Breath Control (Regulation of Respiration): O2 vs. CO2
Breath control (or control of respiration) is accomplished chemically, mainly using CO2 and O2 chemoreceptors. The most important chemical regulator of respiration is either CO2 or O2, since the chemical regulation of breathing is different in healthy and sick people.
Control of breathing in healthy people
Any medical or physiological textbook, which discusses breath control in the human body, states that respiration, in healthy people, is mainly controlled by CO2 concentrations in the brain and arterial blood.
Modern research is focused on specifics and details of this chemical breath control (Binet & Dejours P, 1962; Chernick et al, 1975; Dejours, 1963; Gautier, 2003; Forster & Pan LG, 1994; Heymans C, 1951; Honda Y, 1985; Kiwull-Schune et al, 1994; Lahiri et al, 1978; Lahiri et al, 2003; Murai et al, 1987; Nattie, 1999; Nye et al, 1983; O'Regan & Majcherczyk, 1982; Richerson et al, 2005; Wennergren & Wennergren, 1983).
The main physiological ideas related to regulation of breathing can be found in Chapter 2. The chemical and physiological mechanisms of immediate regulation of breathing (pages 50-59) of my eBook "Normal breathing: the key to vital health". Here is an extract that includes these pages: Breath Control.
Obviously, should carbon dioxide be poisonous, it would be normal to have it as little as possible, but the situation is opposite, since healthy people have higher CO2 concentrations and the "poison" controls breathing or our outer respiration, the fundamental function of the human body.
The breathing centre located near the rear of the brain (medulla oblongata) regulates our breathing movements. This breathing centre (also called the master centre of the body) uses special chemoreceptors to measure CO2 concentrations in the brain and arterial blood. The central chemoreceptors detect changes in the pH of the cerebro-spinal fluid and they are responsible for long-term or slow changes in breathing. Since CO2 dissolves in the blood and can penetrate through the blood-brain barrier, the main reason for pH variations in the brain are changes in CO2 concentrations. Peripheral chemoreceptors monitor immediate changes in CO2, O2, and pH concentrations of the blood and control our breathing in the short run. It is agreed that peripheral chemoreceptors include the carotid and aortic bodies. The carotid bodies, which can sense hypocapnia (low CO2), hypercapnia (high CO2) and hypoxia (low O2) play the main role in humans in comparison with aortic bodies, but immediate changes in CO2 produce the main effects on changes in the involuntary breathing pattern.
The breathing of healthy people during typical daily activities (rest, work, light and moderate exercise, sleeping, etc.) is mainly regulated by the pre-set (or their usual) chemical CO2 concentrations (CO2 breath control).
For example, when a healthy person takes several deep and fast breaths, CO2 in the lungs and blood falls. The breathing centre detects this drop and stops or reduces stimulation and work of the respiratory muscles. The person naturally holds their breath until the CO2 level reaches the initially pre-set value. Conversely, breath holding accumulates more carbon dioxide. The breathing centre senses this increase and intensifies breathing. This overbreathing is going to continue until extra CO2 is removed and the pre-set value is reached again.
We breathe more heavily during physical exercise, when our bodies produce more carbon dioxide. However, the rate of CO2 production matches the rate of CO2 removal in such a fashion that CO2 and O2 values in the arterial blood changes during exercise only slightly.
Do you know by how many times CO2 is more influential that O2 in control of breathing in healthy subjects? This answer (a number) provided right below here as your bonus content.
Breath control in the sick: increased role of O2
Control of breathing of sick people is done, in addition to CO2, by current blood O2 concentrations. The urge for oxygen gets stronger with the advance of many diseases.
The control of the breathing of sick people is accomplished by blood and brain CO2 concentrations and O2 drive, which becomes stronger with progression of chronic diseases and increasing cell hypoxia due to increased respiration (elevated minute ventilation).
In severely sick people, O2 can become the main factor in regulation of respiration.
The change in air composition during human evolution and evolution of animals on the Earth was the key factor that led to appearance of chronic diseases. This is because hyperventilation was beneficial for creatures living in primitive air with very low O2 content and high CO2 content 1-2 millions of years ago, but overbreathing destroys health now.
This YouTube video "Evolution of Air" features Dr. Artour Rakhimov. He explains the key cause of bronchospasm, spasm of blood vessels, reduced Bohr effects and other effects caused by changes in air and low CO2 in modern air.
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* Illustrations by Victor Lunn-Rockliffe