3.4
Absorción de medicamentos: Factores que afectan la absorción gastrointestinal
El proceso de absorción de medicamentos por vía oral puede ser influenciado por varios factores. Los medicamentos débilmente ácidos tienden a ser abso…
La absorción de un fármaco administrado por vía oral está influenciada por varios factores.
Es más probable que los fármacos débilmente ácidos se absorban en el estómago, donde son predominantemente no ionizados.
En comparación, la absorción del intestino superior, que contiene fármacos ionizados, puede ser baja.
En realidad, la capa mucosa del estómago dificulta la difusión. Su superficie más pequeña en comparación con el intestino reduce aún más la tasa de absorción del estómago.
Las vellosidades intestinales proporcionan una mayor superficie, lo que, junto con un mayor flujo sanguíneo, conduce a una mayor absorción del fármaco.
Si los alimentos están en el estómago, los medicamentos pueden absorberse mal, ya que forman complejos con los componentes de los alimentos.
La alimentación también retrasa el vaciado gástrico, es decir, el movimiento de los fármacos hacia el intestino delgado, lo que ralentiza la absorción intestinal.
El movimiento rápido del contenido intestinal durante la diarrea ralentiza aún más la absorción del fármaco.
Además, el epitelio intestinal compuesto por los transportadores de glicoproteína P a menudo bombea los medicamentos de regreso a la luz intestinal. Esto reduce la absorción del fármaco en la sangre.
View the full transcript and gain access to JoVE Core videos
Q1: Why is the small intestine a better site for drug absorption than the stomach?
The small intestine provides superior drug absorption due to its larger surface area created by intestinal villi, combined with higher blood flow. In contrast, the stomach has a smaller surface area and a thick mucous layer that hinders diffusion. These anatomical and physiological features make the intestine the primary site for drug absorption.
Q2: How does food in the stomach affect oral drug absorption?
Food interferes with drug absorption through two mechanisms: drugs form complexes with food constituents, reducing their bioavailability, and food delays gastric emptying, slowing the movement of drugs into the small intestine. This delayed transit further reduces the rate of intestinal absorption, ultimately decreasing overall drug absorption.
Q3: What role do P-glycoprotein transporters play in drug absorption?
P-glycoprotein transporters in the gut epithelium actively pump drugs back into the gut lumen, reducing their absorption into the bloodstream. This efflux mechanism acts as a barrier to drug absorption, limiting the amount of drug that enters systemic circulation from the gastrointestinal tract.
Q4: Why are weakly acidic drugs absorbed more readily from the stomach?
Weakly acidic drugs are predominantly nonionized in the stomach's acidic environment, allowing them to cross the stomach lining more easily through passive diffusion. In contrast, drugs in the upper intestine are often ionized, making them less able to penetrate the intestinal epithelium, resulting in lower absorption rates.
Q5: How does diarrhea impact drug absorption from the gastrointestinal tract?
Diarrhea accelerates the movement of gut contents through the gastrointestinal tract, reducing the time available for drug absorption. This rapid transit limits drug contact with the intestinal epithelium, resulting in decreased absorption and lower drug bioavailability in the bloodstream overall.
Q6: What is the relationship between surface area and drug absorption in the GI tract?
Surface area directly influences absorption rates: the intestine's larger surface area, enhanced by villi, supports greater drug absorption compared to the stomach's smaller surface. Combined with higher intestinal blood flow, this increased surface area facilitates more efficient drug uptake into systemic circulation.
Q7: How do ionization states of drugs affect their absorption at different GI sites?
Drug ionization state determines absorption efficiency at specific GI locations. Nonionized drugs, like weakly acidic drugs in the acidic stomach, cross membranes readily. Ionized drugs in the upper intestine have reduced membrane permeability, though the intestine's superior surface area and blood flow often compensate for lower ionization-based absorption.