  We study the line profiles of a range of transition region (TR) emission
lines observed in typical quiet Sun regions. In magnetic network regions, the
Si IV 1402\AA{}, C IV 1548\AA{}, N V 1238\AA{}, O VI 1031\AA{}, and Ne VIII
770\AA{} spectral lines show significant asymmetry in the blue wing of the
emission line profiles. We interpret these high-velocity upflows in the lower
and upper TR as the quiet Sun equivalent of the recently discovered upflows in
the low corona above plage regions (Hara et al., 2008). The latter have been
shown to be directly associated with high-velocity chromospheric spicules that
are (partially) heated to coronal temperatures and play a significant role in
supplying the active region corona with hot plasma (DePontieu et al., 2009}. We
show that a similar process likely dominates the quiet Sun network. We provide
a new interpretation of the observed quiet Sun TR emission in terms of the
relentless mass transport between the chromosphere and corona - a mixture of
emission from dynamic episodic heating and mass injection into the corona as
well as that from the previously filled, slowly cooling, coronal plasma.
Analysis of the observed upflow component shows that it carries enough hot
plasma to play a significant role in the energy and mass balance of the quiet
corona. We determine the temperature dependence of the upflow velocities to
constrain the acceleration and heating mechanism that drives these upflows. We
also show that the temporal characteristics of these upflows suggest an
episodic driver that sometimes leads to quasi-periodic signals. We suggest that
at least some of the quasi-periodicities observed with coronal imagers and
spectrographs that have previously been interpreted as propagating
magnetoacoustic waves, may instead be caused by these upflows.
