body volume ‐ 70 L The volume of the body of a 70‐kg human is approximately 70 L. This volume is not all water, but it provides a comparison for the total body water value of 38 L. OPTIONAL‐Please participate in the online discussion forum.
Serum binding Background: Approximately 8 wt% of whole blood is protein dissolved in the fluid fraction. Instructions: Read the passage below concerning the influence of blood proteins upon drugs. Learning Goals: To understand the complicating factors of proteins and how they can affect the behavior of drugs in the body. Drugs are designed to target a specific protein in the body. By binding that particular target, the drug gives rise to a biological effect. Just because a drug has a high affinity for one target does not mean that it cannot bind to other proteins. Proteins in the blood have a large effect on the drugs that bind them. Because drugs are transported by the blood, the blood proteins cannot be avoided by drugs. Some of the blood proteins have a very high concentration. Even if a drug has only a low affinity for a protein, if the protein is present in a high enough concentration, the protein will bind a large fraction of the drug. The equilibrium below demonstrates this idea. By Le Chatlier's principle, a higher concentration of protein forces the reaction to shift to the left and form more protein‐drug complex in order to reach equilibrium.
The equilibrium expression below reinforces the concept. A high protein concentration increases the size of the numerator. The value of KD is restored by shifting the equilibrium to the left. This shift raises the concentration of the protein‐drug complex (the denominator) while simultaneously decreasing both the unbound protein and drug concentrations (the numerator).
The interaction of a drug with proteins in the blood affects both how a drug is cleared from the bloodstream as well as a drug's distribution. A drug's clearance and distribution together determine the half‐life of a drug. Both concepts will be formally introduced in Chapter 7 ‐ Pharmacokinetics.