The process, by which waste product of metabolism from the system of an organism are eliminated from the body.

Organs of the Excretory System

  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

Kidneys perform several homeostatic functions:

  1. Maintain volume of extracellular fluid
  2. Maintain ionic balance in extracellular fluid
  3. Maintain pH and osmotic concentration of the extracellular fluid.
  4. Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.

Functioning of the kidney

The kidney removes metabolic and liquid toxic wastes as well as excess water from the organism.

Parts of the kidney

Kidney parts                          kidney and bladder


  • Renal Arteries – 2 renal arteries constantly transport blood to the kidneys.
  • Kidneys – 2 kidneys composed of millions of nephrons constantly filter about 170 to 200 litres of blood to produce about 1.5 to 2 litres of urine daily.
  • Renal Veins – 2 renal veins return useful nutrients back into the bloodstream.
  • Ureters – 2 ureters carry urine from the kidneys to the urinary bladder.
  • Urinary Bladder – The urinary bladder temporarily stores urine until it is released from the body.
  • Urethra – The urethra is the tube that carries urine from the urinary bladder to the outside of the body. The outer end of the urethra is controlled by a circular muscle called a sphincter.

Within each kidney there are an estimated one million microscopic nephrons, where blood filtration takes place. Each nephron contains a cluster of capillaries called a glomerulus. A cup-shaped sac called a bowmans capsule surrounds each glomerulus. The blood that flows through the glomerulus is under great pressure. This causes water, glucose and urea to enter the bowmans capsule. White blood cells, red blood cells and proteins remain in the blood. As the blood continues in the excretory system, it passes through the renal tubule. During this time, reabsorption occurs: glucose and chemicals such as potassium, sodium, hydrogen, magnesium and calcium are reabsorbed into the blood. Almost all the water removed during filtration returns to the blood during the reabsorption phase. The kidneys control the amount of liquid in our bodies. Now only wastes are in the nephron. These wastes are called urine and include urea, water and inorganic salts. The cleansed blood goes into veins that carry the blood from the kidneys and back to the heart.


Each kidney has very minute tubular and convoluted structures known as urniferous tubules. Nephron has a double walled cup shaped structure called bowman’s capsule at its upper end.

The bowman’s capsule has numerous capillaries called glomerulus. The short region after the Bowman’s capsule is called the neck. After this the tubule is narrow and coiled. It consists of a proximal convoluted region, a Henle’s loop and a distal convoluted tubule. The post end of nephron is called collecting tubule.

Collecting tubule opens into the renal pelvis, which opens into the ureter

The waste material along with blood is brought to kidneys by the renal arteries. Blood is filtered out from blood capillaries into Bowman’s capsule under pressure which is known as Ultrafiltration. This filtrate passes through the lumen of tubular parts of nephron. During this useful products such as water, glucose, amino acid, materials, ions etc. are reabsorbedby blood capillaries surrounding the nephron. The remaining fluid contains excretory substance and is called urine. From the ureter urine passes into urinary bladder where is is stored. When the bladder is filled with urine, it contracts and urine passes out of the body.





2 Layers of the Skin

    1. Epidermis – outer protective layer without blood vessels
    2. Dermis – inner layer containing blood vessels, sensory nerve endings, sweat and


Functions of the Skin

  1. Excretion – Wastes such as excess water, salt, urea and uric acid are removed from the body in sweat.
  2. Waterproofing – The skin with its oil glands prevents the entry of water into, and loss of water out of the body.
  3. Protection from Disease – The intact skin prevents invasion of micro-organisms and dust into the body.
  4. Protection from Ultraviolet Rays – Pigments reduce the intake of UV rays.
  5. Regulation of Body Temperature – The thin layer of fat cells in the dermis insulates the body. Contraction of small muscles attached to hairs forms ‘goosebumps’ and creates an insulating blanket of warm air. Also, sweat produced by sweat glands uses excess body heat to evaporate, providing a cooling effect.
  6. Sensory Detection – The nerve endings or receptors in the dermis detect heat, cold, touch, pressure and pain.

Excretion in amoeba

The amoeba lives in fresh water bodies like lakes, slow moving streams and ponds. Amoeba excretes excess carbon dioxide, water and ammonia. Ammonia is excreted across the cell membrane although it can also be excreted by the contractile vacuole when it dissolves in water.

Carbon dioxide is excreted by diffusing across the cell membrane. Osmoregulation is achieved by the contractile vacuole. The excess water collects in the contractile vacuole and then it migrates and fuses with the cell membrane hence releasing the water to the outside.

Excretion in insects

Insects excrete carbon dioxide and uric acid crystals. Uric acid being a nitrogenous waste product is excreted by the malpighian tubules which are found between the junctions of the mid gut projecting into the blood filled cavity.

Excretion in annelids

Excretion in the annelids such as the earthworms is carried out by the niphridia which are found in each segment. They release the waste products through the opening s found on the body surface

Excretion in birds

Birds excrete carbon dioxide and uric acid. They use the lungs to excrete carbon dioxide and the kidney to excrete uric acid.

Excretion in plants

The main excretory products include water and oxygen. These wastes diffuse out of the plant through the stomata and lenticels of stems as they are formed.

Other plant wastes include; tannins, alkaloids, anthocyanins which are converted into insoluble compounds like granules and oil droplets which remain in the cells and are got rid of when certain parts of the plants e.g fruits, leaves and flowers fall off from the plants.

Excretion in plants differs from that in animals because of the following reasons

  1. Plants have got a lower metabolic rate compared to animals therefore the rate of accumulation of metabolic waste is very low
  2. Plants are autotrophs and therefore synthesize their own organic requirements according to the demand for them. There are never excess proteins in plants therefore very little excretion of nitrogenous wastes
  3. Plants have a capacity to store excretory products in some structures where they can be lost at a later stage e.g. fruits, flowers, leaves, barks etc.
  4. Much of the plant structure is based on carbohydrate and not protein. The products of carbohydrates carbon dioxide and oxygen can be used in photosynthesis as raw materials. The oxygen given out as a byproduct of photosynthesis can be used in respiration

Osmoregulation in plants

Plants are divided into four main groups depending on the amount of water available to them. They are; halophytes, mesophytes and hydrophytes, xerophytes.


These are plants which live partially or completely submerged in water. They have thin or no cuticle at all, no vascular tissue and reduced root systems because water is readily available to them, they also have many stomata on the upper surface.


These are plants that live in salty waters; they have special cells which have a higher concentration of solute than those of the ordinary plants. As a result they are able to take up water in the normal way


These are plants which grow in normal water well watered soils and water lost by transpiration is replaced by absorption. They have no special means of conserving water although most of them have a well developed root system.


These are plants which live in arid conditions such as deserts, and have a problem of dehydration.

They have the following adaptations for their survival

  1. Some xerophytes have thick waxy cuticle impermeable to water  e.g. cactus
  2. Some have leaves modified into thorns or spines to reduce the surface area which minimizes on the rate of transpiration
  3. Some shed off their leaves  (deciduous) to reduce on the transpiration through the leaves
  4. Some roll their leaves to trap still and damp air that reduces transpiration.
  5. Some have sunken stomata which are guarded by hairs, the hair traps moisture which reduces on the rate of transpiration
  6. Some have succulent tissues like stems which store water
  7. They have a well developed tap root system for absorbing water from deep areas

46 responses to “Excretion

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