3.25 : 影响溶解的因素：药物 pKa、亲脂性和胃肠道 pH

Drug absorption in the gastrointestinal tract depends mainly on the absorption site's pH, the drug's dissociation constant, and lipophilicity.

The GI tract's pH varies from acidic in the stomach to slightly alkaline in the small intestine. The ionization of drugs is a function of the absorption site's pH and the drug's pK_a.

Very weak acids and bases remain unionized and are usually lipid-soluble at all pH values and are absorbed rapidly independent of gastrointestinal pH.

For acids and bases in specific pK_a ranges, pH changes significantly impact drug dissolution and absorption. Acids are better dissolved and absorbed from the stomach, while bases are better dissolved and absorbed from the intestine.

Stronger acids and bases remain ionized and dissolve poorly throughout the GI tract, leading to poor absorption.

Log P, representing the oil/water partition coefficient, also determines the drug's lipid solubility. Highly lipophilic drugs dissolve in lipid-rich environments, facilitating passage through cell membranes and improving systemic absorption.