3. Diffusion gas exchange in the human lungs - plus comments on COPD and ventilators

The lungs are the means of transferring oxygen from air to the blood stream (blood plasma) and to remove the waste gas carbon dioxide.

Know and understand that in humans:

The surface area of the lungs is greatly increased by the alveoli - millions of tiny air sacs of the end of the tiny bronchiole tubes in the lungs where the gas exchange by diffusion takes place, but some basic stuff before we reach the alveoli.

Bar your head, the thorax is the upper part of you body (between the neck and abdomen) and separated from the lower part of your body by the diaphragm muscle.

The lungs are rather like a large pink 'sponge' protected by the rib cage and surrounded by pleural membranes which aid the movement of the chest wall.

When you breathe in, the air passes down the trachea which splits into two tubes called bronchi, two bronchus, each of which goes to one of your two lungs.

The bronchi split into lots of smaller tubes called bronchioles which finally end in small sacs called alveoli surrounded by tiny blood vessels to effect the gas exchange (O₂ in <=> CO₂ out)..

The diagram shows the connection between the mouth, windpipe (trachea), bronchus, bronchiole, lungs with their alveolus and alveoli sub-structures.

Know and understand that the lungs are in the upper part of the body (thorax), protected by the ribcage and separated from the lower part of the body (abdomen) by the diaphragm.

You should be able to recognise these structures of the lungs in the diagram where the lungs are in the thorax.

The ribcage physically protects the lungs and heart from being easily crushed and damaged.

In between the ribs are the intercostal muscles which help to move air in an out of the lungs (ventilate).

To increase the efficiency of gas exchange in the lungs the bronchus divides in two (the bronchi), so each lung gets a good supply of air.

Each bronchus divides and divides into many bronchioles with a tiny sac at the end of each one - the alveoli - tiny sacs that considerably increases the area for oxygen and carbon dioxide gas exchange.

The 'ventilation' pathway for air (including oxygen) from breathing in through your mouth or nose is:

inhaled air ==> trachea ==> bronchus ==> bronchiole ==> alveoli ==> individual alveolus (air sac) in the lungs

The mouth and nasal passages filter, warm and moisten the inhaled air, before finally reaching the lungs.

Know and understand that the breathing system takes air into and out of the body so that oxygen from the air can diffuse into the bloodstream for respiration, and waste carbon dioxide from respiration, can diffuse out of the bloodstream into the air.

This gas exchange happens in the lungs which has millions of tiny air sacs called alveoli at the ends of the finest bronchiole tubes - a large surface area for gas exchange.

Surrounding the alveoli are many small arteries (fine capillaries) bringing a good supply of 'dark red' deoxygenated blood to the lungs - the thin walls of the fine capillaries of the small arteries mean a short distance to enable faster diffusion rates for the gases and they form a large surface area for gas exchange.

The gas exchange occurs on the specialised moist thin membrane surfaces of the alveoli and the fine blood vessels - the moisture in the membranes is good for dissolving gases and increases the rate of gaseous diffusion.

When the blood from the rest of the body arrives at the alveoli in the lungs it contains a relatively high concentration of carbon dioxide and low concentration of oxygen.

This maximises the diffusion concentration gradients for the gas exchange i.e. the blood to absorb fresh oxygen from the alveoli and the expulsion of carbon dioxide from the blood in breathing out.

Direction of diffusion gradients - from high to low concentration:

When air enters the alveoli it has a greater concentration than the deoxygenated blood.

The steep concentration gradient produces very efficient diffusion of oxygen into the blood.

O₂ air in lungs ==> alveoli ==> blood, favours oxygen transfer by diffusion through the alveoli membranes

Deoxygenated blood has a greater concentration of carbon dioxide than the external air, so it will diffuse out of the blood.

CO₂ blood ==> alveoli ==> air in lungs, favours carbon dioxide transfer by diffusion through alveoli membranes

Therefore the oxygen diffuses out of the air into the blood capillaries of the alveoli (from high to low concentration) and carbon dioxide diffuses out in the opposite direction from the blood to the air in the lungs (again, from high to low concentration).

So, oxygen, from breathing in, is transferred from the air in the alveoli into the fine veins which carry the 'bright red' oxygenated blood away to where it is needed in the rest of the body.

Simultaneously carbon dioxide diffuses in the opposite direction, from the deoxygenated blood into the alveoli and breathed out.

The alveoli are well designed by evolution to perform this gas exchange efficiently - refer to repeated diagrams above.

(from left to right with increasing detail)

 

Alveoli are very efficient exchange surfaces and the adaptations to increase the rate of transfer of gas molecules are:

(i) The alveoli have a huge surface area because of their tiny spherical sac like structure,

 (smaller spheres have a larger surface area : volume ratio than larger spheres. For a given radius: surface area / volume = 3 / r. For more on this see adaptations page.)

(ii) The sac walls are very thin, only one cell thick, to reduce diffusion distance and hence reduce diffusion time - giving a faster rate of gas exchange,

(iii) The cell membrane lining is moist to dissolve gases which can diffuse down their concentration gradients across the exchange surface.

(iv) The alveoli have an excellent blood supply from numerous tiny blood vessels - vein and artery capillaries. Each alveolus is surrounded by blood capillaries that ensure efficient transfer and the gas exchange can function down the steepest concentration gradients.

 

BREATHING (NOT respiration): Know and understand that to make air move into the lungs the ribcage moves out and up and the diaphragm becomes flatter.

Know these changes are reversed to make air move out of the lungs.

Know the movement of air into and out of the lungs is known as ventilation - the mechanism of breathing.

You should be able to describe the mechanism by which ventilation takes place, including the relaxation and contraction of muscles leading to changes in pressure in the thorax.

The lungs consist of soft sponge-like tissue protected by the rib cage.

The diaphragm is a muscle located underneath the ribcage. It moves up when it relaxes and down when it contracts.

In the trachea, or windpipe, there are tracheal rings, also known as tracheal cartilages which are strong and flexible tissue that help support the trachea while still allowing it to move and flex during breathing.

Breathing involves changes in the thorax that produce ventilation of the lung.

As you breathe in, the intercostal muscles contract, expanding the rib cage, and the diaphragm also contracts making it flatter, both of which increase the volume of the lungs.

This has the effect of decreasing the pressure in the lungs and allowing fresh air to be easily drawn in, the air will flow in to the lungs naturally, due to the pressure difference between the air in the lungs (lower pressure) and the 'outside' air (higher pressure).

Overall the air is drawn in down through the trachea which splits in two tubes (bronchi, 2 bronchus) and further splitting of the airways into the smaller tubes of the bronchioles with the tiny air sacs called alveoli at their ends, where the gas exchange takes place.

In breathing out, the intercostal muscles relax (ribcage contracts), the diaphragm relaxes and moves up, so the combined effect is to decrease the volume of the lungs and increase the air pressure and waste air is expelled from the lungs.

 

Measurement of lung volume

Lung volume is the quantity of air you can breathe in for a single breath and varies from person to person e.g.

children will have a smaller lung volume than adults, taller people tend to have larger lung volumes and rib cages.

Lung volume can be measured with a spirometer machine, which, after you breathe in to full capacity, you breathe out through a tube connected to the machine which measures the volume of expelled air.

Your effective lung volume and surface area for gas exchange can be reduced by disease e.g. emphysema or cancer, both can be caused by inhalation of dust (e.g. miners) and smokers (tar and particulates).

 

Breathing rate

Most people, most of the time when at rest, breathe in and out about 12 to 16 times a minute.

Your breathing rate will increase if you are engaged in more intense aerobic activity like running it may increase to 40 to 60 times a minute.

You can do a simple time experiment:

e.g. lets say you breathe in and out 75 times in 5 minutes when just sitting down.

breathing rate = 75 / 5 = 15 times/minute.

Repeat the experiment when briskly walking and you will find your rate of breathing increased because of increasing muscle demands for more oxygen and expulsion of waste carbon dioxide.

 

Chronic obstructive pulmonary disease (COPD)

Chronic obstructive pulmonary disease (COPD) is the name for a group of lung conditions that cause breathing difficulties.

They include emphysema – damage to the air sacs in the lungs and chronic bronchitis – long-term inflammation of the airways

COPD is a common condition that mainly affects middle-aged or older adults who smoke.

The problems are often caused by long-term exposure to irritants (particles in tobacco smoke, any kind of fine dust e.g. mineral or coal, atmospheric particles from vehicle exhaust) which damage and destroy the walls of the alveoli.

This means the gas exchange in the lungs (O₂ <=> CO₂) is not as efficient as it needs to be.

For COPD sufferers, the breathing problems tend to get gradually worse over time and can limit your normal activities (readily become 'out of breath'), although medication treatment can help keep the condition under control.

 

Pandemic footnote in June 2020

As I'm adding this section on COPD, we are in the middle of the Covid-19 coronavirus pandemic.

In the more serious cases, people are suffering from breathing problems due to the virus causing inflammation in the lungs - and this is where artificial ventilation systems using oxygen are used to save lives.

 

Artificial ventilators to aid breathing

Ventilators move air into and out of a persons lungs, where they cannot work unaided. This may be because some injury or medical condition or undergoing an operation, which prevents them from breathing normally.

This used to be done by a large 'capsule' called an 'iron lung' which encased the whole body of the patient except for the head.

The pressure in the capsule is mechanically lowered to allow the lungs to expand and take in air and then raised to make the lungs contract and expel air.

However the blood flow in the lower part of the body can be poor and giving rise to poor circulation side effects.

Modern ventilators work by pumping air in a go/stop cycle, using a mouth piece connection, directly into the lungs to expand them and push out the ribcage.

When the pump temporarily stops, the ribcage relaxes, contracting the lungs and expelling the air.

This is a much more convenient method with a wide range of applications, and, it doesn't interfere with the body's blood supply, but there can be problems if the alveoli (may burst) can't cope with the artificially increased air supply.

 

Extra oxygen supply

In some cases you can use oxygen directly from a cylinder to increase the concentration of oxygen in the air breathed in.

In fact, for many people, with breathing difficulties, may use an extra oxygen supply by connection from a gas cylinder of oxygen to a mouthpiece for breathing from.

 

See also

Human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

Possible practical work

You can use sensors, eg spirometers, to measure air flow and lung volume 

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

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Gas Exchange in the Human Lungs

1. Why Do We Need Gas Exchange?

• The body needs oxygen (O₂) for cellular respiration and must get rid of carbon dioxide (CO₂) - a waste product.

• This exchange happens in the lungs, particularly in alveoli.

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2. Structure of the Lungs (Quick Recap)

• Trachea (windpipe) → bronchi → bronchioles → alveoli

• Lungs contain millions of alveoli—tiny air sacs surrounded by blood capillaries.

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3. How Does Gas Exchange Work?

• Diffusion is the process by which these gases move.

• It occurs down a concentration gradient.

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4. Features of Alveoli That Make Them Good at Gas Exchange

Large surface area

Thin walls (only one cell thick)

Moist lining (gases dissolve before diffusing)

Rich blood supply (maintains steep concentration gradients)

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Medical Conditions Affecting Breathing

1. Chronic Obstructive Pulmonary Disease (COPD)

• COPD is a group of lung diseases including chronic bronchitis and emphysema.

• It causes narrowed airways, damaged alveoli, and difficulty breathing.

Key Effects:

• Less surface area for gas exchange.

• Thicker alveolar walls, reducing diffusion rate.

• Trapped air leads to less oxygen being absorbed.

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2. Asthma

• Airways become inflamed and constricted.

• Triggers include allergens, cold air, and exercise.

• Inhalers deliver bronchodilators to widen airways.

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Medical Treatments and Technologies

1. Medications

• Bronchodilators: Relax airway muscles.

• Steroids: Reduce inflammation in airways.

• Mucolytics: Thins the mucus for easier breathing.

2. Ventilators

Used when people can’t breathe properly on their own:

• Mechanical ventilators move air in and out of the lungs.

• Some patients have to use oxygen via a cylinder and face mask.

• Used during surgery, in ICU, or for severe COPD or COVID-19 cases.

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Exam Tips

Always mention diffusion when explaining gas exchange.

For COPD, link symptoms to reduced surface area and less efficient diffusion.

Practice longer questions, like: “Explain why gas exchange is less effective in someone with emphysema.”

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Summary of learning objectives and key words or phrases

Be able to describe and explain the causes and consequences of suffering from chronic obstructive pulmonary disease (COPD).

Know that artificial ventilators can be used to aid breathing, increase breathing rate and effectively make the lung volume larger.

Be able to describe the gas exchange in human lungs by diffusion.

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