Excretion in plants & animals

Excretory/urinary system consists of :
(1) The kidneys                          : The excretory organ
(2) The ureters                           : The ducts which drain out urine from the kidneys
(3) The urinary bladder              : The urinary reservoir
(4) The urethra                           : The channel to the exterior


The human excretory system
1. The metabolic activities in the body generates many kinds of wastes including nitrogenous wastes which are harmful for the body and hence needed to be removed. Excretion is a process by which these wastes are removed from our body.

2. Unicellular organisms remove these wastes by simple diffusion.


Formation of Urine

  • Each kidney contains many filtration units called as nephrons.
  • Nephrons are made up of a cluster of thin walled capillaries called glomerulus which is associated with a cup like structure called as Bowman’s capsule and the long tube which terminates through this capsule.
  • The renal artery brings oxygenated blood to the kidneys along with the nitrogenous wastes like urea and uric acid and many other substances.
  • The blood gets filtered through the glomerulus and this filtrate enters the tubular part of nephron.
  • As this filtrate moves down the tubular part, glucose, amino acids, salts and excess of water gets selectively reabsorbed by the blood vessels surrounding these tubules.
  • The amount of water reabsorbed depends upon :
    • How much excess of water is there in the body and,
    • How much nitrogenous wastes need to be excreted out.
  • So the fluid now flowing in the tubular part is urine which gets collected in collecting ducts of nephrons.
  • These collecting ducts together leave the kidney at a common point by forming the ureter.
  • Each ureter drains the urine in the urinary bladder where it is stored until the pressure of expanded bladder leads to an urge to pass it out through urethra.
  • This bladder is a muscular structure which is under nervous control.
  • 180 litres of filtrate is formed daily but only 2 litres is excreted out as urine so the rest is reabsorbed in the body.

Functions of Nephron

  • Excretion of nitrogenous wastes.
  • To maintain the water and ionic balance (osmic regulation).


The urine formation involves three steps :
1. Glomerular filtration : Nitrogenous wastes, glucose water, amino acid filter from the blood into Bowman Capsule of the nephron.
2. Tubular reabsorption : Now, useful substances from the filtrate are reabsorbed back by capillaries surrounding the nephron.
3. Secretion : Urea, extra water and salts are secreted into the tubule which open up into the collecting duct & then into the ureter.

Excretion in Plants
Plants use different strategies for excretion of different products :

  • Oxygen and carbon dioxide is diffused through stomata.
  • Excess water is removed by transpiration.
  • Plants can even loose some of their old parts like old leaves and bark of tree.
  • Other waste products like raisins and gums especially in old xylem cells which can also be lost by plants.
  • Plants also secrete some waste substances into the soil around them.

Artificial Kidney
Haemodialysis : The process of purifying blood by an artificial kidney. It is meant for kidney failure patients.