The structural and functional units of the kidney are Nephrons (filtration units). Each kidney consists of a large number of nephrons. Nephrons are composed mainly of two parts - renal corpuscle and renal tubule.

The renal corpuscle is composed of a cup-shaped structure called Bowman’s capsule which is surrounded by a cluster of capillaries that help in the collection of filtered urine.
The composition of urine changes as it passes through the tubular part of the nephron due to the reabsorption of amino acids, glucose, salts, and water.
The reabsorption of the amount of water and other substances depends on their presence in the body.
The urine then enters the collecting duct of the kidney from where it is then transported to the long tube called the ureter.
Lastly, the urine is then collected in the urinary bladder.