The pain and swelling of gout are caused by uric acid crystals that form in the joint. Uric acid is a waste product of purine metabolism.
Uric acid is dissolved in the blood and is excreted through the kidneys into the urine.
In people with gout, the uric acid level increases and uric acid crystals are deposited in joints and other tissues. These needle-shaped crystals trigger an immune response that produces intense local inflammation with severe pain, tenderness and swelling. Tophi often are found in or near severely affected joints, on or near the elbow, over the fingers and toes, and in the outer edge of the ear. Uric acid crystals can also form stones in the kidneys, in the ureters and in the bladder. Stones form when the uric acid concentration in the urine is too high - this is cause by low water intake, diuretics, and overly acidic urine.