Related to the system in your body that moves blood through your heart and blood vessels to deliver oxygen everywhere.
From Latin 'circulare' meaning 'to move in a circle,' derived from 'circulus' (circle). Added '-ory' suffix meaning 'relating to' in medical terminology.
The circulatory system is literally called that because blood goes in circles—out from the heart, through the body, and back again in an endless loop. Ancient doctors watched blood vessels and realized they must be circular, which is why they named it 'circular' even before they understood how it actually worked!
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