The Mechanics of Breathing
The action of breathing in and out is due to changes of
pressure within the thorax, in comparison with the
outside. When we inhale the intercostal muscles
(between the ribs) and diaphragm contract to expand
the chest cavity. The diaphragm flattens and moves
downwards and the intercostal muscles move the rib
cage upwards and out. This increase in size decreases
the internal air pressure and also air from the outside
(at a now higher pressure than inside the thorax)
rushes into the lungs to equalize the pressures.
When we exhale, the diaphragm and intercostal
muscles relax and return to their resting positions. This
reduces the size of the thoracic cavity, thereby
increasing the pressure and forcing air out of the lungs.
Breathing Rate
The rate at which we inhale and exhale is controlled by
the respiratory centre, within the Medulla Oblongata in
the brain. Inspiration occurs due to increased firing of
inspiratory nerves and also the increased recruitment of
motor units within the intercostals and diaphragm.
Exhalation occurs due to a sudden stop in impulses
along the inspiratory nerves.
Our lungs are prevented from excess inspiration due to
stretch receptors within the bronchi and bronchioles
which send impulses to the Medulla Oblongata when
stimulated.
Breathing rate is all controlled by chemoreceptors
within the main arteries which monitor the levels of
Oxygen and Carbon Dioxide within the blood. If oxygen
saturation falls, ventilation accelerates to increase the
volume of Oxygen inspired.
If levels of Carbon Dioxide increase a substance known
as carbonic acid is released into the blood which
causes Hydrogen ions (H+) to be formed. An increased
concentration of H+ in the blood stimulates increased
ventilation rates. This also occurs when lactic acid is
released into the blood following high intensity
exercise.
Respiratory Volumes
Respiratory volumes are the amount of air inhaled,
exhaled and stored within the lungs at any given time
Tidal Volume: The amount of air which enters the lungs
during normal inhalation at rest. The average tidal volume
is 500ml. The same amount leaves the lungs during
exhalation.
Inspiratory Reserve Volume: The amount of extra air inhaled (above tidal
volume) during a deep breath. This can be as high as 3000ml.
Expiratory Reserve Volume: The amount of extra air exhaled (above tidal
volume) during a forceful breath out.
Residual Volume: The amount of air left in the lungs following a maximal
exhalation. There is always some air remaining to prevent the lungs from
collapsing.
Vital Capacity: The most air you can exhale after taking the deepest breath you
can. It can be up to ten times more than you would normally exhale.
Total Lung Capacity: This is the vital lung capacity plus the residual volume
and is the total amount of air the lungs can hold. The average total lung
capacity is 6000ml, although this varies with age, height, sex and health.
Gaseous Exchange
The main function of the respiratory system is
gaseous exchange. This refers to the process of
Oxygen and Carbon Dioxide moving between the lungs
and blood:
Diffusion occurs when molecules move from an area
of high concentration (of that molecule) to an
area of low concentration.
This occurs during gaseous exchange as the blood
in the capillaries surrounding the alveoli has a
lower oxygen concentration of Oxygen than the air
in the alveoli which has just been inhaled.
Both alveoli and capillaries have walls which are only
one cell thick and allow gases to diffuse across
them.
The same happens with Carbon Dioxide (CO2). The
blood in the surrounding capillaries has a higher
concentration of CO2 than the inspired air due to
it being a waste product of energy production.
Therefore CO2 diffuses the other way, from the
capillaries, into the alveoli where it can then be
exhaled.
To demonstrate the use of Oxygen and Carbon Dioxide in respiration you can
look at the amounts of both gases which we inhale and then exhale. The air we
breathe contains approximately 21% Oxygen and 0.04% Carbon Dioxide. When
we exhale there is approximately 17% Oxygen and 3% Carbon Dioxide. This
shows a decrease in Oxygen levels (as it is used in producing energy) and an
increase in Carbon Dioxide due to it being a waste product of energy
production.
VO2 Max
VO2 max is the measure of the peak volume of Oxygen
(VO2) you can consume and use in a minute. It is
measured in ml/kg/min and so you can see that it is
also relative to body weight.
As we already know, Oxygen is needed to produce energy. The harder you
exercise the more Oxygen you use in order to produce sufficient energy.
However, everybody has a maximum level (their VO2 Max), where Oxygen
utilization is at its peak
If exercise intensity increases beyond this point then the anaerobic energy
systems must be used to supply the additional energy. However, anaerobic
metabolism produces lactic acid which causes fatigue and so cannot be
sustained. Anaerobic energy production also results in Oxygen Debt.
Your VO2 Max can be increased through training, as this causes adaptations
within the cardiovascular, respiratory and muscular systems which make the
processes of gas exchange, Oxygen transport and aerobic metabolism more
efficient.
There are a number of ways of testing your VO2 max. The most accurate is in a
laboratory, where exhaled Oxygen and Carbon Dioxide levels are measured
whilst running on a treadmill. This allows us to see how much of the Oxygen
inhaled (we know 21% of the air we inhale is O2) is used for energy production.
VO2 can also be estimated using tests such as a bleep test, or Balke test.
Results vary depending on fitness level, sex, age and genetics. The older you
are the lower your VO2 Max is estimated to be. An average score for a twentysomething male would be 40 ml/kg/min with an excellent score being 52
ml/kg/min. An average score for a female of the same age would be 30
ml/kg/min and an excellent score would be 41 ml/kg/min. Some professional
sports people (involved in endurance activities) have scores in the 80's!