Final answer:
The respiratory tract is the fastest route for toxicants to enter circulation, followed by the GI tract, with the skin being the slowest.
Step-by-step explanation:
To rank the GI tract, respiratory tract, and skin according to how fast a toxicant would appear in circulation:
- Respiratory tract
- GI tract
- Skin
The respiratory tract is typically the fastest route for toxicants to enter the circulation due to the large surface area and high blood flow in the lungs, allowing gases and small particles to be rapidly absorbed into the bloodstream. Poisoning symptoms from inhalation can appear within seconds. The GI tract is the next fastest, as substances can be quickly absorbed through the intestines, though more slowly than through the lungs as the absorption process includes breaking down and passage through the mucous barrier. Finally, the skin is the slowest due to its role as a barrier to protect against external substances; however, some compounds can still penetrate this barrier, particularly lipophilic substances.