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Clinical Pharmacology · Pharmacokinetics
Distribution
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Distribution is what happens after a drug enters the bloodstream but before it reaches its site of action: movement of drug molecules out of the blood and into body tissues. How much reaches a tissue, and how fast, depends on blood flow, capillary permeability, lipid solubility, and plasma protein binding. Only the unbound "free" fraction of a drug can leave the blood and act on receptors, so protein binding quietly controls potency, duration, and interaction risk. Barriers like the blood-brain barrier and placenta further decide which drugs reach the brain or a fetus, making distribution a key gatekeeper of both effect and safety.
The college version
Blood Flow and Capillary Permeability
A drug distributes fastest to highly perfused organs — heart, liver, kidney, brain — because these tissues receive a disproportionate share of cardiac output relative to their mass. Skeletal muscle, skin, and fat fill more slowly, simply because less blood passes through them per minute. Capillary structure adds a second filter: most capillaries have gaps between endothelial cells that let small, water-soluble drugs pass into interstitial fluid; liver sinusoids are especially leaky, while brain capillaries are tightly sealed. Distribution is therefore a sequence, not a single event, with well-perfused organs equilibrating first and poorly perfused tissue filling in over hours.
Lipid Solubility
Once a drug reaches a capillary bed, crossing into cells depends heavily on lipid solubility, since cell membranes are phospholipid bilayers. Lipophilic (fat-soluble) drugs cross membranes readily and distribute widely, including into fat and the central nervous system. Hydrophilic (water-soluble) drugs tend to stay largely within blood and extracellular fluid, since they cannot easily cross lipid membranes without a specific transporter. This is why a lipophilic sedative can act quickly on the brain, while a polar antibiotic stays confined mostly to the vascular and interstitial space.
Plasma Protein Binding
Many drugs travel partly bound to plasma proteins, chiefly albumin (binds many acidic drugs) and alpha-1-acid glycoprotein (binds many basic drugs). Total plasma concentration splits into a bound fraction — pharmacologically inert and unable to cross capillary walls — and a free fraction, the only part able to diffuse into tissue, exert an effect, and be metabolized or excreted. Two drugs with identical total blood levels can act very differently if one is 95 percent bound and the other 50 percent bound, since only unbound drug is active. This underlies a classic interaction: when two highly protein-bound drugs are given together, one can displace the other, transiently raising its free fraction and increasing effect or toxicity. Conditions that lower albumin, such as liver disease or malnutrition, can similarly raise free drug levels and toxicity risk even at standard doses.
Volume of Distribution
Volume of distribution (Vd) relates the total amount of drug in the body to the concentration measured in plasma. It is not a real anatomical space but a useful calculated value: a drug that stays mostly in the bloodstream has a low Vd, close to plasma volume, while a drug that leaves the blood extensively to accumulate in tissue has a high Vd, sometimes exceeding total body water. Highly lipophilic or tissue-bound drugs can have a very large Vd because most of the drug resides outside plasma. Vd guides dosing decisions, including calculating a loading dose needed to reach a target plasma concentration quickly.
Tissue Reservoirs and Barriers
Some tissues act as reservoirs: fat can store lipophilic drugs, and bone can sequester certain agents, prolonging their presence long after dosing stops. Distribution is also limited by selective barriers. The blood-brain barrier's tightly joined capillary endothelial cells block most water-soluble or highly ionized molecules, admitting mainly small, lipophilic drugs unless active transport or inflammation raises permeability. The placental barrier similarly restricts many drugs from reaching the fetus but is not absolute — small, lipophilic, or actively transported substances can cross, so drug safety in pregnancy must be judged individually rather than assumed.
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The same idea, in plain words
Explain it like I’m 10
Imagine the bloodstream is a delivery truck driving around a city of organs. Busy downtown neighborhoods (heart, brain, kidneys) get deliveries fast because the truck passes constantly. Quiet suburbs (fat, muscle) get deliveries slower. Some packages on the truck are handcuffed to a big guard (a protein) who won't let them off at all — only unguarded packages can hop off and do their job. Two neighborhoods also have locked gates that only let tiny, sneaky packages through: the brain's gate (the blood-brain barrier) is super strict, and a pregnant mom's placenta has a gate too, though it isn't perfectly sealed.
Check yourself
2 review questions from the chapter. Try each one, then open the answer.
A patient with severe liver disease has low albumin levels and is started on a highly protein-bound medication at a standard dose. What distribution-related risk should the nurse anticipate, and why?
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Low albumin means fewer "guards" available to hold the drug, so more stays free and active, raising the risk of exaggerated effect or toxicity even at a normal dose.
Fewer handcuffs means more packages hop off the truck and act at once, which can overwhelm the neighborhood (the body) with too much active drug.
Two highly protein-bound drugs are administered together, and one displaces the other from albumin binding sites. What immediate effect would this have on the displaced drug's free concentration and activity?
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The displaced drug's free concentration rises immediately, temporarily increasing its effect or toxicity until a new equilibrium is reached.
One package's guard gets pulled away by a bigger, pushier package, so suddenly more of the first package is loose and running around doing its job — for a while, until things settle down.
Quick check
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A drug with a very high volume of distribution (Vd) most likely:
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Why does the brain typically receive lipophilic drugs more readily than hydrophilic ones?
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