The structure and function of the cardio-respiratory system

• Gas exchange in the lungs

  • It is a 2 way process
  • Oxygen in the alveoli diffuses into the blood in the capillary.
  • The oxygen bonds with haemoglobin to form oxyhaemoglobin

  The oxygen moves along a concentration gradient. There is a higher amount of oxygen in the alveoli (from the air we have breathed in) compared to the small amount in the blood. Therefore it travels from a high concentration to a low concentration.
• Haemoglobin

  A red pigment that is found in red blood cells. It transports oxygen around the body, it also carries carbon dioxide
• Gas exchange

  • Whilst oxygen is diffusing across the membranes, carbon dioxide is moving at the same time
  • The blood in the capillary has a low concentration of oxygen (it has all been used up by the body tissues/muscles) and a high concentration of carbon dioxide (a by product from chemical reactions in the body, e.g. when we exercise)
  • As there is a higher concentration of carbon dioxide in the blood compared to the alveoli, the gas diffuses across the membranes into the lungs where it can be breathed out (high to low concentration)
• Factors that assist gas exchange

  There are millions of alveoli in each lung, this provides a large surface area for the exchange of gases. (The surface area of the alveoli is around the same size as a tennis court!)

  The surface of the alveoli and walls of the blood capillaries are very thin and moist which helps the gasses diffuse more easily.

  The alveoli and capillaries are touching each other so there is a very short distance for diffusion (short diffusion pathway)

  Each alveolus is surrounded by a network of capillaries so there is a rich supply of blood for the gases to diffuse into/from.
• Breathing

  • Breathing is a two-stage process
  • Inspiration (inhalation) the intake of air into the lungs
  • Expiration (exhalation) the expulsion of air from the lungs
• Inspiration

  • Intercostal muscles between the ribs contract and pull the rib cage up and out
  • The diaphragm contracts (flattens)
  • This increases the volume/size of the chest cavity
  • As the chest cavity (thoracic cavity) increases the pressure inside decreases
  • Air is drawn in from the atmosphere
• Expiration

  • Intercostal muscles relax, rib cage goes down and inwards
  • The diaphragm relaxes and returns to its dome/curve shape
  • This decreases the volume/size of the chest cavity (thoracic cavity)
  • As the chest cavity decreases, the pressure inside increases
  • This forces air to be expelled
• Inspiration during exercise

  • More muscles are needed to contract - Sternocleidomastoid & the pectorals 
  • Makes the ribs move up and out more
  • Causing the thoracic cavity and lungs to have an even bigger volume 
  • Air pressure in the lungs is even less than the atmospheric pressure

  Therefore more air forced in
• Expiration during exercise

  During exercise our abdominal muscles contract to pull our rib cage down quicker to force air out quicker.
• Tidal volume


  The volume of air breathed in or out during a normal breath at rest.
• Inspiratory reserve volume

  The amount of extra air that has been forcibly inspired after the inspiration of a normal tidal volume.
• Expiratory reserve volume

  The amount of extra air that can be forcibly exhaled after the expiration of a normal tidal volume.
• Residual volume


  The volume of air that remains in the lungs after maximum expiration.