Comparing Inspired and Expired Air

Section 1 of 11Inspired and expired air

Inspired air

• Also called inhaled air, it is drawn from the atmosphere so its composition matches atmospheric levels.

Expired air

• Also called exhaled air, its composition differs because gas exchange has taken place in the alveoli.

Gas exchange

• Oxygen enters the blood from the alveoli while carbon dioxide and water vapour pass from the blood into the alveoli.

Section 2 of 11Composition compared

Gas	Inspired air (%)	Expired air (%)
Oxygen	21	16
Carbon dioxide	0.04	4
Water vapour	0.2 – 4 (lower)	1 – 5 (higher)
Nitrogen	78	78

Section 3 of 11Reasons for the differences (Supplement)

Gas	Reason
Oxygen	Respiring cells use it, so returning blood has a lower oxygen concentration than alveolar air and oxygen diffuses into the blood
Carbon dioxide	Produced by respiration and carried to the lungs, where it diffuses into the alveoli because it is more concentrated in the blood
Water vapour	Evaporates from the moist lining of the alveoli because of the warmth of the body
Nitrogen	Very stable and cannot be used by the body, so its concentration does not change

Section 4 of 11Limewater test for carbon dioxide

1

Set up two boiling tubes of limewater, tube A for air breathed in and tube B for air breathed out.

2

Breathe in through tube A so atmospheric air bubbles through it.

3

Breathe out into tube B so expired air bubbles through it.

4

Compare the limewater: it stays clear in tube A and turns cloudy or milky in tube B.

5

Conclude that expired air contains a higher percentage of carbon dioxide than inspired air.

Section 5 of 11Investigating the effect of exercise on breathing

1

Count the breaths taken in 1 minute at rest.

2

Measure the average chest expansion over five breaths with a tape measure held around the chest.

3

Exercise for a set time of at least 3 minutes.

4

Immediately afterwards, count the breaths in one minute and measure chest expansion again.

5

Compare: the number of breaths per minute and the chest expansion both increase after exercise.

Section 6 of 11Effect of exercise

Rate and depth

• Exercise increases both the frequency and the depth of breathing.

Recovery

• Breathing stays raised for a time after exercise and then gradually returns to the resting rate.

Section 7 of 11Why breathing increases (Supplement)

1

Muscle cells respire faster, so carbon dioxide levels in the cells rise.

2

Carbon dioxide diffuses into the blood plasma and makes it slightly more acidic.

3

Chemoreceptors in the brain detect the higher carbon dioxide level.

4

The brain sends impulses to the intercostal muscles and diaphragm.

5

The rate and depth of breathing increase, removing carbon dioxide and supplying more oxygen.

Section 8 of 11Oxygen debt (Supplement)

Anaerobic respiration

• If muscles cannot get enough oxygen they also respire anaerobically and produce lactic acid.

Effect of lactic acid

• It lowers the pH of cells and can denature enzymes, so it must be removed.

Repaying the debt

• Lactic acid is removed using oxygen, which keeps breathing fast and deep after exercise.

Measuring it

• The longer breathing takes to return to normal, the more lactic acid was produced and the greater the debt.

Section 9 of 11More from the specification

More from the specification

Respiratory mucus barrier function

• Respiratory mucus barrier function goblet cells secrete sticky mucus whose primary protective function traps airborne particulates before reaching alveoli.

Volume

• The amount of cytoplasm needing supplies and producing waste.

Feature

• An observable characteristic.

Trachea

• The windpipe, held open by C-shaped cartilage rings.

Thorax

• Chest cavity

Exchange surface

• A surface adapted for efficient movement of substances.

Concentration gradient

• The difference in concentration of a substance between two regions.

Inspiration and expiration

• Inspiration and expiration alternative terms for breathing in (inhalation) and breathing out (exhalation) that you may meet in exam questions.

Section 10 of 11Key terms

Term	Meaning
Inspired air	Air breathed in, with atmospheric composition.
Expired air	Air breathed out, with less oxygen and more carbon dioxide and water vapour.
Limewater	A clear solution that turns cloudy with carbon dioxide.
Chemoreceptor	A cell that detects chemical changes such as carbon dioxide level.
Lactic acid	Product of anaerobic respiration in muscles.
Oxygen debt	Extra oxygen needed after exercise to remove lactic acid.

Section 11 of 11Exam technique

1

Saying expired air has no oxygen — it contains about 16% oxygen.

2

Saying nitrogen changes in breathing — it is unused and stays at about 78%.

3

Saying limewater turns cloudy with oxygen — it turns cloudy with carbon dioxide.

4

Saying only depth of breathing changes in exercise — both rate and depth increase.