Breathing rhythms


  • Lung volume
  • The spirometer
  • Homeostasis
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Tidal volume - the volume of air that enters and leaves the lungs at each natural resting breath.

Inspiratory reserve volume (IRV) - the volume of air that can be taken in over and above the normal inspired tidal volume.

Expiratory reserve volume (ERV) - the volume of air that can be expelled over and above the normal inspired tidal volume.

Vital capacity (VC) - tidal volume + IRV + ERV

Residual volume (RV) - the volume of air left in the lungs after the strongest possible expiration.

Total lung capacity (TLC) - the sum of the vital capacity and the residual volume.

Inspiratory capacity (IC)the volume that can be inspired from the end of a normal expiration.

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Ventilation rate

ventilation rate = tidal volume x frequency of inspiration

The ventilation of the lungs needs to supply all the oxygen required by the tissues of the body whatever they are doing.

It is affected by:

  • the amount of air taken into the lungs at each breath
  • the number of breaths per minute
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The Spirometer

Used to measure the inspiratory or expiratory reserves.

  • Person inhales and exhales via a mouthpiece.
  • An airtight chamber is filled with oxygen. When the person inhales and exhales the lid of the chamber moves up and down until all of the oxygen is used up.
  • Has a revolving drum (kymograph) on which the trace is drawn out as the lid moves up and down.
  • Canister of soda lime to remove CO2 from the exhaled air.
  • Any movement is recorded on a chart recorder.
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Control and regulation

The respiratory centre of the medulla sends the stimulus to inhale and exhale. It involves a feedback system based on the stretching of the bronchi during breathing.

The resp centre contains:

  • an inspiratory centre which controls breathing in.
  • an expiratory centre which controls forced exhalation.

We inhale because:

  • impulses from the resp centre travel along sympathetic nerves, causing the intercostal muscles and diaphragm to contract
  • as the lungs inflate stretch receptors in the walls of the bronchi send nerve impulses increasingly rapidly to the resp centre
  • eventually the impulses inhibit the resp centre and it stops stimulating the breathing muscles
  • You stop breathing in - and as the muscles relax, you exhale.
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Breathing and homeostasis

The main stimulus affecting the breathing rate is the level of carbon dioxide in the blood. As the CO2 concentration increases the pH falls, leading to an increase in both the breathing rate and the depth of breathing as the intercostal muscles and diaphragm contract harder and more often.

  • As soon as exercise starts, impulses from the cortex of the brain stimulate the resp centre.
  • This stimulates the resp muscles and increases the rate and depth of ventilation.
  • Chemoreceptors send impulses back to the main resp centre when CO2 levels rise.
  • Impulses are sent to the effectors so the breathing rate changes in a negative feedback system which removes the extra CO2 and matches the body's oxygen needs.
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