text: why does your ankle swell when u sprain it? i need 2 no y ur ankle swells when u sprain it for my hw!! swelling occurs when ligaments are injured because the ligaments are intimately connected with the functioning of the joints. ligaments hold our joints together, helping to maintain the structural integrity of the skeletal system. when a ligament is strained, the synovial membrane of the joint where the ligament is located is "told" by the brain to produce more fluid than is required for proper functioning. this increase in synovial fluid causes the joint to swell. this swelling is our body's way of sending us a strong and important message. the body is saying, "don't move me too quickly or too vigorously, or i will send pain signals to remind you that i need to rest in order to heal."