Crossword Genius

Open space where blood is exchanged? (6)

Ross

I believe the answer is:

atrium

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'open space' is the first definition.
The answer and definition can be both body parts as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'where blood is exchanged?' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

Can you help me to learn more?

(Other definitions for atrium that I've seen before include "Central hall of shopping mall etc" , "Roofed central court" , "High central hall in large building" , "Central court of ancient Roman house" , "Roofed courtyard" .)

I've seen this clue in The Irish Times.
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