Which type tends to be more in plasma than stomach?

Which type tends to be more in plasma than stomach?

• A. Weak bases
• B. Weak acids
• C. Neutral
• D. Strong bases

Answer: (B)

pH-dependent ionization and ion trapping across membranes explains this pattern. The stomach is very acidic (around pH 1–3) while plasma is near neutral (about pH 7.4). For a weak acid, in the acidic stomach the drug is largely nonionized and can cross the gastric mucosa into the bloodstream. Once in plasma, the higher pH makes the drug ionized, and ionized forms cross membranes poorly, so they get trapped in plasma and accumulate there rather than in the stomach. A classic example is a drug like aspirin with a pKa around a few units; it crosses into the blood in the stomach and becomes ionized in plasma, staying there. Weak bases behave oppositely: in the acidic stomach they become ionized and are trapped there, leading to relatively more in the stomach than in plasma. Neutral drugs don’t undergo this ion trapping to the same extent, and strong bases are largely ionized in both environments, reducing cross-membrane movement.

pH-dependent ionization and ion trapping across membranes explains this pattern. The stomach is very acidic (around pH 1–3) while plasma is near neutral (about pH 7.4). For a weak acid, in the acidic stomach the drug is largely nonionized and can cross the gastric mucosa into the bloodstream. Once in plasma, the higher pH makes the drug ionized, and ionized forms cross membranes poorly, so they get trapped in plasma and accumulate there rather than in the stomach. A classic example is a drug like aspirin with a pKa around a few units; it crosses into the blood in the stomach and becomes ionized in plasma, staying there.

Weak bases behave oppositely: in the acidic stomach they become ionized and are trapped there, leading to relatively more in the stomach than in plasma. Neutral drugs don’t undergo this ion trapping to the same extent, and strong bases are largely ionized in both environments, reducing cross-membrane movement.