When they say an injured athlete had fluid drained from a swollen joint such as a knee...?

Question

What's the fluid being drained? Water? Blood? A mix of both?

Answer 1

Normal joints contain a small amount of fluid (called synovial fluid) to lubricate the surfaces of the joint, much like motor oil in an engine.

When a joint is injured, the inflammation causes the production of additional fluid (called an inflammatory exudate) that contains proteins and inflammatory cells.

Injury to the structures in a joint (e.g. ligaments and tendons) could also produce bleeding, which would add to the fluid.

If there is a lot of fluid, the pressure can cause pain which is relieved by removing some of the fluid with a needle.

Answer 2

The body produces water at the point of a swollen joint. Arthritics get fluid on the knees or joint from their disease.

Ibuprofen will accomplish the same thing. It reduces swelling and inflammation. Much less painful than a needle.