Most carcinoid tumours arise in the gut. As such, the hormones they secrete enter a specific part of the circulation known as the portal circulation, illustrated here. This is because the function of the gut is to remove nutrients from the digested food in the gut lumen to release them to the body tissues. The vast majority of the blood draining the gut therefore goes first to the liver, via the portal circulation, where any toxins are removed and nutrients processed for the body tissues. Only then, after it has passed through the liver, does it enter the systemic circulation where the content of this blood can get to the body tissues. Consequently, carcinoid tumours arising in the gut and not in the liver, do not generally produce carcinoid syndrome as the hormones they produce that cause the symptoms are inactivated when they pass through the liver.
The tumours that do produce carcinoid syndrome are therefore extraintestinal tumours including: