While resting, healthy people automatically move between four and six litres of air through their lungs each minute, and by doing so they are able to meet the three primary aims of breathing, which are:
While it is commonly accepted that maintaining the correct concentration of oxygen is almost the sole reason for breathing, and of course without oxygen we would die, this aspect of breathing control is usually of minor importance on a minute-by-minute basis. This is because we can easily obtain oxygen, only using a tiny part of that found in the atmospheric air, and breathing the rest out again; and oxygen needs to drop by approximately one third before it stimulates the automatic breathing pattern.
On the other hand, if carbon dioxide pressure is allowed to become unstable it will severely affect the pH of the blood, and if the narrow acid-alkali range is not maintained then the person will die. To ensure that this does not happen, carbon dioxide only has to increase or decrease by the tiniest amount in order to dramatically alter the amount of air breathed each minute.
Metabolism is the total of the chemical processes occurring in the body at any one time, and because during times of high metabolism extra carbon dioxide is produced by the body, the volume of air breathed each minute automatically increases at the same time.
For example, when a person is running there are more chemical reactions occurring than when the person is sitting in a chair, and so the breathing naturally increases when you run. By automatically following the level of metabolism, the concentration of carbon dioxide is kept more or less constant, all of the time.
The respiratory centre in the brainstem is responsible for controlling your breathing rate - it sends a message to your respiratory muscles telling them when to breathe. This control is automatic and continuous. You do not have to consciously think about it. However, this control can be influenced by emotions, talking and calculated thoughts such as when you dive into a swimming pool.
It "knows" how to control the breathing rate by the amount of carbon dioxide in the arterial blood. As the carbon dioxide levels increase, as it does during exercise, the respiratory centre strengthens the signal - stimulating the breathing. Responding to this stronger signal, the respiratory muscles increase both the speed and depth of breathing.
This increased respiration rids the body of excess carbon dioxide and supplies the body with more oxygen, which is needed when you exercise. When the level of carbon dioxide in the arterial blood returns to normal levels, the respiratory centre is no longer strongly stimulated and breathing becomes quiet and relaxed.
If the breathing is excessive, such as occurs in a habit of snoring during sleep, or when you blast extra carbon dioxide into the atmosphere by sneezing violently three times, the gas concentration drops lower than normal. Now the breathing either stops altogether, or it continues in a very passive, unstimulated way until the concentration of carbon dioxide builds up to the trigger point again.
While maintaining a constant pressure of carbon dioxide is the primary way of controlling breathing, other factors are involved.
Should the concentration of oxygen get too low, then carbon dioxide pressure will be ignored as a marker for breathing until the pressure of oxygen is raised to the normal range. It would be common for the person to hyperventilate in an effort to restore normal oxygen, which of course will also lower carbon dioxide. If this person is given supplemental oxygen, care needs to be taken that the person continues to breathe while the carbon dioxide pressure rises again and takes over the control of breathing.
One part of the respiratory centre instructs the breathing muscles to inhale, and another part puts the brakes on so that the inhalation does not over-inflate the lungs. Stretch receptors in the lungs also help to ensure that a suitable volume of air is inhaled.
Our breath is also under voluntary control to a certain degree. We can exhale for much longer periods that usual while singing and holding long notes for example. We can stop breathing when diving into the ocean, and if there is a nasty smell, we can deliberately breathe less air for a time. We can also deliberately increase the volume of air or push it out of our lungs with force when we wish to blow out several birthday candles for instance.
Another reason for breathing in a particular way is to maintain a steady core temperature, and so if we get too hot, the breathing will increase in an effort to cool us down. Strangely perhaps, we also sometimes increase our breathing when we are cold, which may be due to a degree of bronchoconstriction connected to having a cold face, or in an effort to move warming blood faster around the body.
Drugs can also over-ride the automatic breathing pattern and cause it to either increase or decrease. Morphine and caffeine are examples of this, where morphine slows the breathing and caffeine increases it.
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