Excretion is the removal of toxic and metabolic waste products and excess substances from the body. The accumulation of these substances can interfere with the proper functioning of cells and change the concentration of tissue fluid around the cells.

Metabolic waste includes carbon dioxide, nitrogenous waste and urea.

Toxic material includes alcohol, used drugs, nicotine, etc.

Excess substances include, excess water, minerals, salts, used hormones and amino acids.

Organs of the Excretory System

A variety of excretory products are removed from the body by different excretory organs.

  1. Lungs– removal of excess carbon dioxide.
  2. Liver– produces urea and uric acid as a by-product of the breakdown of proteins.
  3. Skin– removal of excess water, salt, urea and uric acid.
  4. Urinary System– kidneys filter the blood to form urine, which is excess water, salt, urea and uric acid.

Human urinary system


  • Kidney: organ in the urinary system that produces and excretes urine.
  • Ureter: one of the two muscular tubes that take urine from the kidneys to the urine bladder by peristaltic contractions.
  • Urethra: tube that takes urine from the bladder to the outside; it is longer in males and also functions as to carry sperms.
  • Urinary bladder: hollow, muscular organ whose urine is stored before being discharged by way of the urethra.

Most common reasons for kidney failures

  • Hypertension
  • Diabetes
  • Complications from sugary
  • Excessive alcohol intake
  • Severe injury or bacterial infection
  • Drug over use

Kidney transplant

  • A kidney transplant is an operation in which a person with kidney failure receives a new kidney. The new kidney takes over the work of cleaning the blood.
  • Kidney transplant is the best long term solution from kidney failure, but it has its own drawbacks.
  • No enough kidney donors available.
  • No sufficient money for operations

Kidneys may be rejected by the recipient even if the transplant operation is successful.

Kidney Dialysis
Dialysis is the process which involves separation of nitrogenous wastes from the blood artificially. Dialysis is performed using a device which removes nitrogenous wastes from blood in case of kidney failure.

  • An artificial kidney contains a number of tubes with a semi-permeable lining suspended in a tank filled with dialysing fluid.
  • The patient’s blood is passed through these tubes.
  • During this passage, waste products from the blood diffuse into the dialysing fluid.
  • The purified blood is pumped back into the patient.


Disadvantage of dialysis

  • Each patient has to go to treatment at least for three sessions of dialysis per week. Each session is a time consuming treatment.
  • It is very inconvenient and expensive
  • There is risk of getting other diseases, infection of blood like AIDS.