Describe the process of urine formation.

The process of blood filtration by kidneys and urine formation can be explained in the following steps:

1. Filtration — The renal artery brings the blood containing waste material to the kidneys. It then enters the glomerulus under high pressure. Water and small solutes are filtered in the Bowman’s capsule.
2. Reabsorption — The filtrate passes through the thin walls of the Bowman’s capsule into the urinary or renal tubule. As it passes through the tubule, water and many useful substances are reabsorbed by the renal blood capillaries.
3. Secretion — The remaining liquid, along with waste such as urea, uric acid and so on, is called urine and is collected in the urinary bladder.
4. Excretion — Urine leaves the kidneys and passes into the urinary bladder through the ureters. When the urinary bladder is full, urine is passed out through the urethra.

Below diagram shows the process of urine formation: