Wireless mesh networks (WMNs) are practical, simple, and cost effective solution for indoor and outdoor environments. Wireless Mesh networks use the IEEE 802.11 MAC protocol. Since the IEEE 802.11 MAC protocol is originally designed for single hop wireless networks, multimedia transport in WMNs has become a big challenge. In multi-hop wireless networks, throughput decreases significantly with the increase in number of hops, mainly due to the spatial contention and the backoff algorithm adapted in 802.11 MAC layer protocol. In this paper, we introduce an Adaptive Contention Window Algorithm (ACWA) that allows each node to adapt a contention window size, used in its backoff algorithm, based on its location in the WMN. ACWA improves the throughput of each node in the network. In addition, it improves the overall network throughput by 16%, and decreases the end-to-end average delay by about 27%. The results are evaluated and compared with the performance of the default 802.11 MAC protocol.