Urobilin, also known as urochrome, is the tetrapyrrole chemical compound that is primarily responsible for the yellow color of urine. Urobilin is formed through the oxidation of its parent compound uroblinogen. Urobilin is actually generated through the degradation of heme, the red pigment in haemoglobin and red blood cells (RBCs). RBCs have a life span of about 120 days. When the RBCs have reached the end of their useful lifespan, the cells are engulfed by macrophages and their constituents recycled or disposed of.  Heme is broken down when the heme ring is opened by the enzyme known as heme oxygenase, which is found in the endoplasmic reticulum of the macrophages. The oxidation process produces the linear tetrapyrrole known as biliverdin along with ferric iron (Fe3+), and carbon monoxide (CO). In the next reaction, a second methylene group (located between rings III and IV of the porphyrin ring) is reduced by the enzyme known as biliverdin reductase, producing bilirubin. Bilirubin is significantly less extensively conjugated than biliverdin. This reduction causes a change in the color of the biliverdin molecule from blue-green (vert or verd for green) to yellow-red, which is the color of bilirubin (ruby or rubi for red). In plasma virtually all the bilirubin is tightly bound to plasma proteins, largely albumin, because it is only sparingly soluble in aqueous solutions at physiological pH. In the sinusoids unconjugated bilirubin dissociates from albumin, enters the liver cells across the cell membrane through non-ionic diffusion to the smooth endoplasmatic reticulum. In hepatocytes, bilirubin-UDP-glucuronyltransferase (bilirubin-UGT) adds 2 additional glucuronic acid molecules to bilirubin to produce the more water-soluble version of the molecule known as bilirubin diglucuronide. The bilirubin diglucuronide is transferred rapidly across the canalicular membrane into the bile canaliculi where it is then excreted as bile into the large intestine.  The bilirubin is further degraded (reduced) by microbes present in the large intestine to form a colorless product known as urobilinogen. Some of the urobilinogen produced by the gut bacteria is reabsorbed and re-enters the enterohepatic circulation. These urobilinogens are oxidized and converted to urobilin. The urobilin is processed through the kidneys and then excreted in the urine, which causes the yellowish color in urine. Many urine tests monitor the amount of urobilin in urine, as this provides some useful insight into urinary tract function. Normally, urine would appear as either light yellow or colorless. A lack of water intake, for example following sleep or dehydration, reduces the water content of urine, thereby concentrating urobilin and producing a darker color of urine. Obstructive jaundice reduces biliary bilirubin excretion, which is then excreted directly from the blood stream into the urine, giving a dark-colored urine. This dark colored urine has a paradoxically low urobilin concentration.