Urine is yellow primarily because of a chemical called urobilin, which is produced from the breakdown of old red blood cells. The breakdown process begins when the body creates bilirubin from hemoglobin in red blood cells. This bilirubin is secreted into the gut, where bacteria convert it into urobilinogen, which the kidneys then convert into urobilin. Urobilin is the pigment that gives urine its characteristic yellow color. The intensity of the yellow color depends on how concentrated the urine is, which can vary with hydration levels—the more concentrated the urine, the darker the yellow color.
Additional factors like diet, vitamins (especially B vitamins like riboflavin), and medications can influence urine color. For example, excess riboflavin excreted in urine can cause bright yellow urine. Dehydration makes the urine more concentrated and darker yellow, while drinking lots of water dilutes the yellow pigment, making the urine lighter.
In summary, the yellow color of urine is mainly due to urobilin, a byproduct of red blood cell breakdown processed through the liver, gut, and kidneys, with other influences from hydration, diet, and supplements.
