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Cambridge IGCSE0610

Excretion in humans

Biology 0610 Chapter Notes

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Excretion in humans
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In this chapter

  1. 1Excretion by the lungs and kidneys
  2. 2The excretory system
  3. 3Inside the kidney
  4. 4Filtration in the glomerulus
  5. 5Reabsorption and the formation of urine
  6. 6The liver and amino acids
  7. 7Urea and deamination
  8. 8Why excretion matters

1. Excretion by the lungs and kidneys

Excretion is the removal of waste products of metabolism and substances in excess of requirements.

Carbon dioxide is excreted through the lungs. It is produced by (aerobic) respiration.

The kidneys excrete urea, excess water and excess ions.

Examiner insight

“While many candidates realised that the elimination of gas through the alveoli is an example of excretion, a minority believed this was an example of nutrition.” “A significant number of candidates incorrectly believed that undigested food is removed by excretion.”

Worked example 12 marks

Metabolism produces waste substances that are excreted.

Complete Table 2.1 about the excretion of substances.

substanceprocess that produces the substanceorgan that excretes the substance
carbon dioxide…………
……photosynthesis……
  1. 1

    carbon dioxide: produced by (aerobic) respiration and excreted by the lungs [1]

  2. 2

    photosynthesis produces oxygen, excreted by the leaf (accept stem or shoot) [1]

2. The excretory system

Four structures are named on a diagram of the excretory system: the kidney, the ureter, the bladder and the urethra.

A ureter runs from each kidney to the bladder. The urethra is the single tube that leaves the bladder. Spell ureter and urethra with care: they are different structures.

Examiner insight

“The commonest incorrect response was to identify a ureter rather than the urethra.”

Worked example 13 marks

Two tubes run from the kidneys to one organ. A single tube leaves that organ. Name the two tubes, the organ and the single tube.

  1. 1

    two tubes from the kidneys: ureters [1]

  2. 2

    organ: bladder [1]

  3. 3

    single tube leaving it: urethra (not ureter) [1]

3. Inside the kidney

A section through a kidney has two regions to identify. The cortex is the outer region. The medulla is the inner region.

Examiners often see the two names swapped. Fix the pair: outer is cortex, inner is medulla.

When you label a region, end the label line well inside it, not on the boundary.

Examiner insight

“A common error was to give cortex for medulla and vice versa.” “Several candidates placed label lines right on the boundary between the cortex and medulla and could not be awarded credit.”

Worked example 12 marks

Name the outer region and the inner region of a kidney.

  1. 1

    outer region: cortex [1]

  2. 2

    inner region: medulla [1]

4. Filtration in the glomerulus

The glomerulus is part of the nephron. In the glomerulus, water, glucose, urea and ions are filtered from the blood.

Filtration is not diffusion, and it is not reabsorption. Glucose is filtered, so the filtrate does contain glucose.

Examiner insight

“It was common to see the terms filtration and absorption mixed up, or to use diffusion interchangeably with filtration as if they were synonymous.” “Some candidates incorrectly believed that there would be no glucose in the glomerulus and nephron.”

Worked example 11 mark

What is the function of a glomerulus?

A deamination · B filtering the blood · C forming urea · D reabsorbing glucose

  1. 1

    B: filtering the blood. [1]

5. Reabsorption and the formation of urine

As the filtrate passes along the nephron, all of the glucose, some of the ions and most of the water are reabsorbed back into the blood.

The urine that is left contains urea, excess water and excess ions.

The renal artery takes blood to the kidney. The renal vein takes blood away from it.

Examiner insight

“Few explained that all the glucose that is filtered from the blood plasma is reabsorbed and few stated that urine consists of urea, excess water, and excess ions.” “Some incorrectly believed that glucose is only transported in the artery leading to the kidney.”

Worked example 11 mark

A kidney has an artery and a vein joined to it, and a ureter leaving it.

In a healthy person, which structures transport glucose?

A artery only · B artery and ureter · C artery and vein · D ureter and vein

  1. 1

    C: artery and vein. Glucose is reabsorbed back into the blood, so it is in the vein and not in the ureter. [1]

6. The liver and amino acids

The liver takes part in the assimilation of amino acids. It converts amino acids to proteins.

Examiner insight

“A common error was to match assimilation with the ileum.” “Fewer were able to state or describe assimilation or protein synthesis.”

Worked example 11 mark

What is a correct function of the structure?

structurefunction
Akidney glomerulusreabsorption of substances back into the blood
Bkidney nephronfiltration of substances from blood
Cliverassimilation of amino acids to form proteins
Dliverdeamination of proteins to form amino acids
  1. 1

    C: the liver assimilates amino acids to form proteins. [1]

7. Urea and deamination

Urea is formed in the liver from excess amino acids.

Deamination is the removal of the nitrogen-containing part of amino acids to form urea. The urea travels from the liver cells to the kidneys, where it is filtered out of the blood.

Examiner insight

“Some candidates incorrectly believed that urea is made in the kidneys.” “Another common confusion was between the role of the liver in the formation of urea by deamination and the role of the kidney in terms of excretion of urea.”

Worked example 11 mark

The table shows the composition of blood entering and leaving the kidneys and liver.

Which row is correct?

highest concentration of urea in the bloodlowest concentration of urea in the blood
Aentering kidneysleaving liver
Bentering kidneysentering liver
Cleaving kidneysentering liver
Dleaving kidneysleaving liver
  1. 1

    B: the liver forms urea and the kidneys remove it. Blood entering the liver has the lowest concentration and blood entering the kidneys has the highest. [1]

8. Why excretion matters

Excretion is important because urea is toxic. Urea is a metabolic waste product, so the body removes it.

Examiner insight

“Some candidates referred vaguely to waste products rather than metabolic waste products.” “Some candidates referred to toxic products in excess, which is inaccurate as all toxic material is in excess by definition.”

Worked example 12 marks

Urea is formed in the liver. State why urea must be excreted and name the organ that excretes it.

  1. 1

    urea is toxic [1]

  2. 2

    it is excreted by the kidneys (not the bladder, not the liver) [1]

Recap

  • Excretion removes waste products of metabolism and substances in excess of requirements.
  • Carbon dioxide is excreted through the lungs.
  • The kidneys excrete urea, excess water and excess ions.
  • Four structures to name: kidney, ureter, bladder, urethra.
  • Extended: the kidney has an outer cortex and an inner medulla.
  • Extended: the glomerulus filters water, glucose, urea and ions from the blood.
  • Extended: all of the glucose, some of the ions and most of the water are reabsorbed. Urine is urea, excess water and excess ions.
  • Extended: the liver converts amino acids to proteins. Deamination removes the nitrogen-containing part to form urea.
  • Extended: urea is excreted because it is toxic.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the organ that excretes carbon dioxide.1 mark
  2. A student writes that carbon dioxide is removed from the body by egestion. State the correct term.1 mark
  3. A student says that the bladder excretes urea. State the organ that does.1 mark
  4. State two substances, other than urea, that the kidneys excrete.2 marks
  5. (a) Name the single tube that leaves the bladder. (b) Name the pair of tubes that run from the kidneys to the bladder.2 marks
  6. Name the outer region of a kidney and the blood vessel that carries blood away from the kidney.2 marks
  7. The fluid filtered in the glomerulus contains glucose. A healthy person's urine contains no glucose. Explain why.3 marks
  8. A student says that urine is just urea dissolved in water. (a) State the three things that urine contains in a healthy person. (b) Name one substance that is filtered in the glomerulus but is not in urine.4 marks
  9. A doctor tests three fluids from one patient. The table gives the concentration of glucose and of urea in each fluid, in g per dm³. (a) Name the process that moves glucose and urea from the blood into the nephron. (b) One process in this patient's nephrons is not working as it does in a healthy person. Identify the process and use the data to support your answer. (c) A student says the urea concentration is higher in the urine because the nephron makes urea. Explain why the student is wrong and give the correct reason. | fluid | glucose | urea | | --- | --- | --- | | blood entering the glomerulus | 0.9 | 0.3 | | fluid filtered into the nephron | 0.9 | 0.3 | | urine | 0.4 | 20.0 |6 marks
  10. Patient X has a damaged liver that deaminates amino acids more slowly than a healthy liver. Patient Y has a healthy liver, but damaged kidneys that filter the blood more slowly than healthy kidneys. Both eat the same diet, which is rich in protein. (a) Describe what a healthy liver does with amino acids, including those in excess. (b) Predict how the concentration of urea in the blood of each patient compares with that of a healthy person. Give a reason for each prediction. (c) State which patient is at greater risk of harm from urea, and why.8 marks

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