11) Gas exchange in humans

11) Gas exchange in humans


Characteristics of respiratory surfaces:

The exchange of oxygen and carbon dioxide across a respiratory surface, as in the lungs, depends on the diffusion of these two gases. Diffusion occurs more rapidly if:

  • There is a large surface area exposed to the gas.
  • The distance across which diffusion has to take place is small.
  • There is a good blood supply.
  • There is a big difference in the concentrations of the gas at two points brought about by ventilation.


Lung structure:

  • The lungs are enclosed in the thorax.
  • They have a spongy texture and can be expanded and compressed by movements of the thorax in such a way that air is sucked in and blown out.
  • The lungs are joined to the back of the mouth by the windpipe or trachea.
  • The trachea divides into two smaller tubes, called bronchi, which enters the lungs and divide into bronchioles.
  • These small branches end in microscopic air sacs called alveoli.
  • The epiglottis and other structures at the top of the trachea stop food and drink from entering the air passages when we swallow.
  • The larynx manipulates pitch and volume.
  • The diaphragm is a sheet of tissue that separates the thorax from the abdomen.
  • Ribs form a cage to protect the lungs and heart, and to move to ventilate the lungs.
  • Intercostal muscles are muscles between the ribs which raise the ribcage by contracting and lower it by relaxing.


Ventilation of the lungs:

  • The movement of air into and out of the lungs, called ventilation, renews the oxygen supply in the lungs and removes the surplus carbon dioxide.
  • Horseshoe-shaped hoops of cartilage are present in the trachea and bronchi to prevent them collapsing when we breathe in.
  • The lungs contain no muscle fibres and are made to expand and contract by movements of the ribs and diaphragm.



  1. the internal intercostal muscles relax and the external intercostal muscles contract, pulling the ribcage upwards and outwards
  2. the diaphragm contracts, pulling downwards
  3. the volume in the thorax gets bigger, forcing the lungs to expand, and the air pressure inside decreases
  4. air is pushed into the lungs



  1. the external intercostal muscles relax and the internal intercostal muscles contract, pulling the ribcage downwards and inwards
  2. the diaphragm relaxes, moving back upwards
  3. The lungs are elastic and shrink back to their relaxed volume and the air pressure inside increases
  4. air is pushed out of the lungs

Gaseous exchange:

Gaseous exchange refers to the exchange of oxygen and carbon dioxide, which takes place between the air and the blood vessels in the lungs.

Gas exchange in the lungs happens in the alveoli. Some of the features of alveoli include:

  • thin walls (just one cell thick)
  • large surface area
  • moist surface
  • many blood capillaries
    • Some of the oxygen is absorbed into the bloodstream when it enters the alveoli, resulting in a reduction of oxygen.
    • Cells of the body produce CO2 as a waste product during aerobic respiration. The bloodstream carries CO2 to the lungs for excretion and diffuses across the walls of the alveoli to be expired, resulting in a 100% increase of carbon dioxide breathed out.
    • The lining of the alveoli is coated with a firm of moisture in which oxygen dissolves. Some of this moisture evaporates into the alveoli and saturates the air with water vapour. The air you breathe out, therefore, always contains a great deal more water vapour than the air you breathe in.


    Limewater turns milky in the presence of carbon dioxide, so it can be used to show the differences between inhaled (inspired) air and exhaled (expired) air. The lime water immediately turns milky on contact with exhaled air.


    Breathing rate and exercise:

    • The increased rate and depth of breathing during exercise allows more oxygen to dissolve in the blood and supply the active muscles.
    • The extra carbon dioxide that the muscles put into the blood is detected by the brain, which instructs the intercostal muscles and diaphragm muscles to contract and relax more rapidly, increasing the breathing rate.
    • Carbon dioxide will be removed by the faster, deeper breathing.
    • The rate of breathing can be measured by counting the number of breaths in one minute. The depth of breathing can be measured using a spirometer (a device that measures the volume of air inhaled and exhaled).
    • To investigate the effects of exercise on breathing, record the rate of breathing for a few minutes when the person is at rest. After they do some exercise, record their rate of breathing every minute until it returns to the normal resting value.


    Protection of the gas exchange system from pathogens and particles:

    Pathogens are present in the air we breathe in and are potentially dangerous of not actively removed. There are two types of cells that provide mechanisms to help achieve this.

    • Goblet cells are found in the epithelial lining of the trachea, bronchi and some bronchioles of the respiratory tract. Their role is to secrete mucus. The mucus forms a thin film over the internal lining. This sticky liquid traps pathogens and small particles, preventing them from entering the alveoli where they could cause infection or physical damage.
    • Ciliated cells are also present in the epithelial lining of the respiratory tract. They are in a continually flicking motion to move the mucus, secreted by the goblet cells, upwards and away from the lungs. When the mucus reaches the top of the trachea, it passes down the gullet during normal swallowing.