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In current Retina models air is sucked in through the vents on the sides under the computer, and exhausted through the hinge.

Other models might work differently, but my understanding is, that generally the vents in the hinge are exhaust.
 
Since the 2012 Retina Macbook Pro's release, the retina models have vents on the left and right side of the base that sucks in air and it is vented through the hinge. (amazing design for a laptop imo)

Old Macbook Pro models as well as the Macbook Air's are sucked in one half of the hinge and exhausted through the other half of the hinge.
 
Since the 2012 Retina Macbook Pro's release, the retina models have vents on the left and right side of the base that sucks in air and it is vented through the hinge. (amazing design for a laptop imo)

Old Macbook Pro models as well as the Macbook Air's are sucked in one half of the hinge and exhausted through the other half of the hinge.

Thanks :)
 
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