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Excretion is the process by which waste products and toxic materials are removed from the body of an organism.

Excretory organs:

  • Kidneys
    • The kidneys remove urea and other nitrogenous waste from the blood. They also expel excess water, salts, hormones and drugs.
  • Lungs
    • The lungs supply the body with oxygen, but they are also excretory organs because they get rid of carbon dioxide. They also lose a great deal of water vapour, but this loss is unavoidable and is not a method of controlling water content of the body.
  • Liver
    • The liver breakdown haemoglobin to produce yellow/green bile pigment, bilirubin. Bilirubin is excreted with the bile into the small intestine and expelled with the faeces. The pigment undergoes changes in the intestine and is largely responsible for the brown colour of the faeces.
  • Skin
    • Sweat consists of water, with sodium chloride (salt) and traces of urea dissolved in it. These substances are excreted when we sweat. But, sweating is a response to rise in temperature and not a change in the blood composition. Therefore, skin is not an excretory organ.


  • Structures
    • Capsule
    • Cortex
    • Medulla containing pyramids and renal pelvis
    • Nephron or kidney tubule consisting of
      • Malpighian corpuscle (renal capsule and glomerulus)
      • proximal convoluted tubule
      • loop of Henle
      • distal convoluted tubule
      • collecting duct
  • Functions
    • Excretes excess water, salts and metabolic waste products such as urea, uric acid and creatinine in the form of urine
    • Two main processes involved in formation of urine
      • ultrafiltration
      • selective reabsorption
    • Acts as osmoregulators by regulating the water and solute levels in the blood to maintain a constant water potential in the body
    • The reabsorption of water by the kidney tubules is controlled by antidiuretic hormone (ADH)
      • Failure to function leads to kidney failure

Kidney failure

  • Kidney failure may result from an accident involving a drop in blood pressure, or from a disease of the kidneys.
  • A dialysis machine is required to replace the function of the kidneys should the kidneys are damaged.


1. What happens to blood as it passes through a kidney machine?
a. carbon dioxide is removed
b. glucose is added
c. oxygen is added
d. urea is removed

2. Some of the structures in the excretory system are listed
1 bladder
2 ureter
3 urethra

In which order does a molecule of urea pass through these structures?
        First -----------> Last
a.        1        2        3
b.        1        3        2
c.        2        1        3
d.        3        1        2

3. How is a working kidney dialysis machine similar to a healthy kidney?
a. it takes sugar molecules out of the blood
b. it regulates the concentration of the blood
c. it deaminates amino acids to urea
d. it removes large molecules from the blood

4. What is an example of excretion?
a. release of adrenaline from the adrenal glands
b. release of sweat from the sweat glands
c. removal of carbon dioxide from the lungs
d. removal of faeces from the alimentary canal

5. What passes through the membranes of a kidney machine?
a. protein and red blood cells
b. urea and red blood cells
c. water and protein
d. water and urea

6. The most accurate description of excretion is that the body is eliminating
a. waste products of metabolism
b. undigested food materials from the intestines
c. unwanted products of respiration
d. water and urea from the kidneys

7. What happens when a person drinks a large volume of water?
 amount of ADH secretedre-absorption of water from kidney tubulevolume of urine produced
alessless     more

MCQ Answers

1. d
2. c
3. b
4. c
5. d
6. a
7. a

Structured Question Worked Solutions

1. Describe how the body removes urea.

  • Urea is formed in the liver by the deamination of excess amino acids
  • It is then transported in the blood to the kidneys where it is removed through the formation of urine.
  • As blood passes from the renal artery into the glomerulus in the renal capsules, water and small molecules - glucose, amino acids, mineral salts, and nitrogenous waste products - are forced out of the glomerular blood capillaries into the renal capsule by the high blood pressure in the glomerulus, forming the filtrate.
  • The filter in the glomerulus is a partially permeable membrane that wraps around the glomerular blood capillaries and allows only water and very small molecules to pass through. This is ultrafiltration.
  • Useful materials are then selectively reabsorbed as the filtrate passes through the tubule.
  • Most of the water, some mineral salts and all the glucose and amino acids are reabsorbed through the walls of the tubule into the surrounding blood capillaries, while nitrogenous waste products such as urea, and excess water and mineral salts pass along the kidney tubule and out through the collecting duct into the renal pelvis as urine.
2. Explain what happens to urine volume when
a. a person exercises vigorously
b. a person eats a meal with high salt content


a. When a person exercises vigorously, he/she will lose a lot of water and some salts through perspiration. Urine volume will decrease as the body reabsorbs more water from the glomerular filtrate.

b. Eating a meal with high salt concentration will lower the water potential of the blood. Urine volume will decrease as the kidneys reabsorb more water from the glomerular filtrate. More salts will also be excreted through the urine.