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  1. Identify on diagrams and name the larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries.


The capillary network around the alveoli are the associated capillaries.


  1. List the features of gas exchange surfaces in animals.
  • Alveolar walls are very thin – only a single cell thick – reducing the diffusion distance.
  • These walls are moist, to prevent the cells from drying out, and to allow the gases to dissolve – making diffusion easier.
  • They have a very high surface area, making it possible for large amounts of gas to diffuse at the same time.
  • They have very steep concentration gradients maintained by the movement of blood and the inspiration and expiration of air.

During gas exchange, oxygen diffuses from the alveoli, across the alveolar membrane and capillary wall, into the bloodstream, to be picked up by the RBCs. Carbon dioxide diffuses from the blood into the alveoli. This causes the partial pressure of oxygen in the alveoli to dwindle and the partial pressure of carbon dioxide in the alveoli to increase.

Note: partial pressure is the pressure of one gas in a mixture of gases. It is proportional to its concentration.

Therefore, inspiration allows the dwindling supply of oxygen in the alveoli to be replenished, and expiration allows the maintenance of a low carbon dioxide concentration.

The steady flow of blood prevents oxygen from building up and keeps bringing more carbon dioxide close to the alveoli.

These two processes help maintain the steep oxygen and carbon dioxide concentration gradient.


  1. Explain the role of mucus and cilia in protecting the gas exchange system from pathogens and particles.

A thin layer of mucus lines your trachea, bronchus and bronchioles. It is a sticky substance produced by cells called goblet cells. This sticky substance traps dust particles, smoke particles and pathogens.

Cilia are the small finger-like projections found on the cell membranes of the epithelial cells that line the upper respiratory tract. The cells with cilia are ciliated epithelial cells. These cells are found from your nose to your bronchi, and in some bronchioles.

Cilia sweeps mucus up and out of the respiratory tract by a beating motion, and into your mouth so that it can be swallowed into your alimentary canal. This helps destroy any pathogens trapped in the mucus and prevents the build-up of mucus and pathogens in the respiratory tract.


  1. Describe the effects of tobacco smoke and its major toxic components (tar, nicotine, carbon monoxide, smoke particles) on the gas exchange system.

Tar is a carcinogen – it increases the risk of cancer. It is deposited along the airways. It irritates goblet cells, causing them to produce more mucus, and damages/ paralyses ciliated epithelial cells, so mucus builds up and blocks the airways. This can result in chronic bronchitis.

Nicotine is an addictive substance. It increases heart rate and blood pressure.

Carbon monoxide is a toxic gas. It combines permanently with haemoglobin, preventing it from binding to and transporting oxygen.

Smoke particles irritate the air passages. This increases mucus production and causes the airways to inflame, possibly resulting in blockages. This could also result in chronic bronchitis. The presence of smoke particles in the alveoli may also lead to emphysema – the breaking of alveoli walls. Smoke particles also result in heavy coughing – the body’s attempt to expel these particles.


  1. State the differences in composition between inspired and expired air.

Inspired and expired notes


  1. Use limewater as a test for carbon dioxide to investigate the differences in composition between inspired and expired air.

We use lime water (Ca(OH)2), because it turns cloudy/ milky when carbon dioxide is bubbled through. How milky it appears is proportional to the amount of carbon dioxide bubbled through it.

Inspired air is the same as the air around us so we can fill a balloon with a known volume of the air around us.

In order to collect expired air, we can fit a balloon to one end of a glass tube and breathe into the other end, in order to fill the balloon with the air we exhale.

Set up apparatus so that we have two containers filled with limewater, each with a delivery tube that has one end submerged in the limewater.

Using a gas syringe, we can take a known volume of gas from each balloon, and feed it into their respective delivery tubes. The milkier limewater has more carbon dioxide.


  1. Investigate and describe the effects of physical activity on rate and depth of breathing.

To measure the rate of breathing, simply use a stopwatch, and count the number of breaths (one breath being one inspiration and one expiration) that the person takes in one minute. You now have the number of breaths this person takes per minute.

Physical activity results in an increasing in breathing rate.

Physical activity also results in an increase in breathing depth.


In normal breathing, the volume of air breathed in and out is usually about 0.5 litres (this is the tidal volume), and the breathing rate is about 12 breaths per minute.

During exercise, the inspired and expired volume increases to about 5 litres – this depends on the age, sex, size and fitness of the person.

The maximum amount of air breathed in and out in one breath is the vital capacity of a person.

The breathing rate can increase to over 20 breaths a minute.

The total lung capacity is greater than the vital capacity because some air always remains in the lungs (this is the residual volume), otherwise, the airways would collapse.


  1. Explain the effects of physical activity on rate and depth of breathing.

Breathing rate and depth increase during exercise, so that more oxygen can be absorbed per unit time, as exercising muscles need to respire more to produce more energy.



Notes  Submitted by Sarah.

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