What happens during breathing?
During exhalation the breathing muscles become relaxed: the diaphragm curves arch shaped into the ribcage, the outer intercostal muscles contract and lower the ribcage by becoming relaxed. The breastbone sinks and the lung tissue contracts elastically. Through restriction of the torso cavity, excess pressure arises therein and the available respiration air flows out.
Illustration 5: The bellows operational principle of the lungs (according to A. Fallen The human body (Der Korper des Menschen))
In the breathing pause the breathing muscles as well as the lung tissues are released.
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The breastbone remains lowered or sinks further down. The state existing during exhalation remains the same or is even increased. The exhalation movement does not quite abate, but continues further to a small extent.
The breathing centre can to a large degree be influenced through the carbon dioxide and oxygen content of the blood. With increased carbon dioxide content in the blood it becomes stimulated, with a lowered content it is inhibited. The breathing centre is supported through the automatic controlling mechanism of the vagus nerve (10th cranial nerve), which regulates the expansion state of the lung tissue through impulses from mechanical receptors. Apart from the direct chemically acting influences of the breathing centre by means of the blood, also reactive influences through chemical receptors in the carotid artery and the large body arteries are known to us, which alarm the breathing centre when an oxygen deprivation takes place. Apart from these automatic control mechanisms the breathing can also be influenced through other systems such as for example the mid brain and voluntarily through the cerebral cortex. We can volitionally stop our breathing, increase or decrease the breathing rate or deepen our breathing. Furthermore, we can have a stimulating or inhibiting effect on the breathing through emotional influences (such as fear or fright), through thermal impulses (like cold temperature or heat), or through skin stimuli (such as e.g. cold showers and Kneipp treatments), through pain, motor Impulses (such as work and movement) and also through medicinal products.
The principle of respiration.
When we examine the composition of the inhaled and exhaled air, then we will find that the inhaled air contains approximately 20% oxygen, 80% nitrogen, as well as traces of carbon dioxide, whereas in the exhaled air there is only 16% oxygen, and instead about 4% carbon dioxide and roughly the same amount of nitrogen as before.
During breathing the air exchange in the lungs is not complete: only about 2/3 of the oxygen from the inhaled air reaches the lungs during an inhalation, 1/3 of the used air remaining in the respiratory tract from the last exhalation is added, so that it comes to a mixture of 2/3 fresh air and 1/3 residual air in the lungs.