Mammalian Gas Exchange

Test your knowledge with free interactive questions on Seneca — used by over 10 million students.

Structure

Mammalian gas exchange takes place in specialised organs called the lungs. The lungs are specialised for the quick exchange of oxygen and carbon dioxide with the bloodstream.

Structure

The structure of the lungs is designed for efficient gas exchange.
Air enters the body through the trachea and travels through a highly branched system where surface area is maximised for exchange of oxygen and carbon dioxide.

Trachea

The trachea is the entrance to the mammalian gas exchange system.
When you breathe in, air flows through the trachea.
Ridges of cartilage surround the front of the trachea to provide protection and structure.
- There is no cartilage at the back of the trachea so that the oesophagus (the tube where food travels) is not constricted.

Bronchi

The trachea divides into two bronchi.
Air flows along each bronchus to a lung.
The bronchi are made from cartilage and smooth muscle.
The cartilage provides support. Smooth muscle can contract and dilate to control the size of the airway.

Bronchioles

Each bronchus divides into many smaller bronchioles.
The many bronchioles branch throughout the lungs into small air-sacs called alveoli.
Bronchioles do not contain any cartilage. Patency is maintained via elastic fibers. Bronchioles are surrounded by a thin layer of smooth muscle.
- Smooth muscle contracts to limit airflow to the alveoli. Smooth muscle dilates to increase airflow to the alveoli.

Alveoli

The alveoli are sacs that fill with air when you breathe in.
Oxygen in the alveoli diffuses into the bloodstream and carbon dioxide in the bloodstream diffuses into the alveoli.
The alveoli provide a large surface area for gas exchange.
Alveoli have a very good blood supply, with alveolar sacs being right next to capillaries.
Alveoli do not have any cartilage or smooth muscle.
They contain elastic fibres which stretch during inhalation to allow the alveoli to fill with air. They recoil during exhalation to expel air.

Ciliated epithelium

The trachea, bronchi, and large bronchioles are lined with ciliated epithelium.
Goblet cells are the mucus-secreting cells of ciliated epithelium. This mucus traps dust, particles, and pathogens.
Ciliated cells have multiple, thin projections from their surface ('cilia").
- Cilia contract and "waft" the mucus produced by goblet cells up the trachea to the back of the throat, where it is swallowed.
Any swallowed pathogens are destroyed by stomach acid.

Alveoli

Alveoli are the millions of air sacs within the lungs where gases are exchanged with the bloodstream. They provide a large surface area for gas exchange.

Capillaries

Each alveolus is surrounded by a network of capillaries.
The many capillaries provide a large surface area for gas exchange between the alveoli and the bloodstream.

Alveolar epithelium

The alveolar epithelium is made up of a single layer of squamous epithelial cells that line the walls of the alveoli.
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus.
In the alveoli, the squamous cells are arranged in a single layer to facilitate gas diffusion.
- The squamous epithelium provides a very short diffusion distance from the alveoli to the capillaries which maximises the rate of gas exchange.

Concentration gradient

The capillaries supply carbon dioxide to the alveoli and oxygen is rapidly carried away from the alveoli.
The quick transport of gases in the bloodstream maintains a steep concentration gradient of oxygen and carbon dioxide.
The steep concentration gradient allows quick diffusion of gases into and out of the bloodstream.