Assertion (A): A complex of veins beginning and ending in capillaries through which blood circulates before returning to the heart is called a portal system.
Reason (R): All mammals have a hepatic portal system.
Both A and R is true and R is the correct explanation of A.
A portal vein is a complex of veins which begin and end in capillaries. It collects blood from a particular organ and distributes the blood to some other organ instead of sending it directly to the heart. All mammals have a hepatic portal system which carries blood from the digestive tract to the liver for various metabolic activities. Therefore, both A and R are true.