Human Lung System

Concept 3: Overview of the Human Lung System

Success Criteria

  • I can briefly describe the ecological niche of humans.

  • I can describe ventilation and gas exchange in the human lung system.


Do Now

Do Now in your books:
One of the characteristics of an efficient gas exchange syst
em is a 'steep concentration gradient'.

List the OTHER THREE characteristics of an efficient gas exchange system.

Ms. Adviento's Video of Concept 3: Overview of the Human Lung System

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Watch these videos to broaden your understanding.

Fantastic explanation of ventilation, and the journey of air and oxygen through the lung system.

Ecological Niche of Humans (Mammal)

A mammal is a taxonomic group that is characterised by a lung gas exchange system (among other things). One example of a mammal, is the HUMAN.

(Remember: Ecological niche includes where animals live (their habitat) and the resources that are obtained from that habitat (like their source of oxygen).

Humans live on land, which means they are TERRESTRIAL animals. Because they live on land, they obtain their oxygen from the oxygen that is in the air. Like all mammals, humans have a lung system as their gas exchange system, that has adaptations to try to get as much of the 21% oxygen in air as possible.

Because the air is so dry, the lung system has adaptations to help it stay moist and prevent DESSICCATION (drying out). Because air contains dust, debris, and pathogens like bacteria, fungi, and viruses (like COVID-19), the lung system has adaptations to prevent infections. You will read more about these adaptations below.

What are the parts of the human lung system?

The human lung system consists of a nasal cavity, connected to a trachea, which is connected to two bronchi - each bronchus goes into one of the two lungs. Then the bronchi are connected to around 60,000 bronchioles (300,000 per lung), which is then connected to around 600,0000,000 alveoli (300,000,000 per lung). Underneath the lungs is one diaphragm.

The lungs are enclosed inside the thoracic cavity, which is inside and protected by the rib cage. Between each rib are intercostal muscles, which work with the diaphragm to control ventilation of the lungs. You will read more about ventilation of the lungs below.

For now, here is a quick description of each of these organs in the lung system:

  • nasal cavity - where air enters and exits the lung system.

  • trachea - wide tube/airway that connects the nasal cavity to bronchi.

  • bronchi - medium-sized tubes/airways that connect the trachea to bronchioles in the lungs.

  • bronchioles - narrow tubes/airways that connect the bronchi to the alveoli.

  • alveoli - specialised respiratory surface, where gas exchange between air and blood happens.

  • lungs - respiratory organs of mammals, containing bronchi, bronchioles, alveoli, and blood vessels.

  • diaphragm - large, strong sheet of muscle below the lungs that cause ventilation.

  • intercostal muscles - muscles between ribs that cause ventilation.

  • capillaries - tiny blood vessels surrounding alveoli, transporting blood.

What's the journey of air in and out of the lungs like?

When the diaphragm contracts, air enters the lung system through the nasal cavity. Air travels down a relatively wide tube called the trachea.

The trachea is lined with MUCOUS to humidify the dry air that is coming in. The walls of the trachea are also lined with cells that have finger-like projections called CILIA that trap dirt, dust, and debris. These projections can sweep these unwanted particles stuck in the mucous, back up the trachea, to be coughed out or swallowed with saliva. It is important to prevent foreign particles going any further down into the lung system because they can cause infections.

The trachea is also very distinct because it is surrounded by RINGS OF CARTILAGE, which is not a bone, but a bendy structure that feels like your ears. These cartilage rings keep the trachea open and stops them from collapsing in during inhalation.

After the trachea, air either goes into the left bronchi or the right bronchi. This image to the left shows a resin cast of the airways and blood vessels. The yellow-tinged tubes represent the extensive branching of the airways. You can see the relatively wide tube that is the trachea, and how it splits into two smaller but still relatively wide tubes that are the left and right bronchi.

There are two bronchi because one goes into the left lung, and the other one goes into the right lung. The inside of each bronchus is lined with more MUCOUS to keep humidifying the dry air that is coming in. Each bronchus is also lined with CILIATED CELLS to sweep up unwanted particles trapped in mucous back up towards the mouth to be coughed out or swallowed. And bronchi are also lined with CARTILAGE RINGS to prevent them from collapsing in when we breathe in.

All of the air humidifying and filtering from unwanted particles happens in the trachea and bronchi because that’s where the mucus and ciliated cells are. After the bronchi, air splits off and goes into many of the bronchioles that branch off from the bronchi. Bronchioles are like the trachea and bronchi in that they are moist with mucus to humidify the air, but they are different to the trachea and bronchi in that they do not have ciliated cells or have rings of cartilage around them. After the bronchioles, air finally makes it to the specialised respiratory surface within the lungs, called alveoli.

Alveoli are really tiny air sacs that look like a bunch of grapes, surrounded by tiny blood vessels called capillaries. The walls of the alveolus AND the capillary form the specialised respiratory surface where oxygen from the inhaled air dissolves in the thin layer of fluid on the alveoli, diffuse across 1 alveolus cell and 1 capillary cell to get into the blood. The opposite happens for carbon dioxide. Carbon dioxide already dissolved in the blood, diffuses across 1 capillary cell and 1 alveolus cell to get into the air. Each alveolus is like a cul-de-sac or a “No Exit” road, because there’s only one road in and one road out, or one airway in and one airway out. So after gases have been exchanged at the alveolus, we have to exhale or breathe out to get rid of “old air” that consists mostly of carbon dioxide, in order to breathe “new air” that will have more oxygen in it.

How are the lungs ventilated?

In terms of the lungs, ventilation is another word for BREATHING. Basically, the diaphragm and the intercostal muscles work together to increase and decrease the volume inside your chest cavity, and therefore causing air to flow in and out of the lung system.

The diaphragm is clearly shown in this GIF - it is a very strong dome-shaped sheet of muscle that forms the floor of the chest cavity. This is what I mean by the diaphragm being dome shaped when it is relaxed. But as soon as it contracts during inhalation, it flattens down.

During inhalation, the diaphragm contracts and flattens and moves downwards and the intercostal muscles, (which are found in between the ribs) contract and move the ribs up and outward. This increases the volume inside the chest cavity, and decreases the pressure inside the chest cavity creating a suction force that causes air to flow in through the mouth - this is inhalation.

During exhalation, the diaphragm relaxes and returns to its original dome shape, and the intercostal muscles also relax, moving the ribs down and inward. This decreases the volume inside the chest cavity, and increases the pressure inside the chest cavity, creating a blowing force that causes causing air to flow out through the mouth - this is exhalation. I’m not sure if ‘blowing force’ is a thing, but when pressure increases inside the chest cavity, there’s nowhere else for air inside the chest cavity to go, but out into the surrounding air.

Concept 3 Task 1: Complete one of these worksheets on OneNote.

Worksheet 1: Sky Level

Worksheet 2: Sun Level (SciPAD Internals)

Concept 4: Human Lung System Adaptations & Limitations

Success Criteria

  • I can explain how specific adaptations of the lung system enable humans to survive in their terrestrial niche.

  • I can discuss the advantages and limitations of the human lung system.


Breathing / ventilation: Movement of air/water in and out of the gas exchange system.

Cartilage rings: Rigid structures that prevent the trachea and bronchi from collapsing during inhalation

Cilia: The hair-like projections on the outside of cells that move in a wavelike manner.

Desiccation: Drying out

Human: Example of a mammal with a lung system for gas exchange.

Mucous: A slimy substance produced in the nose and throat to moisten and protect them.

Terrestrial: To live on land

Do Now:

Do Now in your books:

List all of the lung system structures you would expect to see with the naked eye during our sheep lung dissection today.

Ms. Adviento's Video of Concept 4: Human Lung System Adaptations & Limitations

Click here to subscribe and stay updated with 12BIO videos:

Watch this video to broaden your understanding.

Comprehensive video in terms of adaptations that help with ventilation and gas exchange. Even gives detail about cartilate, mucociliary escalator, and limitations of tidal ventilation (dead end alveoli).

Adaptations for Ventilation and Maintaining a Steep Concentration Gradient

Diaphragm and intercostal muscles

So what specific adaptations crucial for ventilation? The first are the muscles, the diaphragm and intercostal muscles which drive the process of ventilation. Without these muscles, there's nothing causing oxygen poor air to go out and oxygen rich air to come into the lungs.

Cartilage rings around the trachea and bronchi

Trachea and bronchi are held open by cartilage rings. They’re like the rings around a vacuum cleaner hose that keeps the tube open when you’re running the vacuum cleaner. Why do they need to be held open? Because during inhalation, the suction is force is so large that without strong and rigid cartilage rings, the airways would collapse while breathing in. It’s like if the vacuum cleaner hose was made of soft plastic, the hose would collapse once you turned on the vacuum cleaner.

The concentration gradient of oxygen and carbon dioxide across the specialised respiratory surface is what drives diffusion and therefore, gas exchange. The steeper the concentration gradient across the alveoli, the more efficient gas exchange is. The human lung system has 2 adaptations for maintaining a steep concentration gradient across the alveoli.


The diaphragm and intercostal muscles work together, contract rhythmically to breathe in new oxygen-rich air to the alveoli, and breathe out old oxygen-poor air from the alveoli. Ventilation ensures that the alveoli always get a supply of new oxygen-rich air, so that the concentration of oxygen is always highest in air in the alveoli.

The opposite happens for carbon dioxide.

Circulatory system

The heart contracts rhythmically to pump keep blood flowing through the capillaries in the alveoli. Oxygen-rich blood is constantly being pumped away from the alveoli, and oxygen-poor blood is constantly being pumped towards the alveoli. The circulatory system ensures that the alveoli always get a supply of oxygen-poor blood, so that the concentration of oxygen is always lowest in the blood.

The opposite happens for carbon dioxide.

The concentration of oxygen is always highest in the air inside the alveoli, and lowest in blood inside the capillaries - so a steep concentration gradient is maintained. Because of ventilation and the circulatory system, oxygen will always diffuse from air to blood. The opposite happens for carbon dioxide.

GCSE Biology - Gas Exchange and Lungs (Cognito). Nice video that gives a clear and simple description of gas exchange in lungs.
Oxygen’s surprisingly complex journey through your body (TED-Ed). This is a very comprehensive video of oxygen transport. It covers more than what you need to know, but if you're interested in this topic, you should definitely watch this video.

Adaptations for a Large SA : V

A large SA : V is a requirement for efficient gas exchange, because the more sites for gases to enter and exit the alveoli, the faster the rate of diffusion. The human lung system has 3 adaptations to maximise the SA : V.

Extensive branching of airways / extensive folding of alveoli

There is extensive branching of the airways - as in 1 trachea branches into 2 bronchi, which branches into 60,000 bronchioles, before leading to 600,000,000 alveoli. If there was no branching of these airways, then there would be much less alveoli present. There would also be less alveoli present if there is less folding of the alveoli. If there were less alveoli, there would not be enough sites for oxygen and carbon dioxide to enter and exit the blood via diffusion.

Blocked airways and lung infections can reduce SA : V. So it is a problem that air contains debris and pathogens (e.g. viruses, bacteria, and fungi) because they can block airways or cause infections. in fact, infectious organisms thrive in the warm and moist environment of the lungs. Blocked airways prevent air from getting to alveoli, while infections destroy alveoli - significantly reducing the SA : V of alveoli. The next adaptations maintain a large SA : V by preventing airway blockages and lung infections.

Nose hairs

Small hairs inside the nostrils trap dirt, dust, debris, and pathogens (e.g. bacteria, viruses, fungi) floating in the air as you breathe in. There is also a thin layer of sticky mucus inside your nostrils that collect these unwanted particles, for you to blow out or swallow.

Mucociliary escalator

  • Muco = mucus that lines the trachea and bronchi.

  • Ciliary = cells with cilia (cells that have finger-like projections that can 'beat' or 'sway').

  • Escalator = refers to movement of unwanted particles up towards the throat.

Cilia 'beat' or 'sway' to transport a sheet of sticky mucus that has collected debris and pathogens from inhaled air. This 'beating' or 'swaying' transport back up towards the back of the throat to be swallowed. Once swallowed, these debris and pathogens will get destroyed by the stomach.

Adaptations for Moisture

Gases must first dissolve in water before they can diffuse across the specialised respiratory surface, therefore the alveoli must be kept moist. So it is a problem for humans that air is dry, because inhaling dry air could risk drying out the alveoli. The human lung system has 3 adaptations to keep the alveoli moist.

Internal lungs

The lungs are internally located inside the chest cavity, to protect them from physical damage and drying out. If the lungs were located outside of the body, they would dry out very quickly.


There is mucus lining the nasal cavity, trachea, and bronchi. This mucus contains water, so as air passes through the airways, the air mixes with water vapour from the mucus, moistening/humidifying the air. So even though the air breathed in is dry, by the time inhaled air reaches the alveoli, the air should be adequately moistened/humidified and not be as dry.


Alveolus cells secrete/release surfactant, which is a fluid that keeps the alveolus surface moist. Surfactant contains water, so it allows gases to dissolve into it, in order to diffuse across the alveoli.

Adaptations for a Short Diffusion Distance

Since diffusion is a passive process, it actually takes a relatively long time for gases to diffuse from one area to another. Diffusion of oxygen and carbon dioxide is only efficient when the diffusion distance is short. The human lung system has 2 adaptations to keep the diffusion distance short.

Thin respiratory surface

The specialised respiratory surface is only 2 cells thick - that's very thin! There are only two layers of cells that separate air from blood: one alveolar cell and one capillary cell. For oxygen to diffuse from air to blood, it only has to cross one alveolar cell, and one capillary cell (and vice versa for carbon dioxide).

Remember from Video 2: Characteristics of an Efficient Gas Exchange System, that as organisms get larger, the diffusion distance from the respiratory surface to cells of the body gets farther. Humans are actually very large animals. If the human lung system just relied on diffusion to move oxygen from alveoli to the cells in your feet, your toes will fall off because diffusion would take too long, and the cells in your toe would not get the oxygen they need for aerobic respiration.

How do humans reduce the diffusion distance from respiratory surface to cells?

Circulatory system

The circulatory system is made up of the:

  • Heart - a really strong muscle, that is constantly pumping.

  • Blood - contains red blood cells that carry oxygen and carbon dioxide.

  • Blood vessels like capillaries - tubes/pipes that blood flows through.

Blood inside the capillaries surrounding alveoli participate in gas exchange by releasing carbon dioxide waste, and absorbing oxygen. Then the heart pumps this oxygen-rich blood around the body. Oxygen is released to cells that need it for aerobic respiration. Carbon dioxide is picked up by blood, and the heart pumps this carbon dioxide-rich blood back to the lungs.

This way of transporting oxygen from alveoli to cells, and carbon dioxide from cells to alveoli is much much much faster than diffusion. If oxygen was not pumped to the rest of the body by the heart, the diffusion distance would be too far, and diffusion would take too much time. Cells would not get the oxygen they need for aerobic respiration.

With the circulatory system, the diffusion distance is greatly reduced because passive diffusion would only have to take place across the very thin alveoli (respiratory surface).

Limitations of the Lung System

There are 3 limitations of the lung system:

1) Incompatibility with water

One of the most obvious limitations of the lung system is that it cannot be used to "breathe under water". The lung system is incompatible with water for two reasons.

First, is that water is too dense/viscous to be ventilated by the diaphragm and intercostal muscles. These muscles are just not strong enough to move water in and out of the lung system. Without ventilation, gas exchange would stop because a concentration gradient of oxygen and carbon dioxide would not be maintained across the alveoli - there would be nothing driving the diffusion of oxygen and carbon dioxide.

Second, is that the lungs would not be able to exchange gases efficiently enough with water to meet the oxygen demands of aerobic respiration, and sustain life. This is because the SA : V of alveoli is not large enough to absorb enough oxygen from the 1% of oxygen available in water. The SA : V of the lung system is not compatible with the extremely low oxygen availability in water; it can only efficiently carry out gas exchange with air, which has 21% oxygen availability.

2) Tidal ventilation and residual volume

The term tidal ventilation describes how air enters and exits the lungs through the same way, just like an ocean tide comes and goes along the same beach. Air enters from the nasal cavity - trachea - bronchi - bronchioles - alveoli ... and exits through the same way from alveoli - bronchioles - bronchi - trachea - nasal cavity.

Tidal ventilation is a limitation of the lung system because the new oxygen-rich air breathed in mixes with old air trapped in the lungs. This old air is called the residual volume because it is the volume of air that is always left behind (residue) in the lungs, even after forcefully exhaling. Because it is left behind in the lungs, it is oxygen poor and carbon dioxide rich.

This mixing of new and old air during tidal ventilation lowers the oxygen concentration of the air that reaches the alveoli, thus reducing the concentration gradient across the alveoli.

3) Dead space

Dead space refers to the 30% of air that enters the lung system, but remains in the airways (nasal cavity, trachea, bronchi, and bronchioles)- so the air that never makes it to the alveoli to participate in gas exchange.

Dead space also refers to the air that makes it to the alveoli, but doesn’t participate in gas exchange. In fact, of the 70% of air that actually make it to the alveoli, only ¼ of oxygen in that air actually diffuses into the blood.

Dead space is a limitation because not all of the air inhaled gets to participate in gas exchange. Specifically, this is the limitation of the SA : V of the lung system. Not all all volume of air inhaled will have enough surface area on the alveoli to participate in gas exchange.

Concept 4 Task 1: Complete one of this worksheet on OneNote.

Concept 4 Task 2: Complete Checkpoint #1