Several heavy metals-such as cadmium, lead, and mercury-are excreted by the kidneys. These metals are bound to plasma proteins. The protein metal complex has a molecular weight and is able to pass through the glomerulus to the nephron. However, this complex may be reabsorbed by active transport mechanisms in the proximal convoluted tubules. The metals are stored in the kidneys for prolonged periods of time, or they may reach a concentration high enough to produce toxic effects causing tubule cell death and, subsequently, kidney failure.