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|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||-|
|STOMACH||1 - 3
decomposition, weak acid unionized
|normal||good||small||30 - 40 minutes, reduced absorption||no|
|DUODENUM||5 - 7
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|
Table 22.3.1 GI Physiology and Drug Absorption
Figure 22.3.1 Dependence of Peak Acetaminophen Plasma Concentration as a Function of Stomach Emptying Half-life
Redrawn from Heading, R.C., Nimmo, J., Prescott, L.F. and Tothill, P. 1973.
The dependence of paracetamol absorption on the rate of gastric emptying,
Br. J. Pharmacol., 47, 415-421
Generally drugs are better absorbed in the small intestine (because of the larger surface area) than in the stomach, therefore quicker stomach emptying will increase drug absorption. For example, a good correlation has been found between stomach emptying time and peak plasma concentration for acetaminophen. The quicker the stomach emptying (shorter stomach emptying time) the higher the plasma concentration, Figure 22.3.1.
Also slower stomach emptying can cause increased degradation of drugs in the stomach's lower pH; e.g. l-dopa.
|Volume of Ingested Material||As volume increases initially an increase then a dcrease. Bulky material tends to empty more slowly than liquids|
|Type of Meal|
|Temperature of Food||Increase in temperature, increase in empyting rate|
|Body Position||Lying on the left side decreases emptying rate. Standing versus lying (delayed)|
|Anticholinergics (e.g. atropine)||Decrease|
|Narcotic (e.g. morphine)||Decrease|
|Analgesic (e.g. aspirin)||Decrease|
Table 22.3.2 Factors Affecting Gastric Emptying
From Mayersohn, M. 1971.
Physiological Factors Influencing Drug Absorption,
Can. Pharm. J., 164-169
Figure 22.3.2 Effect of Fasting versus Fed on Propranolol Concentrations
Melander, A., Danielson, K., Schersten, B. and Wahlin, E. 1977.
Enhancement of the bioavailability of propranolol and metaprolol by food,
Clin. Pharmacol. Ther., 22, 108-112
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.
Food Retards transit
Copyright 2001-3 David W. A. Bourne (email@example.com)