Stimulated by carbon dioxide accumulating in the bloodstream. When levels build up, this triggers the breathing centres in the brain, which in turn stimulates the muscles used to inhale. As activity increases, more CO2 is released into the blood, causing the levels to rise faster and therefore increasing the rate of breathing. This is why it is very hard to hold your breath until you pass out as little kids often threaten to do. Of course, anyone stupid enough to counteract their own bodily defenses may deserve to pass out occasionally.

The best way to breath, according to many esoteric chi gung (qigong) practitioners (and some others), is with ones belly as opposed to one's chest. When breathing, one's shoulders and upper chest shouldn't move at all - it's all in the belly.

When breathing in...
The intercostal muscle between your ribs and the diaphragm contracts, and the thorax volume increases. Pressure outside is now greater than pressure in the thorax, hence air rushes in. The lung's exchange services, alveoli, absorb the oxygen into the bloodstream and transfer waste products out. Then...

When breathing out...
The intercostal muscles and diaphragm relax, decreasing the thorax volume, pushing the ribs and sternum in. Air rushes out due to the pressure difference.

And that's how we breathe.

We feel "out of breath" when the body detects that CO₂ levels in the blood have risen, indicating that more Oxygen will be needed. An above post suggests that the receptors for this are in the brain, but they are in fact in the blood vessels close to the heart, the carotid arteries and aorta to be precise. Chemoreceptors in the walls of these blood vessels monitor the pH of the blood. When CO₂ levels rise, the levels of carbonic acid also go up, as CO₂ dissolves, forming this weak acid. The pH of the blood therefore falls, and a negative feedback mechanism sends a message to the brain that there are higher levels of CO₂ in the blood. The brain then sends signals down the accelerator nerve to the sino-atrial node in the heart, which stimulates a faster heart rate (for information on the sino-atrial nerve and heart beat, see heart).

Similarly, when carbonic acid levels fall, the pH of the blood rise and so the Chemoreceptors relay this information to the brain, which sends more signals, this time down the vagus nerve, which tell the sino-atrial node to stop beating so fast.

Normally if someone is forced to breathe the same air over and over again (eg through a balloon), they will begin to feel out of breath, as respiration uses up Oxygen and produces CO₂, activating the Chemoreceptors and making us breathe faster. However, if a chemical such as Potassium Hydroxide is put into the air being breathed, the CO₂ will continuously be removed from the air. The subject will therefore not feel out of breath, and so can quite happily sit there breathing air with virtually no Oxygen in it and not notice.. that is until they collapse and fall unconscious. Some more sadistic teachers conduct this experiment on their students, asking them to ride exercise bikes and monitoring breathing rates when forced to breathe the same air, which has the CO₂ removed with each breath. The teacher will ask the subject to stop at some point, hopefully before they have fainted!

Breath"ing (?), n.

1.

Respiration; the act of inhaling and exhaling air.

Subject to a difficulty of breathing. Melmoth.

2.

Air in gentle motion.

3.

Any gentle influence or operation; inspiration; as, the breathings of the Spirit.

4.

Aspiration; secret prayer.

"Earnest desires and breathings after that blessed state."

Tillotson.

5.

Exercising; promotion of respiration.

Here is a lady that wants breathing too; And I have heard, you knights of Tyre Are excellent in making ladies trip. Shak.

6.

Utterance; communication or publicity by words.

I am sorry to give breathing to my purpose. Shak.

7.

Breathing place; vent.

Dryden.

8.

Stop; pause; delay.

You shake the head at so long a breathing. Shak.

9.

Also, in a wider sense, the sound caused by the friction of the outgoing breath in the throat, mouth, etc., when the glottis is wide open; aspiration; the sound expressed by the letter h.

10. Gr. Gram.

A mark to indicate aspiration or its absence. See Rough breathing, Smooth breathing, below.

Breathing place. (a) A pause. "That caesura, or breathing place, in the midst of the verse." Sir P. Sidney. (b) A vent. -- Breathing time, pause; relaxation. Bp. Hall. -- Breathing while, time sufficient for drawing breath; a short time. Shak. -- Rough breathing (spiritus asper) (). See 2d Asper, n. -- Smooth breathing (spiritus lenis), a mark (') indicating the absence of the sound of h, as in 'ie`nai (ienai).

 

© Webster 1913.