A2/A-level Biology - Regulation of Ventilation Rate

• The rate at which someone breathes is the ventilation rate.

• The volume of air we breathe in and out at each breadth.

• The maximum volume of air we can inhale and exhale.

• The volume of air taken into the lungs in one minute (also known as the ventilation rate).

Minute ventilation = tidal volume (average volume of one breath) x breathing rate (number of breaths per minute)

• The fall in the trace is due to the consumption of oxygen by the subject.
• The rate of oxygen consumption can be calculated by dividing the decrease in volume by time for the fall.
• A person using a spirometer breathes in and out of an airtight chamber causing it to move up and down and leaving a trace on a kymograph.
• This can be used to investigates effects of exercise by connecting it to someone doing exercise e.g. on a treadmill.

• The ventilation centre in the medullar oblongata of the brain controls breathing.

Inhalation

• The ventilation centre sends nerve impulses to the external intercostals muscles and diaphragm muscles.
• Both these sets of muscles contract causing inhalation.
• During deep inhalation, the neck and upper chest muscles are brought into play.

Exhalation

• As the lungs inflate, stretch receptors in the bronchioles are stimulated.
• These receptors send inhibitory impulses back to the ventilation centre, therefore the impulses to the muscles stop, stopping inhalation and allowing exhalation.
• Exhalation is caused by the elastic recoil of the lungs and by gravity helping to lower the ribs. Not all air leaves, the residual air mixes with the inhaled air with each breath.

The most important stimulus controlling the breathing rate and depth of breathing is the concentration of dissolved carbon dioxide in arterial blood, via its effect on pH:

1.    CO2 dissolves in the blood plasma, making carbonic acid.
2.    Carbonic acid dissociates into hydrogen ions and hydrogencarbonate ions, thereby lowering the pH of the blood.
3.    Chemoreceptors sensitive to hydrogen ions are located in the ventilation centre. They detect the rise in hydrogen ion concentration.
4.    Impulses are sent to other parts of the ventilation centre.
5.    Impulses are sent from the ventilation centre to stimulate the muscle involved in breathing.

• There are also chemoreceptors in the walls of the carotid artery and aorta.

A small increase in CO2 causes a large increase in ventilation.

• Increasing CO2 and the associated fall in pH leads to an increase in rate and depth of breathing, through more frequent and stronger contractions of the appropriate muscles. This in turns ensures efficient removal of CO2 and uptake of oxygen.
• The opposite response occurs with a decrease in CO2 – it’s an example of homeostasis operating via negative feedback.
• The motor cortex of the brain controls movement.
• As soon as exercise begins, impulses from the motor cortex have a direct effect on the ventilation centre, increasing ventilation sharply.
• Ventilation also increases from stretch receptors in tendons and muscles involved in movement.

That's the end of the topic!

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