What is an open circulatory system?

When a vein,instead of carrying blood directly into the heart,first carries it to an intermediate organ like the liver,it is known as:

The portal system in humans is defined as a system where:

Name the vascular connection that exists between the digestive tract and liver.

The circulatory pattern,in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses,is called $.......$ and it is found in $........$

The systemic circulation provides nutrients, $A$ and other essential substances to the $B$ and takes $C$ and other harmful substances away for elimination. Choose the correct option for $A, B$ and $C$.