$A$ portal system is a system in which

• A
  $A$ vein starts from an organ and ends up in the heart.
• B
  An artery breaks up in an organ and restarts by the union of its capillaries.
• C
  The blood from the gut is brought into the kidney before it is poured into the posterior vena cava.
• D
  $A$ vein breaks up in an organ into capillaries and restarts by their union as a new vein in the same organ.