CORE, Excretion in Humans, EXTENDED, IGCSE Biology, IGCSE Revision

Excretion in Humans

For those writing CIE IGCSE examinations, this section covers both the CORE and SUPPLEMENTARY components (TOPIC 13) of the 0610 syllabus (accurate in 2017). Please note that these revision notes SHOULD NOT be used as a substitute for comprehensively studying the course, but are advised to be used as revision tools to help with concepts and ‘cramming’. As always – enjoy the resource, study hard and all the best!

Due to the toxicity of Carbon Dioxide (CO2) and urea, humans need to remove these products from the body as quickly as possible. The removal of these waste products is called excretion.

Excretion is defined as “the removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements”.

Excretion is is carried out by the lungsskin, and the kidneys.

The Lungs

In animals, Carbon Dioxide (CO2), from respiration, is EXCRETED by the lungs, gills, or other gas exchange surfaces.

  • CO2 is made in a cell, during respiration, as the result of a metabolic reaction. As it is a waste product (something the body cannot use), it is excreted.

Nitrogenous waste is also produced, from excess proteins and amino acids.

  • To remove this waste from the body, urea is formed in the liver from the excess amino acids.

Deamination is “the removal of the nitrogen-containing part of amino acids to form urea.


  1. After consumption, protein is taken to the alimentary canal.
  2. During digestiondigestive enzymes in the iliumduodenum and stomach break down the protein molecules into amino acids.
  3. These amino acids are then absorbed into the the blood capillaries in the villi of the ilium.
  4. Blood capillaries join up to the hepatic portal vein, which take the absorbed food to the liver.
  5. Amino acids that are needed are released into the blood. If not needed (in excess), they are deaminated.
  6. The role of the liver is to assimilate amino acids by converting them to proteins, including plasma proteins (such as fibrinogen); enzymes in the liver split each amino acid molecule: energy is kept, turned into carbohydrates and stored in the liver; that which contains nitrogen is turned into ammonia and then urea.
  7. The urea is then dissolved in the blood plasma and taken to the kidneys; some of this urea is excreted in sweat.

The Kidneys

There are two kidneys at the back of the abdomen, behind the intestines. They have 3 main parts: the cortexmedulla and pelvis (from which a tube called the ureter stems).

kidneys makes urine >> ureter carries urine >> bladder stores urine >> urethra excretes urine.

Each kidney is full of thousands of tubules or nephrons, which loop from the cortex to medulla, back to the cortex, through the medulla to the pelvis, where the nephrons join up with the ureter.

The kidneys are excretion organs and part of the urinary and excretory systems. The kidneys  function as filters; they filter things out of the blood under high pressure and then reabsorb the useful things. The resulting product (after this filtration) is urine.

The kidneys filter:

  1.  urea
    • urea is produced in the livefrom excess amino acids.
    • kidneys remove urea from the blood 
  2. excess water
    • kidneys adjust water content of the blood
  3. salts
    • kidneys adjust salt levels of the blood

Filtration takes place in the renal capsules (thousands of which reside within the cortex of each kidney).

The kidneys are full of tubulesKidney tubules/ Nephrons play an imperative role in the filtration process.

Nephrons are the filtration units in the kidneys. As blood passes through them, they go through a process of ultrafiltrationreabsorption and the release of wastes.


  1. Ultrafiltration
    1. Blood from the renal artery flows through the glomerulus (a bundle of capillaries at the start of the nephron).
    2. The high pressure, which is built up, squeezes waterureasalts and glucose out of the blood and into the Bowman’s Capsule (renal capsule).
    3. Between the blood vessels in the glomerulus and the Bowman’s Capsule are membranes that act as filters. These membranes ensure that big molecules, such as proteins and blood cells, are not squeezed out; they stay in the blood. The remaining filtered liquid in the Bowman’s Capsule is known as the glomerular filtrate.
  2. Reabsorption
    1. As the filtrate flows through the nephron, useful substances are selectively reabsorbed back into the blood, through capillaries:
      • all glucose is reabsorbed from the proximal convoluted tubule (involves active transport)
      • sufficient salt is reabsorbed
      • sufficient water is reabsorbed from the collecting duct into the bloodstream
  3. Release of Wastes
    1. Remaining substances (inc. watersalts, and urea) form urine*.
    2. This flows out of the nephron, through the ureter, down to the bladder (where it is temporarily stored), before being released through the urethra.

When the bladder is full, the sphincter muscle opens so that the urine flows out of the body.

The volume and concentration of urine produced is affected by water intake, temperature and exercise.

*In the urine of a healthy person, there will be no concentration of protein.

>> Ensure you know and can identify on drawings, diagrams and images, the uretersbladder and urethra.


kidney machine or dialysis unit is used to filter the blood, as the kidneys would.

  • Urea diffuses out of the blood
  •  The amount of other substances (such as glucose and salt) in the blood can be regulated by controlling the dialysis fluid (This takes place through diffusion, down a concentration gradient).

There are a number of advantages and disadvantages for kidney transplants and dialysis:

  • ADVANTAGES for transplants
    • dialysis no longer has to take place
  • DISADVANTAGES for transplants
    • tissue type of donor must closely match or the recipient’s body (immune system) will reject the transplanted organ.
    • the recipient is given immunosuppressants in order to stop the immune system from rejecting the organ; these immunosuppressants stop the immune system from working properly, which means that the recipient is more susceptible to disease.
    • These drugs (immunosuppressants) have to be taken for the rest of the recipient’s life.
  • ADVANTAGES for dialysis
    • does the work of damaged kidneys
  • DISADVANTAGES for dialysis
    • patients have to be treated 2/3 times a week and the process takes several hours.



Egestion is NOT excretion.

Some food cannot be digested (such as cellulose). These substances go straight through the alimentary canal and out of the anus as faeces.

Substances that are egested are not excreted, as they have never been involved in any metabolic reaction and pass unchanged thorough the digestive system.


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