Excretory System - Nephrons and Urine Formation

The Nephron

  • Nephrons are the functional unit of the kidney.
  • There are over one million nephrons responsible for removing waste in each kidney.
  • The vital job results in the formation of urine.
  • Also responsible for purification and filtration of the blood.
  • When blood enters through the arteriole of the nephron, the wastes in the blood are filtered into the collecting duct. The blood free of wastes now exits through the venule.

[[http://www.coolschool.ca/lor/BI12/unit14/U14L02/Nephron.swf -click this link to quiz yourself over the structure of a nephron.

Process of Filtration

  • As blood flows through the glomeruli the fluid containing waste is diffused and filtered through the membrane into Bowman's capsule. Approximately 20 percent of the blood gets filtered through walls of the glomerulus and the bowmans capsule.
  • The glomerulus is a concentrated network of capollaries located in the upper region of the nephron.
  • The Bowman's capsule is a hollow cup shape that encases the glomerulus and adsorbs the waste in the blood from the glomerulus.
  • The materials that are filtered from the blood are called the filtrate, which contains water, urea, glucose, salts, amino acids, and some vitamins.


  • Most of the materials that are removed from the blood at the Bowman's capsule at some time return to the blood through the process of reabsorption.
  • Reabsorption is when liquid is absorbed back into a vessel.
  • 99% of the water that enters the Bowman's capsule makes its way back into the blood.

Urine Formation

  • The liquid that is left after filtration and reabsorption is called urine.
  • Urine is made up of urea, excess salts, and water.
  • The urine becomes concentrated in the loop of Henle which is a long tubule located in the nephron.
  • In the loop of Henle, water is conserved and the urine is minimized.
  • The purified blood returns to the circulation process throughout the body.
  • Urine moves to the urinary bladder and is released from the body through a tube called the urethra.