On the contrary, spoken English is normally more 'awkward' than spoken English, so your utterance sounds OK, and I don't see your 2nd sentence as an improvement; the parallel structure is better. I see 'their brain' and the countability of 'circulation' as greater style problems, and the whole sentence could be tightened— 'people', for instance, is a generally useless word—and made more formal. The professor states that the differences between extroverts and introverts result from differences in the blood circulation in the brain.
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