21 Questions
What percentage of the total body weight represents the plasma volume in a person weighing 70 kg?
6%
What is the total body water volume in a person weighing 70 kg?
42 L
What is the formula to calculate the Apparent Volume of Distribution (Vd)?
Vd = Dose/Conc.
If a drug has a dose of 100 mg and a concentration of 20 mg/L, in which volume would it be distributed?
4.2 L
What is the term for the fluid that is in between cells?
Interstitial fluid
What is the percentage of the total body weight that represents the extracellular fluid?
20%
What is the primary factor that explains the short duration of propofol's anesthetic effect after intravenous injection?
High blood flow to the brain
Which of the following drugs is most likely to penetrate the CNS?
Lipid-soluble anaesthetic
What is the effect of binding of a drug to plasma proteins on its distribution?
Decreases the volume of distribution
Which of the following is an example of a drug that is concentrated in a specific tissue due to its high affinity to a cellular constituent?
Tetracycline in bone and teeth
Which of the following plasma proteins binds acidic drugs?
Albumin
What is the primary factor that determines the ability of a drug to diffuse across cell membranes?
Lipophilicity of the drug
What is the primary characteristic of a drug that is bound to plasma protein?
It is not active and cannot pass blood vessels
A drug with high molecular weight, such as heparin, would have a Vd of approximately:
4.2 L
What is the primary importance of Vd calculation in pharmacology?
It estimates the extent of tissue uptake of drugs
What is the effect of a drug's binding to cell and tissue constituents on its volume of distribution?
Increases the volume of distribution
What is the primary factor that determines the concentration of a drug in the CNS after its initial distribution?
Diffusion of the drug out of the CNS
A drug with a small Vd, such as frusemide, would indicate:
Limited tissue uptake
In cases of drug toxicity, dialysis is most useful for drugs with a:
Low Vd
What is the formula to calculate the loading dose (LD) of a drug?
LD = Vd x Css
What is the minimum Vd for any drug?
Not specified
Study Notes
Body Water Compartments
- Plasma volume: 6% of total body weight (4.2 L in a 70 kg person)
- Interstitial fluid: 20% of total body weight (14 L in a 70 kg person)
- Total body water: 60% of total body weight (42 L in a 70 kg person)
Apparent Volume of Distribution (Vd)
- Vd: the volume that would accommodate the entire amount of drug in the body if its concentration throughout the body was the same as that in plasma
- Vd = Amount of drug in the body / Plasma concentration
- Vd = Dose / Concentration
Factors Affecting Drug Distribution
- Perfusion: blood flow to the organs (e.g., brain, liver, kidney > skeletal muscles > adipose tissue)
- Diffusion: ability of the drug to diffuse across cell membranes, governed by lipophilicity
- Capillary Permeability: affects drug distribution
- Binding of drug to plasma proteins: decreases its diffusion to tissues, decreasing Vd
- Binding of drug to cell and tissue constituents: increases Vd due to high affinity
Binding to Plasma Proteins
- Albumin: binds acidic drugs
- Lipoprotein and α 1 acid glycoprotein: binds basic drugs
- Globulin: binds thyroxin and sex hormones (steroid)
- Drug bound to plasma protein: not active, cannot pass blood vessels, and not highly distributed
Importance of Vd Calculation
- Estimates the extent of tissue uptake of drugs
- Small Vd: limited tissue uptake, e.g., frusemide
- Large Vd: extensive tissue uptake, e.g., digoxin
- Minimum Vd for any drug: 4.2 L (plasma volume)
- Maximum Vd for any drug: total body water (42 L in a 70 kg person)
Clinical Applications
- In cases of drug toxicity: dialysis is not useful for high Vd drugs, but useful for low Vd drugs
- Vd can be used to calculate the loading dose (LD): LD = Vd x Steady state plasma concentration (Css)
This quiz covers the importance of calculating volume of distribution, properties of drugs highly bound to plasma protein, and predicting the risk of displacement interaction. It is part of the BMS161 course, taught by Professor Ahmed Nour Eldin Hassan and others.
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