Urea is freely filterable by the kidney and the excretion of this urea constitutes a large osmotic load to the kidney. Most solutes excreted in such large amounts would obligate large amounts of water excretion by causing an osmotic diuresis.

What do the kidneys excrete?

The most common waste products are urea and creatinine, but there are many other substances that need to be eliminated. The kidneys act as very efficient filters for ridding the body of waste and toxic substances, and returning vitamins, amino acids, glucose, hormones and other vital substances into the bloodstream.

How is urea excreted?

The urea and water are released from the liver cells to the bloodstream and transported to the kidneys where the blood is filtered and the urea is passed out of the body in the urine. Urea is very soluble and a small molecule, so it is relatively easily passed out by the kidneys as a solution in water.