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It is from this video. It is at around 6 minute. Here is the context:
This happens because when blood and other fluids block your circulation the heart is forced in overtime.
The word overtime has a few meanings according to The Oxford Dictionary, and it seems to me none fits.
When certain employers require their workers to do more than 40 hours in a week, they are required to pay a higher wage, referred to as overtime. Employers do not like to pay 1.5 or 2 times the usual wage, so in most cases overtime happens only during extraordinary circumstances: when a big, time-sensitive job must be completed right away or when the business is short-staffed and has not been able to hire workers to meet its needs.
There is a sense, then, that workers doing overtime and overworked and frantically trying to get the job done to keep the business from going under. The word is often used in a more general context. When your circulation is blocked, the heart must work at a frantic pace, with an unusual load. It has to work overtime to keep you alive.
