Formation in the liver
Proteins in food are digested to amino acids, which are absorbed into the blood from the small intestine. The blood carries them to the liver via the hepatic portal vein. The body uses amino acids for protein synthesis, but any excess cannot be stored.
In the liver, excess amino acids undergo deamination: the amino group is removed. This produces ammonia, which is toxic. The liver immediately converts ammonia to urea, which is less toxic and can safely dissolve in the blood.
Transport to the kidneys
Urea dissolves in blood plasma and leaves the liver in the hepatic vein. It travels through the circulatory system to the renal artery, which carries blood into the kidneys.
Inside the kidney, blood enters the glomerulus: a knot of capillaries inside each Bowman's capsule. The high blood pressure forces small molecules, including urea, water and mineral ions, through the capillary walls and into the nephron. This is ultrafiltration.
Excretion in urine
Urea is not reabsorbed from the nephron: it remains in the tubular fluid. As the fluid passes through the nephron, useful substances are selectively reabsorbed, but urea continues into the collecting duct. It is carried with excess water and ions into the renal pelvis, through the ureter to the bladder, and eventually leaves the body as urine through the urethra.
The complete pathway: liver (urea formed) → hepatic vein → heart → renal artery → kidney → glomerulus → nephron → collecting duct → renal pelvis → ureter → bladder → urethra.