Question

Blood flowing away from stomach and intestine is put into circulation via the liver not directly

Answer 1

First of all follow the diagram of hepatic portal system. The veins starting from the stomach and intestine donot directly convey the blood to the posterior venacava. Instead they first enter the liver as a combined hepatic portal vein.

This is why, liver acts as a store house to regulate the quantity of nutrients flowing into the general blood circulation.

The liver performs many functions some of which are glucogenesis, glycogenolysis and detoxification.

Glucogenesis is the conversion of excess sugar (that are absorbed from the intestinal wall) into glycogen with the help of insulin hormone.

Glycogenolysis is the conversion of glycogen into glucose with the help of glucagon hormone.

again liver cells detoxify any toxic molecules that are absorbed into body through food.

for the above reasons, blood flowing away from stomach and intestine is put into circulation via the liver but not directly.