This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
When a soluble drug is introduced into the stomach, it will undergo absorption, and the whole of it may possibly be absorbed without any portion of it even passing into the intestine. After absorption into the blood it will either remain in the plasma or form a compound with the corpuscles. It will thus be carried to the liver, where part of it may be retained (vide p. 39). Such portions as pass through the liver will then be carried to the right side of the heart, to the pulmonary circulation, and then, passing to the left side of the heart, will be distributed to all parts of the body. As absorption continues, the quantity of the drug in the stomach will gradually diminish, while that in the circulation will increase to a certain extent; this extent, however, will depend upon the activity of the eliminating organs. The drug will be carried to all parts of the body, both to the eliminating organs and to those connected with the other functions of the organism. It will enter into combination, more or less firm, with all those organs which have any attraction for it, and will more or less modify their functional activity. In the processes of tissue-change, which are constantly going on, the combination between the drug and the organs will be gradually destroyed; and, being again returned to the circulation, it will undergo gradual elimination. The method in which elimination occurs will also depend, to a certain extent, on the selective action of the eliminating organs; thus soluble substances are usually eliminated most readily by the kidneys, while salts of the heavy metals, which form insoluble compounds with albumen, are eliminated to a great extent by mucous membranes.