmacokinetics
- MSD Manual Professional Edition. Drug Distribution to Tissues. msdmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-distribution-to-tissues
- Deranged Physiology. Volume of distribution. derangedphysiology.com/main/cicm-primary-exam/pharmacokinetics/Chapter-202/volume-distribution
- Deranged Physiology. Maintenance dose and loading dose. derangedphysiology.com/main/cicm-primary-exam/pharmacokinetics/Chapter-401/maintenance-dose-and-loading-dose
- OpenStax. Anatomy and Physiology 2e, 26.1 Body Fluids and Fluid Compartments. openstax.org/books/anatomy-and-physiology-2e/pages/26-1-body-fluids-and-fluid-compartments
- LITFL. Ethanol toxicity (total body water and Vd). litfl.com/ethanol-toxicity/
- Wikipedia (secondary). Volume of distribution; Loading dose. en.wikipedia.org/wiki/Volume_of_distribution · en.wikipedia.org/wiki/Loading_dose
- Clinical Significance of Volume of Distribution in Pharmacotherapy. Europe PMC PMID 31424864. europepmc.org/article/MED/31424864
Compartment sizes of 3 L, 14 L and 42 L are for a 70 kg adult and are rounded teaching values. Drugs X and Y, the 350 mg hook patient and all dose and concentration values are invented teaching numbers.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (cropped, recoloured, outlined): male silhouette, syringe, fat tissue, elderly person. "protein-1" icon by Servier via Bioicons (bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/), adapted (recoloured, outlined). Blood sample icon by Marcel Tisch via Bioicons (bioicons.com), CC0. All diagrams, compartment shading, bars, graphs and handwriting are drawn in code (original).
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-PHARM-003.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Potency vs efficacy: EC50, Emax and agonist types5:17
- EP 02Competitive vs noncompetitive antagonists: effect on EC50 and Emax5:01
- EP 03Therapeutic index explained: TD50 / ED505:01
- EP 04First-pass metabolism and bioavailability: calculating oral F from AUC5:18
- EP 05Volume of distribution: why Vd can be bigger than the body5:17
- EP 06Clearance and half-life: t½ = 0.693 × Vd / CL5:01
- EP 07Zero-order vs first-order elimination explained5:12
- EP 08Loading dose and maintenance dose, explained5:02
- EP 09CYP450 inducers and inhibitors: how they change drug levels5:16
- EP 10Autonomic receptors: alpha, beta and muscarinic5:33
- EP 11Beta blocker selectivity: beta-1, ISA and alpha blockade5:24
- EP 12Vasopressors compared: receptors, pressure and heart rate5:46
- EP 13Organophosphate poisoning: muscarinic vs nicotinic signs5:36
- EP 14Receptor desensitization, downregulation and upregulation5:01
- EP 15Beta-lactams and vancomycin: enzyme versus substrate5:04
- EP 16Protein synthesis inhibitors: 30S vs 50S antibiotics5:07