 Dear students, in this topic, we shall take an overview of mechanism of urine production. The kidneys act as excretory as well as osmoregulatory organs. Both these actions of excretion and osmoregulation are performed by forming urine. The excretive functions of kidneys include the filtration of nitrogenous wastes and their removal in urine. While the osmoregulatory functions include controlling the amount of water, salts and ions excreted in the urine. Kidneys produce dilute urine when body is flooded with water, while they form concentrated urine when body is dehydrated. Dear students, urine formation involves three main processes, glomerular filtration, tubular reabsorption and tubular secretion. The glomerular filtration or ultrafiltration is the first step in urine formation. It results in the formation of an ultrafiltrate. It takes place in the renal carpacil under high pressure. The filtration pressure comes from exceptionally high blood pressure in the glomerular capillaries. Due to high filtration, pressure, water and small solute molecules are filtered out of the glomerular capillaries. While blood cells, platelets and large molecules, such as proteins, are not filtered. The filtrate is collected by the surrounding Bowman's capsule. This filtrate is called glomerular filtrate. It has a chemical composition which is very similar to that of the blood plasma. Dear students, now we shall discuss the second step of urine formation, that is tubular reabsorption or selective reabsorption. The tubular part of the nephron selectively reabsorbs about 99% of water and most of the useful substances from the ultrafiltrate. This absorption occurs into the blood by the surrounding peritubular capillaries. The major reabsorbed molecules are the glucose, amino acids, vitamins and various inorganic salts. Due to reabsorption, only nitrogenous wastes and other metabolic wastes remain and ultrafiltrate converts into the form of urine. Dear students, epithelial cells of convoluted tubules also secrete some substances by active transport into the ultrafiltrate. Purpose of this tubular secretion is to excrete toxic substances which are not filtered. As such, toxic substances which are not filtered are actively secreted into the outgoing urine. The second purpose is the maintenance of the secretion of pH of the urine.