The Human Lung System (+ sheep lung dissection)

Navigate the knowledge tree: 🌿 Biology ➡ NCEA Level 2 Biology ➡ 2.3 Plant & Animal Adaptations

Learning Intentions

By the end of this topic, you should be able to:

Explain aspects of the human ecological niche, that is relevant to gas exchange.
Describe the parts of the human lung system and where gas exchange takes place.
Explain how the human lung system is ventilated.

Glossary

Key terms and definitions.

alveoli: Millions of tiny air-filled sacs in the lungs where gas exchange occurs in mammals.

breathing: Movement of air in and out of the lungs using respiratory muscles.

bronchi: Two medium-sized airways that connect the trachea to bronchioles in each lung.

bronchioles: Many narrow, highly branched airways that connect the two bronchi to millions of alveoli.

capillaries: Tiny blood vessels that form a network surrounding the alveoli, transporting oxygenated blood away from the lungs and deoxygenated blood toward the lungs.

cartilage rings: Rigid C-shaped structures that prevent the trachea and bronchi from collapsing during inhalation.

cilia: Finger-like projections on cells that line the trachea. They move in a wave-like motion to sweep mucus and trapped dust and debris up the trachea.

desiccation: Drying out

diaphragm: A large, strong sheet of muscle below the lungs that cause ventilation in mammals.

ecological niche: The functional position of an organism in its environment (the environment being the habitat and the resources available in this habitat).

intercostal muscles: Muscles between the ribs that cause ventilation in mammals.

lungs: The respiratory organ of mammals; contains bronchi, bronchioles, alveoli and blood vessels.

mucus: A slimy substance produced in the nasal cavity and trachea to moisten and protect them.

nasal cavity: Hollow space behind the nose lined with mucus.

pressure: The force exerted by a gas (or liquid) per unit of area.

trachea (mammal): The wide airway that connects the nasal cavity to bronchi.

volume: The amount of three dimensional space a substance takes up.

Ecological Niche of Humans (Mammal)

Mammals are a taxonomic group 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 desiccation. 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.

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

www.youtube.com/watch?v=9xhxALk9gm8&t=205s&ab_channel=WeTheCurious

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 mucus 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 mucus, 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 cartilage rings, 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 mucus 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 red in the GIF below.

During exhalation, the diaphragm is relaxed and becomes dome-shaped. Do you notice how the space in the lungs gets smaller?

During inhalation, the diaphragm contracts and flattens down. Do you notice how the space in the lungs gets bigger?

This change in shape of the diaphragm changes both the volume of the chest cavity and air pressure inside the lungs, causing ventilation.

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.

More on The Human Lung System

📕 article 📸 image 🎮 interactive 🎦 video

Glossary

Key terms and definitions.

alveoli: Millions of tiny air-filled sacs in the lungs where gas exchange occurs in mammals.

breathing: Movement of air in and out of the lungs using respiratory muscles.

bronchi: Two medium-sized airways that connect the trachea to bronchioles in each lung.

bronchioles: Narrow, highly branched airways that connect the two bronchi to millions of alveoli.