The portal vein system is to feed the absorbed nutrition and toxin (defined as "substance" here) to liver first for it to take them up as fast as possible, before they reach other organs which may be harmful.
For this to be effective, liver should take as much substance as possible in one pass. For example, if the liver only take up 10% of the substance in one pass, then the portal vein system won't make much difference, since most of the substance will circulate to other organs anyway and get taken up by liver in the following circulation passes.
On the other hand, it seems to be very difficult for an organ to take up most substance in one pass since this is not a "effective design" (i.e. over designed).
How much percentage of substance does liver take in one pass? And if the percentage is low, why is the portal vein architecture important?