The low-power communication in wireless sensor networks can be impacted by severe channel impairments. In this paper, to address this problem and achieve high network goodput, we propose that the medium access control protocol takes into consideration the error performance of the underlying wireless links. We combine a distributed back-off strategy regulated by the wireless link quality with Carrier Sense Multiple Access with Collision Avoidance. We integrate our cross-layer operational approach in the IEEE 802.15.4 standard, taking advantage of existing functionality and signaling to avoid network overhead and achieve simplicity in implementation. Our performance evaluation indicates that our scheme is more effective, achieving up to 69% higher goodput, and more efficient, delivering up to 154% more data bits per unit of energy consumed in the network, at the expense of an up to 18% degradation in fairness, compared to the basic 802.15.4.