Biology AS AQA: The Lungs

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The Lungs

Basics and the Alveoli

Two basic functions of lungs and surrounding structures:

· Get as much air as possible in close contact with blood

· To ventilate the gas exchange surfaces, the alveoli, so they have a constant supply of fresh air with high oxygen and low carbon dioxide concentration.

The numerous alveoli are the actual respiratory surface. These are essentially air sacs supplied with gases via a system of tubes, the trachea and eventually the two bronchi.

The walls of the alveoli are essentially a single layer of flattened epithelial cells as are the walls of the capillaries so those makes for a thin layer so gases can diffuse quickly.

The alveoli are kept moist with water which diffuses into alveoli from alveoli cells. This water also contains a soapy surfactant which works to stop the alveoli collapsing and to reduce surface tension. The alveoli contain phagocyte cells which kill any bacteria not trapped by mucus.

Oxygen dissolves into this water and diffuses through the cells into the blood where it is absorbed by the haemoglobin in the red blood cells. The blood supply is extensive and so oxygen is quickly carried away round the body.

How are the lungs attached to ribs?

Lungs are simply elastic sponges, can’t move on their own. Only move because of movement of ribs and diaphragm. Each lung is covered by a thin pleural membrane, ribs also lined with this. Between the two is the pleural cavity, a fluid-filled space.

This sticks the lungs to the ribs and so when the ribs move, as do the lungs, they will slide over each other but difficult to pull them apart, like wet glass sheets.

When the ribs and diaphragm contract, the volume of the thorax is increased and the lung expands and air fills the alveoli. The rib cage protects the lungs.


Secreted by special cells in the alveolar walls, this lowers the surface tension of the fluid layer lining the alveolus, keeping the alveoli open and so reducing the amount of effort needed to breathe in and inflate the lungs.

Lung Tissues

· Ciliated Columnar epithelium: Lines larger tubes such as bronchi, prevents bacteria and dust from entering lung tissue so the alveoli continue working efficiently.

· Squamous Epithelium: These thin cells form the alveoli and allow the rapid diffusion of oxygen and carbon dioxide into the blood capillaries.

Gaseous Exchange

Breathing is essentially down to pressure, air moves down a pressure gradient = from high pressure to low pressure areas.

The steep concentration gradient across the respiratory surface is maintained by:

· Blood flow on one side

· Air flow on the other

This means that oxygen can always diffuse down its concentration gradient from the air to the blood and simultaneously, carbon dioxide can diffuse down its concentration gradient from the blood to the air. This movement of respective elements is known as ventilation and has two stages, inspiration and expiration. This is controlled by nervous impulses from


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