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Following wikipedia:
Cases of systemic toxicity from accidental swallowing are rare, and attempted suicide via intravenous injection does not appear to result in systemic toxicity,[1] though it is still causes damage by physicaly blocking blood vessels both at the site of injection and the lungs.
Shouldn't both inhalation and IV injection of elemental mercury produce the same result, namely, mercury in the bloodstream?
I guess the answer is that mercury atoms stick with each other very well (e.g. high surface tension in comparison to most other liquids $\gamma_{293K}^{\ce{Hg}} = 0.47 \text{ N/m}$, where $\gamma_{293K}^{\ce{H2O}} = 0.073 \text{ N/m}$ [1]) and that this continues to happen in the blood stream. Such clumps of mercury are not able to pass the blood brain barrier (BBB) effectively, especially in comparison to finely divided dissolved vapor, which is transported trough the BBB by the formation of a complex with cysteine.
The simple answer is that elemental, metallic mercury isn't very soluble in either the blood or in the contents of the digestive system. If ingested, metallic mercury will simple pass through the gut with little effect on the body and with very little being absorbed into the places where it can do harm. The same is true in the blood.
Mercury causes harm when it interacts with essential enzymes in the body and makes them malfunction. For it to do this, mercury atoms or ions have to be able to reach a significant concentration in bodily fluids. There are two ways for this to happen easily. Mercury vapour is absorbed into the bodily system (but this requires some effort such as combining bad ventilation with finely divided mercury droplets and heat or bad ventilation and a long time period as early mercury chemists discovered). Many mercuric compounds, especially organic ones, are also readily absorbed (either because they are soluble, as many salts are, or because they are volatile, as many organic mercury compounds like methyl mercury are.)
In fact the worst case of human mercury poisoning (the Minamata disaster) resulted from the release of methyl mercury into the environment where it accumulated in fish ultimately eaten by people.
