Original question.
Is this true ? Illustrate the statement.
The assertion is too absolute: a remedy can cause a distant response through a reflex or another intervening physiological change without first being absorbed into the blood. It is substantially right for the direct action of a circulating drug on a distant target, but does not describe every remote effect of treatment.
Three mechanisms should be distinguished:
- Direct action after drug absorption. An orally administered drug that acts directly in the brain must gain access to that tissue. Absorption from the gastrointestinal tract, transport in the blood, and passage to the target explain such a systemic action. Changing the route changes the first step: a subcutaneous medicine must leave its local depot, whereas an intravenous medicine is introduced directly into the circulation. The literal word “absorbed” is consequently too restrictive even before considering reflexes.
- Local action followed by a neural response. Stimulation of sensory nerves at one surface can cause a response elsewhere through the nervous system. Gastric stimulation can evoke vomiting, including coordinated movement of respiratory and abdominal muscles; sensory input from the gastrointestinal tract reaches brainstem circuits through vagal afferents. No requirement follows that the original gastric irritant must itself circulate to every contracting muscle. A swallowed emetic can have more than one mechanism, however: an observed instance of vomiting alone does not prove that absorption played no part.[ref]Nausea and Vomiting Related to Cancer Treatment by the National Cancer Institute. “Pathophysiology of N&V”; visceral afferent inputs and output to abdominal musculature and respiratory centres
- Local action followed by a change in the body's contents. As a present-day illustration, an osmotic laxative can retain water in the intestinal lumen and promote evacuation with limited systemic absorption. Any secondary change in fluid balance or symptoms is then a consequence of the local intestinal action, not proof that the laxative has directly acted on a remote organ. The polyethylene glycol 3350 information from MedlinePlus describes retention of water in stool; the regulatory monograph records minimal absorption.[ref]Polyethylene Glycol 3350 by MedlinePlus. “Why is this medication prescribed?”; retention of water in stool[ref]PegLyte prescribing information. “Action and clinical pharmacology”, p. 9; mechanism and pharmacokinetics of PEG 3350
The distinction is between transport of the medicinal substance and transmission of its consequences. A reflex transmits activity in neurons; a locally acting intestinal agent changes a physical condition; a circulating drug delivers molecules to its distant target. The same remedy may combine these routes, and an apparently rapid effect does not by itself settle which route caused it.
A useful experimental design would therefore compare remote responses with measurements of the agent in blood, and would test the proposed neural pathway separately. In an appropriate physiological preparation, recording the response while interrupting a relevant afferent pathway can test whether that pathway is necessary; preserving the circulation while interrupting nerve conduction distinguishes neural transmission from vascular transport. Conversely, a remote effect after vascular transfer supports a circulating agent, provided contamination and other transferred mediators are controlled. Failure to detect a drug is only as strong as the analytical detection limit.
Absorption is one route to remote action, not a universal prerequisite for every distant physiological consequence. The historical language “animal economy” means the functioning organism; it should not conceal the distinction between a direct pharmacological effect and a secondary physiological response.
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