Why do pre-term infants often exhibit higher heart rates?

Pre-term infants displaying higher heart rates can primarily be attributed to the developed sympathetic nervous system, which is not yet balanced by adequate parasympathetic regulation. In the developing fetus, the autonomic nervous system consists of both the sympathetic and parasympathetic branches, with the sympathetic system responsible for the "fight or flight" response, which includes increasing heart rate.

In pre-term infants, the sympathetic nervous system is more developed and tends to dominate because the parasympathetic system, which has a calming effect on heart rate and is more mature in full-term infants, is still in the process of maturing. This imbalance leads to a higher baseline heart rate in pre-term infants.

The other options do not provide a comprehensive explanation for the elevated heart rates. Immature lung function can affect oxygenation but is not a direct cause of increased heart rate. An inadequate oxygen supply tends to trigger a reflex bradycardia rather than tachycardia in many cases. Lastly, overactivity of the parasympathetic nervous system would result in a reduced heart rate, not an increased one. Therefore, option C effectively captures the physiological dynamics responsible for the higher heart rates observed in pre-term infants.