Excretion in Humans Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • The lungs excrete carbon dioxide; the kidneys excrete urea, excess water and excess ions in urine.
  • The urinary system is kidneys, ureters, bladder and urethra.
  • The kidney has an outer cortex (ultrafiltration) and an inner medulla (water reabsorption).
  • In the nephron, ultrafiltration removes small molecules from the blood, then selective reabsorption returns all glucose, some ions and most water.
  • Excess amino acids are deaminated in the liver; the toxic ammonia is made into urea.

Excretory organs

The lungs excrete carbon dioxide, and the kidneys filter the blood to make urine, which passes to the bladder and out through the urethra.

Link each part of the urinary system to its function: kidneys filter, ureters carry urine, the bladder stores it and the urethra carries it out of the body.

  1. 1

    Kidneys

    Filter the blood and make urine

  2. 2

    Ureters

    Carry urine to the bladder

  3. 3

    Bladder

    Stores urine

  4. 4

    Urethra

    Carries urine out of the body

The path of urine

Which organ stores urine?

Kidney structure

The outer cortex is where blood is filtered; the inner medulla is where water is reabsorbed.

The cortex is the outer layer. It contains the first part of the nephrons, where ultrafiltration happens. The medulla is the inner region with cone-shaped structures. It contains loops of Henle and collecting ducts, where water reabsorption is regulated. Do not mix the regions up.

Labelled diagram of a nephron
Bowman's capsule and the convoluted tubules sit in the lighter outer cortex; the loop of Henle and collecting duct run into the darker medulla.

Cortex

Position:
Outer layer
Function:
Contains the first part of nephrons: ultrafiltration

Medulla

Position:
Inner region
Function:
Loops of Henle and collecting ducts: water reabsorption

Where does ultrafiltration take place?

The nephron

The nephron filters small molecules out of the blood, then takes back everything useful.

In ultrafiltration the glomerulus forces small molecules into Bowman's capsule. In selective reabsorption all the glucose, some ions and most of the water return to the blood. What remains (urea, excess water and ions) is urine. Large proteins are not filtered.

  1. 1

    Ultrafiltration

    Glomerulus forces small molecules into Bowman's capsule

  2. 2

    Selective reabsorption

    All glucose, some ions and most water return to the blood

  3. 3

    Urine formed

    Urea, excess water and excess ions remain

  4. 4

    Collecting duct

    Urine passes to the ureter

How the nephron makes urine

Filtered?

Glucose:
Yes
Urea:
Yes
Water:
Yes
Large proteins:
No

Reabsorbed?

Glucose:
All of it
Urea:
No
Water:
Most of it
Large proteins:
Not applicable

In urine?

Glucose:
No
Urea:
Yes
Water:
Excess only
Large proteins:
No

Glucose is found in a patient's urine. Why is this abnormal?

Urea

The liver breaks down excess amino acids and makes the toxic ammonia into urea.

The body cannot store excess amino acids. In the liver the nitrogen-containing part is removed (deamination), forming ammonia, which is very toxic. The liver converts it to urea, which is carried in the blood to the kidneys and excreted. The remainder of the amino acid is used in respiration.

  1. 1

    Excess amino acids

    Not needed for protein synthesis

  2. 2

    Deamination

    Nitrogen-containing group removed

  3. 3

    Ammonia

    Highly toxic

  4. 4

    Urea

    Less toxic; carried to the kidneys and excreted

Making urea

What happens to excess amino acids in the liver?

Why excretion matters

Waste products are toxic or upset the balance of the body, so excretion keeps the internal environment stable.

Urea is toxic to cells, especially the nervous system. Carbon dioxide would lower blood pH and disrupt enzymes. Excess water and ions must go so that cells do not burst or shrivel. Link excretion to a stable internal environment for enzymes and cells.

  1. 1

    Metabolism makes wastes

    Urea, carbon dioxide, excess water and ions

  2. 2

    Wastes build up

    Urea is toxic; CO2 lowers blood pH; water and ions upset cells

  3. 3

    Excretion removes them

    Kidneys remove urea, water and ions; lungs remove CO2

  4. 4

    Internal environment stays stable

    Enzymes and cells keep working

Why wastes must be removed.

Urea

Excreted by:
Kidneys
Why it must be removed:
Toxic to cells

Carbon dioxide

Excreted by:
Lungs
Why it must be removed:
Lowers blood pH and disrupts enzymes

Excess water and ions

Excreted by:
Kidneys
Why it must be removed:
Stop cells bursting or shrivelling

What can happen if the kidneys fail to excrete urea?

Try an exam question

Describe how urea is formed in the body and how it is removed.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.