What is the human circulatory system described as?

The Circulatory system is 'closed' and is a 'double system', what does this mean?

Fill in the blanks:
The _____ the surface area to _____ ratio, and the more _____ the organism, the greater the need for a specialised ___________ system with a ______

Whether or not there is a specialised transport system depends on two factors, name them.

What is reduced when blood is passed through the lungs?

Why after passing through the lungs is blood returned to the heart?

Blood being returned to the heart after passing through the lungs ensures what?
Why is this important for mammals?

Why is the final exchange of substances from the capillaries to the cells so rapid?