Excretion

The Process of Excretion

  • Excretion is a vital biological process in living organisms where metabolic wastes and toxins are removed from the body.
  • It should not be confused with egestion, which refers to the removal of undigested food material from the body.
  • Major waste products excreted by humans include carbon dioxide, urea, excess water and salts which are synthesised during various metabolic activities.
  • The organs primarily involved in excretion are the lungs, kidneys, skin, and the liver.

Role of Different Organs in Excretion

Lungs

  • The lungs excrete carbon dioxide and water vapour as by-products of respiration.
  • Oxygenated blood is pumped to each cell where oxygen is used up and carbon dioxide is produced.
  • This carbon dioxide is then transported back to the lungs via the bloodstream and exhaled, completing the process of excretion by the lungs.

Kidneys

  • The kidneys play a major role in excreting urea, excess water and salts through the creation of urine.
  • Urea is formed in the liver when excess amino acids are broken down––a process known as deamination.
  • This urea is then transported by the blood to the kidneys where it’s removed along with excess water and salt to form urine.

Skin

  • The skin helps in removing excess water, salts and a small amount of urea through sweat.
  • Sweating is also an important process for temperature regulation in mammals, including humans.

Liver

  • The liver is the site of detoxification wherein harmful substances like drugs and alcohol are converted into less toxic forms.
  • It is also where excess amino acids are broken down in the process of deamination, creating urea.

Functioning of Nephrons

  • Within the kidneys, the process of excretion takes place in numerous tiny filtration units known as nephrons.
  • They function by filtering blood under high pressure, forcing small molecules such as water, glucose, urea and salts into the Bowman’s capsule––a process known as ultrafiltration.
  • Essential substances like glucose, some salts and water are selectively reabsorbed back into the blood in the proximal convoluted tubule.
  • Further down the nephron––in the loop of Henle and the distal convoluted tubule––more water and salts are reabsorbed based on the body’s requirements.
  • The final product, urine––consisting of urea, excess water and salts––collects in the bladder through the ureters, before it’s finally excreted through the urethra.