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B. Gastrointestinal physiology

1. Characteristics of G- I physiology

Table XI-1, GI Physiology and Drug Absorption

pHMembraneBlood SupplySurface AreaTransit TimeBy-pass liver
BUCCALapprox 7thinGood, fast absorption with low dosesmallShort unless controlledyes
ESOPHAGUS5 - 6Very thick, no absorption-smallshort-
STOMACH1 - 3 decomposition, weak acid unionizednormalgoodsmall30 - 40 minutes, reduced absorptionno
DUODENUM6 - 6.5 bile duct, surfactant propertiesnormalgoodvery largevery short (6" long), window effectno
SMALL INTESTINE7 - 8normalgoodvery large 10 - 14 ft, 80 cm 2 /cmabout 3 hoursno
LARGE INTESTINE5.5 - 7-goodnot very large 4 - 5 ftlong, up to 24 hrlower colon, rectum yes

2. Gastric emptying and motility

Peak Concentration versus Stomach Emptying Half-life

Figure XI-1, Showing Dependence of Peak Acetaminophen Plasma Concentration as a Function of Stomach Emptying Half-life[1]

Generally drugs are better absorbed in the small intestine (because of the larger surface area) than in the stomach, therefore increasing stomach emptying will increase drug absorption. For example, a good correlation has been shown between stomach emptying time and peak plasma concentration for acetaminophen. The quicker the stomach emptying the higher the plasma concentration.

Also slow stomach emptying can cause increased degradation of drugs in the stomach's lower pH; e.g. l-dopa.

Table XI-2, Factors Affecting Gastric Emptying[2]

pH Membrane Blood Supply Surface Area Transit Time By-pass liver
BUCCAL approx 6 thin Good, fast absorption with low dose small Short unless controlled yes
ESOPHAGUS 6 Very thick, no absorption - small short -
decomposition, weak acid unionized
normal good small 30 - 40 minutes, reduced absorption no
bile duct, surfactant properties
normal good very large very short (6" long), window effect no
SMALL INTESTINE 6 -7 normal good very large 10 - 14 ft, 80 cm 2 /cm about 3 hours no
LARGE INTESTINE 6.8 - 7 - good not very large 4 - 5 ft long, up to 24 hr lower colon, rectum yes

3. Food

Propranolol Concentrations in Fed and Fasted Subjects

Figure XI-2, showing the Effect of Fasting versus Fed on Propranolol Concentrations[3]

Food can effect the rate of gastric emptying. For example fatty food can slow gastric emptying and retard drug absorption. Generally the extent of absorption is not greatly reduced. Occasionally absorption may be improved. Griseofulvin absorption is improved by the presence of fatty food. Apparently the poorly soluble griseofulvin is dissolved in the fat and then more readily absorbed.

Propranolol plasma concentrations are larger after food than in fasted subjects. This may be an interaction with components of the food.

4. Other factors

Intestinal Motility and Transit Time [2]

Food Retards transit

This page was last modified: 12 February 2001

Copyright 2001 David W.A. Bourne

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