Structure of the human breathing system

Air passes through a branching set of tubes to reach tiny air sacs in the lungs. Blood vessels surround these sacs, bringing blood close to the air.

Air passages and alveoli

Air passes through the larynx and down the trachea, a tube in the neck and chest. The trachea divides into two bronchi; each bronchus leads into one lung.

Inside the lungs, the bronchi branch into smaller tubes called bronchioles. These end in tiny air sacs called alveoli. A network of small blood vessels called capillaries surrounds the alveoli, placing the blood close to the air inside them.

In a diagram, the trachea can be recognised as the single tube above the point where it divides into the two bronchi. Follow the branches inside a lung to find the narrower bronchioles. Alveoli may be shown as clusters of small sacs at their ends, with capillaries around them.

Structures around the lungs

The ribs surround the chest. Intercostal muscles lie between neighbouring ribs. Beneath the lungs is the diaphragm, a sheet of muscle that separates the chest from the abdomen.

In a diagram, look for the ribs around the lungs, the intercostal muscles between them, and the diaphragm forming a boundary below the lungs.

Features of human gas exchange surfaces

The alveoli are tiny air sacs in the lungs, surrounded by blood capillaries. Oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli. Diffusion is the net movement of particles from a higher concentration to a lower concentration.

Features that make gas exchange efficient

• Large surface area: The lungs contain many alveoli, providing a large total surface over which gases can diffuse.
• Short diffusion distance: The walls of the alveoli and surrounding capillaries are very thin, so oxygen and carbon dioxide have only a short distance to travel between the air and the blood.
• Good blood supply: Blood flowing through the capillaries carries oxygen away and brings carbon dioxide to the alveoli. This helps maintain the concentration differences that drive diffusion.
• Good ventilation: Breathing replaces air in the alveoli, bringing in oxygen and removing carbon dioxide. This also helps maintain the concentration differences between the air and the blood.

Together, these features allow oxygen and carbon dioxide to diffuse efficiently in opposite directions.

Inspired and expired air

Inspired air is air breathed into the lungs; expired air is air breathed out. Expired air contains less oxygen and more carbon dioxide and water vapour than inspired air.

In the lungs, oxygen passes from the air into the blood. At the same time, carbon dioxide passes from the blood into the air in the lungs and is breathed out. Air also gains water vapour as it passes through the breathing system.

Testing air for carbon dioxide

Limewater turns cloudy when carbon dioxide is passed through it. This can be used to compare the carbon dioxide in inspired and expired air.

Pass inspired air through one sample of fresh limewater and expired air through another. Use equal amounts of limewater and pass comparable volumes of air through them at the same rate. The limewater receiving expired air turns cloudy more quickly, showing that expired air contains more carbon dioxide.

Effect of physical activity on breathing

During physical activity, breathing becomes faster and deeper. Breathing rate is the number of breaths taken in a given time. Breathing depth is the amount of air moved in each breath.

Working muscles respire more rapidly than they do at rest. They therefore need more oxygen and produce more carbon dioxide. Faster, deeper breathing moves more air into and out of the lungs, helping the body take in more oxygen and remove more carbon dioxide.

Investigating breathing after physical activity

Breathing rate and depth can be compared at rest and immediately after physical activity:

1. At rest, count the breaths taken in a set time. Observe or measure breathing depth.
2. After a period of physical activity, count the breaths over the same length of time and observe or measure depth in the same way.
3. Compare the resting and post-activity observations.

The methods must be consistent for the comparison to be meaningful. For example, if chest movement is used to indicate breathing depth, observe it in the same way both times. Greater chest movement can indicate deeper breathing, but it does not give an exact volume of air.

Mechanics of ventilation

Ventilation moves air into and out of the lungs. Muscles change the volume of the thorax (the chest cavity), causing the lungs to expand or become smaller. This changes the air pressure inside the lungs: air flows in when that pressure falls below atmospheric pressure and flows out when it rises above atmospheric pressure.

The intercostal muscles lie between the ribs in two layers. The external intercostal muscles are nearer the outside of the chest wall, while the internal intercostal muscles lie beneath them. The diaphragm is a sheet of muscle beneath the lungs.

Inhalation

The external intercostal muscles contract, moving the ribs upwards and outwards. At the same time, the diaphragm contracts and flattens. These movements increase the volume of the thorax, so the lungs expand and the pressure of the air inside them falls below atmospheric pressure. Air then enters the lungs.

Exhalation

The diaphragm relaxes and becomes more dome-shaped, while the ribs move downwards and inwards. The volume of the thorax decreases, so the lungs become smaller and the pressure of the air inside them rises above atmospheric pressure. Air then leaves the lungs.

During active exhalation, the internal intercostal muscles contract to pull the ribs downwards and inwards, helping to reduce thoracic volume.

Keeping the airway open

Air passes through the trachea as it enters and leaves the lungs. Cartilage supports the trachea and prevents it from collapsing, keeping this airway open.

Carbon dioxide control of breathing

During physical activity, working muscles respire more and produce more carbon dioxide. This carbon dioxide enters the blood, increasing its concentration there.

The brain detects the increased carbon dioxide concentration and causes breathing to become faster and deeper. More air moves into and out of the lungs, so more carbon dioxide is removed from the blood and breathed out.

Mucociliary defence of the breathing system

The airways contain cells that trap inhaled material and move it away from the lungs. This helps stop particles and pathogens, such as disease-causing microorganisms, from reaching the gas exchange surfaces.

Goblet cells produce mucus, a sticky substance that coats the airway lining. Inhaled particles and pathogens become trapped in the mucus rather than continuing deeper into the lungs.

Ciliated cells have tiny hair-like structures called cilia on their surfaces. The cilia beat to move the mucus towards the throat, carrying the trapped material away from the lungs. This combined action of mucus and cilia is called mucociliary defence.