Excretory Products and their Elimination

Excretion is the process of elimination of metabolic waste from the body. This process is strictly found only in animals.

The excretory products include amino acids, urea, uric acid, carbon dioxide, water, and ammonia.

Some Molluscs and Echinoderms excrete waste products from the body in the form of amino acids.

Ammonia is the primary excretory product in animals, it is derived from the proteins present in the food we eat. It is the most toxic nitrogenous waste and is excreted by diffusion by most of the amphibians and aquatic animals.

Mammals excrete nitrogenous waste as urea. It is less toxic and comparatively less soluble in water.

Birds and reptiles excrete nitrogenous waste as uric acid.

Human Excretory System

The human excretory system comprises of the following organs:

  • A pair of kidneys
  • A pair of ureters
  • Urinary Bladder
  • Urethra


These are the primary excretory organs in humans located on the left and right side of the spine, near the liver. They are divided into the renal cortex, renal medulla, and renal pelvis. Renal pelvis carries urine from the kidney to the ureter. A nephron is the functional unit of a kidney.


It is a thin tube that comes out of each kidney carrying urine to the urinary bladder.

Urinary Bladder

It is a sac-like structure that stores urine until micturition (expelling urine from the body).


It is a tube-like structure that extends from the urinary bladder and helps in expelling urine outside the body. It is usually shorter in females and longer in males.

Mechanism of Excretion

Urine Formation

The urine is formed in the nephrons and involves the following steps:

  • Glomerular Filtration
  • Tubular Reabsorption
  • Secretion

Glomerular Filtration

It is the primary step in urine formation. In this process, the excess fluid and waste products from the kidney are filtered out of the blood into the urine collection tubules of the kidney and eliminated out of the body.

The amount of filtrate produced by the kidneys every minute is known as Glomerular Filtration Rate.

Tubular Reabsorption

It is the absorption of ions and molecules such as sodium ions, glucose, amino acids, water etc. Water involves passive absorption, while glucose and sodium ions are absorbed by an active process.


Potassium ions, hydrogen ions, and ammonia are secreted out to maintain the equilibrium between the body fluids.

The functions of the various tubules involved in the process are:

  • Glomerulus filters the blood
  • Proximal Convoluted Tubules reabsorb ions, water, and nutrients, removes toxins, and maintains the pH of the filtrate.
  • Descending Loop of Henle allows water to pass from the filtrate into the interstitial fluid through aquaporins.
  • Ascending Loop of Henle reabsorbs sodium and chloride ions from the filtrate into the interstitial fluid.
  • Distal Tubule reabsorbs and secretes selective ions and maintains the pH of the blood.
  • Collecting Duct, solutes, and water is reabsorbed from the filtrate by the collecting duct.


The urinary bladder is stretched and gets filled with urine formed in the nephrons. The receptors present on the walls of the urinary bladder send signals to the Central Nervous System, thereby, allowing the relaxation of sphincter muscles to release urine. This is known as micturition.

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