A liquid by-product of metabolism in the human body is called human urine. Urine is expelled from kidneys and flows through the ureters to the urinary bladder and excreted from the body through the urethra which is called urination. Normal human urine is a transparent, colorless or pale yellow colored fluid with a characteristic aromatic odor. It is slightly acidic. Under normal conditions, the volume, pH, color and percentage composition of the constituents of urine may vary within physiological limits depending on the amount of water intake, nature of diet, ambient temperature, state of physical exercise etc. Normal human adults excrete about 1.5 liters of urine per day, on an average.
Human urine consists of 90-95% of water and each liter of urine contains roughly 50 g of solids. The urinary solids include various organic and inorganic constituents as given below. Human urine consists primarily of water (91% to 96%), with organic solutes including urea, creatinine, uric acid, and trace amounts of enzymes, carbohydrates, hormones, fatty acids, pigments, and mucins, and inorganic ions such as sodium (Na+), potassium (K+), chloride (Cl-), magnesium (Mg2+), calcium (Ca2+), ammonium (NH4+), sulfates (SO42-), and phosphates ( PO43-).
Normal organic constituents of human urine: -
Non-protein nitrogenous (NPN) compounds
The chief organic constituents of human urine are nitrogenous compounds which rather than being proteins, are derivatives or protein metabolism. Such materials are referred to as non-protein-nitrogenous compounds or the NPN compounds.
The principal NPN compound of human urine is urea which is produced in liver from degradation of amino acids. Urea constitutes almost half of the total urinary solids. Other NPN compounds-present in normal urine include uric acid, creatinine, allantoin, hippuric acid, amino acids and ammonia.
Normally fresh urine contains ammonia in traces only but if urine is stored for a long time, it gives a pungent smell of ammonia which is evolved from breakdown of urea by the active bacteria.
Non-nitrogenous organic acids
Human urine contains small quantities of various non-nitrogenous organic acids such as lactic acid, oxalic acid, citric acid, glucoronic acid, some fatty acid etc.
Other substances in human urine
Traces of certain other organic compounds such as ketone bodies which are derived from fat metabolism and pigments like bilinogens, bilirubin etc.which are derived from degradation of hemoglobin are found in human urine. These pigments are responsible for the color of urine. Degradation products of hormones and vitamins are also excreted through urine of human body.