Structure of the Human Gas Exchange System

  • Created by: Han2812
  • Created on: 13-01-14 10:28

Reasons for Gas Exchange in Mammals

All aerobic organisms need a constant supply of oxygen to release energy in the form of ATP during respiration. CO2 produced by this process needs to be removed as a build up could be fatal.

The volume of oxygen that has that has to be absorbed and the volume of carbon dioxide that must be removed are large in mammals because:

  • They are relatively large organisms with a large volume of living cells
  • They maintain a high body temperature and have a high metabolic and respiratory rates

Because of this, mammals have developed lungs, to have efficient gas exchange between the air and blood

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Structure of the Lungs

The lungs:

  • Pair of lobed structures made up of series of branched tubules, bronchioles
  • End in tiny air sacs, alveoli


  • Flexible airway, supported by rings of cartilage - prevents the trachea collapsing as the air pressure falls inside when breathing
  • Walls are made up of muscle, lined with epithelium and goblet cells - produce mucus that traps dirt particles and bacteria when air is breathed in
  • The cilia moves the mucus down to the throat, passes down to oesophagus to the stomach

The Bronchi:

  • Two divisions of the trechea - each leading to the lungs. Pretty large
  • Similar in structure to trachea + produces mucus 
  • The larger bronchi are supported by cartilage, although the amount of cartilage is reduced as the bronchi get smaller
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Structure of the Lungs cont.

The Bronchioles:

  • Series of branching subdivisions of the bronchi
  • Walls made up of muscle lined with epithelial cells
  • The muscle allows the to constrict to control the airflow in and out of the alveoli

The Alveoli:

  • Tiny air sacs - diameter 100 micrometers and 300 micrometers
  • At the end of the bronchioles
  • Contain some collagen and elastic fibres - lined with epithelium
  • The elastic fibres allow the alveoli to stretch as they fill with air when breathing in
  • They spring back during breathing out - to expel the CO2 rich air
  • The alveolar membrane is the gas-exchange surface
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Mechanism of Breathing - Inspiration


Breathing is an active process (it uses energy)


  • The external intercostal muscles contract, while the internal intercostal muscles relax
  • The ribs pull upwards and outwards, volume in thorax increases
  • The diaphragm muscles contract, flatterning it - increasing the volume of the thorax
  • The increased volume of the thorax results in the reduction of pressure in the lungs (LOWERS AIR PRESSURE IN LUNGS)
  • Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into the lungs
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Mechanism of Breathing - Expiration


Breathing out is largerly a passive process (it does not require much energy):

  • The internal intercostal muscles contract, the external intercostal muscles relax
  • The ribs move downwards and inwards - decreasing volume of the thorax
  • The diaphram muscles relax, returning it to its domed position - decreasing the volume in the thorax
  • The decreased volume increases pressure in the lungs
  • The pulmonary pressure is now greater than the atmosphere - air is forced OUT of the lungs

During normal quiet breathing, the recoil of the elastic fibres is less, forcing less air out.

During exercise or strenuous activities do the various muscles play a part

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Pulmonary Ventilation

To work out how much air is taken in, expelled and left in the lungs can be worked out by using pulmonary ventilation

Pulmonary ventilation is the total volume of air that is moved into the lungs during one minute. To calculate this we multiplay two factors:

  • Tidal Volume - the volume of air normally taken in at each breath when the body is at rest. Usually at 0.5 dm3
  • Ventilation (breathing rate) - the number of breaths taken in a minute or a time frame to work out to a minute. For 1 min - usually 12-20 breaths for an adult

Pulmonary Ventilation = Tidal Volume x Ventilation Rate

           (dm3 min-1)                  (dm3)                (min-1)

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Essential Features of Exchange Surfaces

To have efficient gas exchange, exchange surfaces need to have these characteristics:

  • Large Surface Area : Volume ratio --> speed up rate of exchange
  • Very Thin --> Keep diffusion pathway short so quicker rate of exchange
  • Partially Permeable --> Allow selected materials to diffuse across
  • Movement of the Environmenal Medium --> Eg. AIR - To maintain a diffusion gradient
  • Movement of the Internal Medium --> E.g BLOOD - maintain a diffusion gradient

Diffusion is proportional to :



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Role of the Alveoli in Gas Exchange

There are around 300 million alveoli in each human lung --> Total surface area = 70m2 

Around each alveolus is linned with epithelial cells and surronded by a network of CAPILLARIES

Diffusion here is quick because:

  • Capillaries are so narrow (7-10um) that the red blood cells are pushed up and flatterned out against the walls --> distance is reduced 
  • Walls of capillaries and red blood cells are thin --> short distance for diffusion
  • Alveoli and pulmonary capillaries have very LARGE TOTAL SURFACE AREA
  • Blood flow in capillaries is slow --> more time for diffusion
  • Steep concentration gradient --> blood flow of heart + ventilation from breathing is constant
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Some spelling issues

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