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Regulation of ventilation rate The rate at which someone breathes is the ventilation rate.
TIDAL VOLUME: the volume of air we breathe in and out at each breadth.
VITAL CAPACITY: the maximum volume of air we can inhale and exhale.
MINUTE VENTILATION: 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 control of breathing
The ventilation centre in the medullar oblongata of the brain controls breathing
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.
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.
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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.