Rheumatoid arthritis causes inflammation in the synovium It is also referred to as the synovial membrane,is the soft tissue that lines the entire inner surface of the joint,except where the joint is lined is lined with cartilage.The result is very similar to inflammation that you may have seen if you've had an infected cut or wound-it goes red,swells,produces extra fluid and hurts.The redness is caused by the flow of blood increasing.As a result,the inflamed joint may feel warmer than usual.The inflammation is caused by a build-up of fluid and cells in the synovium.Your joint hurts for two reasons.