Is it true that the urge to breathe is triggered only by high CO2 levels?
Yes, mostly so. There might be trace effects from other stuff, but CO2 is the major trigger AFAIK.
(CO2 as dilute carbonic acid (Carbon dioxide dissolved in aqueous solution), the same stuff makes caves in limestone rock).
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Wikipedia:
Carbon dioxide dissolved in water is in equilibrium with carbonic acid... Carbonic acid has the formula H2CO3. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H2CO3. The salts of carbonic acids are called bicarbonates and carbonates. It is a weak acid. the majority of the carbon dioxide is not converted into carbonic acid and stays as CO2 molecules. CO2 + H2O ⇌ H2CO3
Role of carbonic acid in blood
Carbonic acid is an intermediate step in the transport of CO2 out of the body via respiratory gas exchange. The hydration reaction of CO2 is generally very slow in the absence of a catalyst, but red blood cells contain carbonic anhydrase which both increases the reaction rate and disassociates a hydrogen ion (H+) from the resulting carbonic acid, leaving bicarbonate (HCO3-) dissolved in the blood plasma. This catalysed reaction is reversed in the lungs, where it converts the bicarbonate back into CO2 and allows it to be expelled.
Carbonic acid also plays a very important role as a buffer in mammalian blood. The equilibrium between carbon dioxide and carbonic acid is very important for controlling the acidity of body fluids, and the carbonic anhydrase increases the reaction rate by a factor of nearly a billion to keep the fluids at a stable pH.
[ame=http://en.wikipedia.org/wiki/Carbonic_acid]Carbonic acid - Wikipedia, the free encyclopedia[/ame]
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Air hunger
Air hunger is the sensation of the urge to breathe. It is usually caused by the detection of high levels of carbon dioxide in the blood by sensors in the carotid sinus and is one of the body's homeostatic mechanisms to ensure proper oxygenation. Natural chemicals in the blood such as epinephrine (adrenaline) can also induce an urge to breathe by a separate pathway. Insufficient pulmonary minute ventilation, a sustained breath-hold, constriction of the alveoli of the lungs as in asthma, or high ambient levels of carbon dioxide in the air breathed can cause air hunger resulting in a respiratory distress condition characterized by dyspnea, labored breathing or gasping. Air hunger can be extremely distressing and triggers panic and strong reactions to restore breathing.
In mammals the breathing reflex is triggered by excess of carbon dioxide rather than lack of oxygen. In particular, this means that air hunger is not always experienced during asphyxiation. In oxygen-deprived environments, respiration continues to cycle out carbon dioxide but does not bring in sufficient oxygen. Without the required carbon dioxide accumulation, victims may not realize they are being asphyxiated until other symptoms appear, or at all.
[ame]http://en.wikipedia.org/wiki/Air_hunger[/ame]
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Parts of the brain which react to high CO2 (hypercapnea):
http://www.uthscsa.edu/opa/issues/new34-9/PET.html
They induced breathlessness in healthy volunteers by adding a small amount of carbon dioxide (CO2) to the subjects' inspired air. The resulting PET images revealed consistent blood flow changes (a measure of brain activity) in the amygdala, anterior cingulate cortex, cerebellum and other brain areas. These changes then were correlated with measures of the participants' physiological and subjective responses to better characterize the brain circuitry involved in unconscious and conscious detection of air hunger.
The amygdala, a brain structure that is present in all animals and regulates the subconscious ability to perceive threat, was strongly activated by CO2 inhalation. However, when subjects felt breathless, blood flow to the anterior cingulate cortex increased dramatically. "We believe this reflects a key role for the anterior cingulate in mediating conscious awareness of breathlessness," stated Dr. Liotti.
"Our physiological drives run on autopilot to a certain point," continued Dr. Brannan. "We breathe without thinking; we eat when it seems time; we drink when we feel thirsty. What we have identified here is part of an adaptive brain circuit involved in automatic respiratory control and conscious perception of air hunger. The amygdala continuously senses our internal states and detects potential threats to our survival. The anterior cingulate sounds the alarm when there is conscious perception of immediate danger."