Modelling of asymmetries in spectral lines with different formation heights (K I 7699, Fe I 5576 and Fe I 5635A), observed in the quiet Sun at the disk center, has been performed with the aim of reproducing the absolute line bisectors obtained from low resolution observations. The influence of a number of physical and observational factors on the bisector shape and line shifts, acting together or separately, is discussed, e.g. mesogranular flows, waves, microturbulence, defective temporal and spatial averages, etc. The simulations are based on the VAL-C model (Vernazza et al. 1981) representing the mean photosphere, and the granular model by Nelson (1978). Following evidence in the literature, the best match to the observed absolute mean bisectors is obtained by including in the numerical code a gravity wave and a strengthening of the microturbulence in the lower layers of the intergranule.