Pharmacology
Summary
The volume of distribution (Vd) represents the hypothetical volume a drug would distribute into if its concentration throughout that volume was the same as the plasma concentration. Not a true volume, but a ratio expressed in units of volume such as liters, it gives insight into the distribution characteristics of a drug in the body. In calculating Vd, the total amount of drug absorbed into the body is divided by the plasma concentration of the drug. A high volume of distribution signifies that a drug is prevalent beyond the plasma, mainly in the tissues. Such drugs often have a Vd greater than 40 liters, indication of existing extensively in tissues, such as muscle tissue or fat stores. On the contrary, low volume of distribution shows that a drug largely exists within the plasma, having little to no distribution into the tissues. Importantly, the volume of distribution can be influenced by conditions that alter body volumes, or tissue or protein binding, as well as changes in plasma protein concentrations and capillary permeability.
Lesson Outline
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FAQs
The apparent volume of distribution (Vd) refers to a hypothetical volume that a drug would distribute into if the drug's concentration throughout that volume was the same as the plasma concentration. Note that Vd is not a physiological volume but rather a ratio expressed in units of volume, specifically liters (L).
The volume of distribution has a direct effect on the size of the dosage required. When drugs have a high Vd, they will distribute into tissues such as fat and muscle. As a result, larger doses are often needed to reach the desired plasma concentration. By contrast, drugs with a low Vd tend to remain primarily in the plasma and therefore require smaller doses to reach the desired plasma concentration.
Typically, drugs with a high molecular weight have a lower Vd. For example, the Vd of heparin, which has a large molecular weight, is around 3L. On the other hand, drugs with a low molecular weight tend to have a higher Vd.
Drugs often bind to plasma proteins in the bloodstream. When there's a change in the concentration of these binding proteins, it can shift the balance between the bound and unbound fractions of the drug. This unbound fraction is pharmacologically active and can distribute throughout the body. Therefore, variations in plasma protein levels can directly influence the volume of distribution, determining how widely and effectively a drug spreads within the body.
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