Gas exchange

  • Created by: AIV17
  • Created on: 17-03-21 06:06

Human gas exchange system

*Larynx is where trachea is marked

** Trachea is red while cartilage is white

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Gas exchange surfaces and air composition

Features of gas exchange surfaces

  • They are thin so that gases diffuse across them quickly, are close to a transport system to take gases to and from the exchange surface, have large surface area so a lot of gas diffuses at the same time and have good supply of oxygen

Differences in composition between inspired and expired air

Inspired air: 21% O₂, 0.04 % CO₂, variable water content (humidity). Expired air: 16% O₂, 4% CO₂, high humidity

Reasons: Oxygen is absorbed across the surface, then used by cells in respiration. CO₂ is made inside respiring cells and diffuses out across the gas exchange surface. Gas exchange surfaces are made of living cells, so must be moist; but some of it evaporates into air.

Test for CO₂ to investigate differences in composition: Set up 2 tubes with limewater (clear to cloudy) or hydrogencarbonate indicator solution (red to yellow), and a rubber tubing (must be sterilised). Then breathe in and out gently (not too hard!) through the tubing until the liquid on one of the tubes changes colour

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Investigation and explanation of effects of exerci

  • Measuring rate of breathing: Use stopwatch and count number of breaths in one minute
  • Exercise increase rate and depth of breathing
  • Normal breathing: volume of air breathed is 0.5 litres and the rate is 12-14 breaths/minute.
  • Exercise: Volume of air breathed is 5 litres depending on age, gender, size and fitness, and the rate increases to over 20 breaths/minute


  • The muscles work harder and respire aerobically, so they need more oxygen to continue working
  • Extra energy is produced by anaerobic respiration, also releasing lactic acid. This is removed by combining it with oxygen in the liver. The breathing rate does not decrease until all the acid is used up; repaying the oxygen debt

In terms of CO2 concentration, more is produced as the respiration rate increases, entering blood. It causes pH to fall (also lactic acid), affecting enzymes. When blood passes through brain, the increase stimulates receptor cells to send nerve impulses to diaphragm and intercostal muscles to contract faster and more strongly, increasing frequency and depth of breathing until CO2 concentration is lowered enough

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  • It can cause chronic obstructive pulmonary disease (COPD), lung cancer and coronary heart disease
  • Nicotine is addictive, and a stimulant for the brain. It alters circulatory system by narrowing blood vessels and increasing its pressure -causing hypertension
  • Tar has carcinogens, which make cells in the respiratory passages and the lungs divide, forming a tumour. If malignant, then it is cancer. Cells may break away from it and expand to other parts of the body, where new tumours will grow.
  • Carbon monoxide combines with haemoglobin - less oxygen can be carried, ending up with deprivation.
  • Smoke particles get trapped in lungs, so white blood cells secrete chemical to remove them, but these damage the lungs, causing COPD. The walls of the alveoli tend to break down, reducing surface area for gas exchange (also oxygen). This is emphysema
  • Some chemicals make goblet cells produce more mucus, sweeping it to the lungs with infectious bacteria that can also difficult diffusion of O₂ and CO₂
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Goblet cells and alveoli

  • Goblet cells secrete mucus, in which microorganisms and particles of dust get trapped.
  • Cilia sweep the mucus up towards the back of the throat, where it is swallowed
  • Mucus protects the lungs from pathogens and too many particles which can cause inflammation
  • Capillaries surround the alveolus, O₂ diffuses across the walls of the alveolus into blood and CO₂ diffuses the other way

The walls of the alveolus

  • Very thin (1 cell thick). Capillary walls are the same, so O₂ molecule only has to diffuse across this thickness to get into the blood
  • They have an excellent transport system, for the capillaries are constantly pumped with blood from the lungs. CO₂ can diffuse out into air spaces and O₂ into blood, which is then taken back to the heart
  • Large surface area
  • Good supply of O₂ due to breathing movements
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