Urine is a waste product of metabolism and is formed in the kidneys to be excreted normally through urination or micturition. In mammals, including humans, urine is in a liquid form whereas in birds and reptiles, it is solid or semisolid. The biological system responsible for the production of urine is the urinary system. It is comprised of kidneys, ureters, urinary bladder, and urethra. The processes that lead to the formation of urine are filtration, reabsorption, and tubular secretion. From the kidneys, the urine passes through the ureter and stored in the urinary bladder. When it is to be excreted from the body, the urine passes through the urethra.
Human urine is chiefly comprised of water (91 to 96 %). Other components are inorganic salts, proteins, hormones, and other metabolites. It has a specific gravity ranging from 1.003–1.035. The pH ranges from 5.5 to 7. It is typically slightly transparent to amber in colour. Colorless urine may indicate over-hydration whereas dark yellow urine may indicate dehydration. Hematuria (bloody urine) and melanuria (a black or dark-colored urine) are possible symptoms of certain underlying medical conditions.
Urine produced by an adult healthy person is, on average, is about 1.4 L in a day. The amount of urine excreted depends on the health and medical condition of a person. Excessive urine production (i.e. >2.5 L/day) is referred to as polyuria. In contrast, the condition wherein less than 100mL urine is produced per day is referred to as anuria.