[Darren]

I have learnt that in some organisms such as fish, blood is not separated from the products of digestion and there is mixing of blood and food products through the entire circulatory system of the fish

I have learnt that in humans or other mammals, such mixing does not occur and blood isseparated from the digestive tract. If that is the case, how is disgested material transferred through the body? After nutrients are absorbed by the small intestine, where do they go to be transferred around the body?

[Borek]

blood is not separated from the products of digestion

Please define what you mean by "products of digestion" here.

Nutrients - like glucose and aminoacids - are absorbed into the bloodstream and transported by it both in fish and mammals. If they are the "products of digestion" they are not separated from the bloodstream in neither.

[Arkcon]

I'm assuming Darren: that you're asking about the differences between open and closed circulatory systems.  If you use the correct terms, all the answers you want are a Google away:

http://www2.gsu.edu/~bioasx/closeopen.html

http://en.wikipedia.org/wiki/Circulatory_system#Other_animals