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!

The human body requires many things to survive, and one major requirement is oxygen. Every moment of every day, the human body feeds itself oxygen by breathing air. Breathing involves external respiration and internal respiration. The former is the act of bringing air into the lungs for gas exchange to occur within the lungs, and the latter is the exchange of gases within the cells of all body tissues. Both processes are essential for delivering oxygen to the brain and thus preventing tissue damage or death.

Air enters the body through the nose or mouth and travels through the nasal cavity to the pharynx. The air reaches a crossroad at the end of the pharynx where it must choose between the trachea or the esophagus. Air can only travel through the trachea, so this is where it goes.

The larynx, also known as the voice box, is between the trachea and pharynx. During speech, the vocal cords within the larynx vibrate in response to air passing through this area.

At this point, the air needs to enter the lungs, which will extract oxygen for the use of the body. This is made possible by the action of the diaphragm. The diaphragm is a muscle that contracts during inhalation, creating a vacuum within the thoracic cavity which draws air through the pharynx, trachea, bronchi, and lungs. Each contraction and relaxation of the diaphragm is responsible for inhalation and exhalation.

After the diaphragm contracts, the air travels along the bronchi; the bronchi consist of two main branches with each branch going into each lung. Within the lungs, the bronchi divide into smaller branches called bronchioles. At the end of each bronchiole are alveoli, or air-sacs. The exchange of oxygen and carbon dioxide occur between the walls of the alveoli. Once the necessary oxygen has been extracted from the main air supply, it leaves the walls of the alveoli to enter the surrounding capillaries which lead to pulmonary veins. The oxygen is now part of the bloodstream and ready for transport to the heart.

Oxygen is transported to the cells of the body by erythrocytes, also known as red blood cells. Red blood cells are the most abundant of cells in the blood. The hemoglobin portion of the red blood cells holds oxygen. The erythrocytes travel to the body cells through blood vessels such as veins, arteries and capillaries.

The heart receives oxygen from the lungs via pulmonary veins. The heart is the control center of the blood vessel network which is the life source for all tissue within the body. In other words, it is responsible for delivering oxygen to all body tissues and helping to remove waste gases via blood vessels. The heart itself is located in the mediastinum, which is the cavity between the lungs. This location allows for efficient exchange of gases between heart and lungs. The oxygenated blood passes through the pulmonary veins into the left atrium chamber of the heart, then the left ventricle chamber, and finally exiting the heart through ithe aorta. The aorta is the largest artery in the body and from here, the oxygen-rich blood enters arterial circulation.

The oxygen-rich blood travels through various arteries, into arterioles, then into capillaries. Oxygen and waste exchange occurs through the walls of the capillaries between the tissues and cells of the body. The red blood cells deliver oxygen and pick up gases such as carbon dioxide to carry back through the capillaries. From the capillaries, the deoxygenated blood enters venous circulation and travels back to the heart and lungs.

Ever wonder why nurses and phlebotomists only take blood samples from your veins and not your arteries? It's because the blood in your veins has not been recently pumped from the heart. As a result, veins don't pulsate in the way that arteries do. When taking a blood sample from an artery, there is an increased risk of blood spurting uncontrollably from the artery once punctured by the needle. It also takes longer for artery walls to heal compared to the walls of veins. For this reason, arterial blood samples are rarely taken. When they are, it is usually to check for abnormal gas percentages in the blood.

The red blood cells re-enter the heart via the superior and inferior vena cava, emptying into the right atrium chamber, then the right ventricle chamber. It exits through the pulmonary arteries to enter the air spaces of the lungs. Here, carbon dioxide and other waste gases are extracted from the blood to be exhaled from the body.

The entire process is called inhalation and exhalation. Each beat of the heart is one cardiac cycle.

Breath"ing (?), n.


Respiration; the act of inhaling and exhaling air.

Subject to a difficulty of breathing. Melmoth.


Air in gentle motion.


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


Aspiration; secret prayer.

"Earnest desires and breathings after that blessed state."



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.


Utterance; communication or publicity by words.

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


Breathing place; vent.



Stop; pause; delay.

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


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.

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