Without going into great detail - yes, but not by much. Each data fragment (packet) you upload has to be acknowledged by the server you are uploading to, so your PC knows it got there. This acknowledgement is a small amount of data downloaded to your PC. This is an overhead so it impacts on your available download bandwidth. In practice you will see between 1% and 5% decrease in your download if uploading to the max at the same time. Hardly noticeable.