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  1. Name and identify the lungs, diaphragm, ribs, intercostal muscles, 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 humans, limited to large surface area, thin surface, good blood supply and good ventilation with air
  • Alveolar walls are very thin – only a single cell thick – reducing the diffusion distance.
  • Capillary walls are very thin – only a single cell thick – reducing diffusion distance.
  • Alveolar walls are moist, to prevent the cells from drying out and to allow the gases to dissolve in the water on the alveolar walls. This reduces diffusion distance.
  • Alveoli have a very high surface area: volume ratio, making diffusion easier.
  • Collectively, the alveoli have a huge surface area, making it possible for large amounts of gas to diffuse at the same time.
  • Good blood supply and proper ventilation ensure that steep carbon dioxide and oxygen concentration gradients are maintained.

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 enables 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. State the differences in composition between inspired and expired air limited to oxygen, carbon dioxide and water vapour

Inspired air vs expired air table


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

The air in our atmosphere is typically made up of 21% oxygen and 0.04% carbon dioxide. The amount of water vapour in our air can vary, depending on where we are in the world, the climate, etc.

When you inspire air, you are breathing in the air in our atmosphere. This is why inspired air has 21% oxygen, 0.04% carbon dioxide and variable water vapour levels.

While that air is in your alveoli, it loses oxygen to your blood and picks up carbon dioxide from your blood. Because the inside of your body is moist – your mouth has saliva, your nasal cavity and airways are lined with mucus, and your alveoli have a thin layer of water inside – a lot of the water evaporates into the air that you inspired.

So when you expire, the air that you expired has less oxygen, more carbon dioxide and more water vapour than atmospheric air. This is why expired air has 16% oxygen, 4% carbon dioxide and is saturated with water vapour.

Note: inspiration (noun) (verb = inspire) is when you breathe in air. Expiration (noun) (verb = expire) is when you breathe out air. A saturated solution is a solution containing the maximum amount of solute. Air saturated with water vapour means the air contains the maximum amount of water vapour it can carry.


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

We use limewater (Ca(OH)2), because it turns cloudy/ milky when carbon dioxide is bubbled through it. 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. Clip the end of the balloon closed.

To collect expired air, we can fit a balloon to one end of a glass tube and breathe into the other end, to fill the balloon with the air we exhale. Clip the end of the balloon closed to prevent the expired air from escaping.

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 increase 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 – 14 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 in terms of the increased carbon dioxide concentration in the blood, causing an increased rate of breathing

When we exercise, our muscles are working harder. This means they need more energy, so their rate of respiration increases.

Remember, the equation for respiration is

Glucose + oxygen -> carbon dioxide + water (+ energy)

As the rate of respiration increases, the amount of CO2 produced increases. This CO2 is picked up by the blood, increasing the carbon dioxide concentration in the blood. Specialised sensory nerve cells called chemoreceptors pick up this increase in carbon dioxide concentration and sends this information to the brain. The brain sends nerve impulses to the muscles that control ventilation to increase the rate and depth of your breathing.

Increased rate of breathing means you can expel carbon dioxide from your body faster, making the carbon dioxide concentration gradient steeper. This helps carbon dioxide diffuse out of your blood more quickly.

Increased depth of breathing means that you can breathe in a higher volume of oxygen per breath, increasing your oxygen concentration gradient. This allows more oxygen to diffuse into your blood.

Note: the main muscles that control ventilation are your intercostal muscles and your diaphragm. During inspiration, your diaphragm contracts, making it move down. Your external intercostal muscles also contract, causing your ribs to move outwards and upwards. This causes the volume of your lungs to increase, making the air in the atmosphere rush inside to fill up the excess space.

During expiration, your diaphragm relaxes and your internal intercostal muscles contract. This makes your diaphragm move up, and your ribs move down and inwards. This causes the volume of your lungs to fall, pushing the excess air out into the atmosphere.


  1. Explain the role of goblet cells, mucus and ciliated cells 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 sweep mucus up and out of the respiratory tract by a beating motion, and into your mouth. That mucus can then 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. State that tobacco smoking can cause chronic obstructive pulmonary disease (COPD), lung cancer and coronary heart disease

Tobacco smoking can cause COPD, lung cancer and coronary heart disease (CHD).


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

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 COPD.

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.



Notes submitted by Sarah

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