Let me answer this like you answer everyone elses questions:
In mammals, breathing in, or inhaling, is usually an active movement, with the contraction of the diaphragm muscles needed. This is known as negative pressure breathing. The diaphragm's normal relaxed position is that of a recoiled one (decreasing the thoracic volume) whereas in the contracted position it is pulled downwards (increasing the thoracic volume). Relaxation of the diaphragm compresses the lungs, effectively decreasing their volume while increasing the pressure inside them. With a pathway to the mouth or nose clear, this increased pressure forces air out of the lungs by diffusion. Conversely, contraction of the diagraphm increases the volume of the (partially empty) lungs, decreasing the pressure inside, which creates a partial vacuum. Environmental air then follows its concentration gradient down to fill the lungs.
In amphibians, the process used is positive pressure breathing. Muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils (which uses the same mechanics - pressure, volume, and diffusion - as a mammalian lung). With the nostrils and mouth closed, the floor of the oral cavity is forced up, which forces air down the trachea into the lungs.
At rest, breathing out, or exhaling, is a combination of passive and active processes powered by the elastic recoil of the alveoli, similar to a deflating balloon, and the contraction of the muscular body wall. The following organs are used in respiration: the mouth; the nose and nostrils; the pharynx; the larynx; the trachea; the bronchi and bronchioles; the lungs; the diaphragm; and the terminal branches of the respiratory tree, such as the alveoli.
 Gas exchange
Breathing is only part of the processes of delivering oxygen to where it is needed in the body and removing carbon dioxide waste. The process of gas exchange occurs in the alveoli by passive diffusion of gases between the alveolar gas and the blood passing by in the lung capillaries. Once in the blood the heart powers the flow of dissolved gases around the body in the circulation.
As well as carbon dioxide, breathing also results in loss of water from the body. Exhaled air has a relative humidity of 100% because of water diffusing across the moist surface of breathing passages and alveoli.
 Control of breathing
Main article: Control of respiration
Breathing is one of the few bodily functions which, within limits, can be controlled both consciously and unconsciously.
 Conscious control
Conscious attention to breathing is common in many forms of meditation, specifically anapana and other forms of yoga. In swimming, cardio fitness, speech or vocal training, one learns to discipline one's breathing, initially consciously but later sub-consciously, for purposes other than life support.
 Unconscious control
Unconsciously, breathing is controlled by specialized centers in the brainstem, which automatically regulate the rate and depth of breathing depending on the body’s needs at any time. When carbon dioxide levels increase in the blood, it reacts with the water in blood, producing carbonic acid. Lactic acid produced by anaerobic exercise also lowers pH. The drop in the blood's pH stimulates chemoreceptors in the carotid and aortic bodies in the blood system to send nerve impulses to the respiration centre in the medulla oblongata and pons in the brain. These, in turn send nerve impulses through the phrenic and thoracic nerves to the diaphragm and the intercostal muscles, increasing the rate of breathing.
This automatic control of respiration can be impaired in premature babies, or by drugs or disease.
For instance, while exercising, the level of carbon dioxide in the blood increases due to increased cellular respiration by the muscles, which activates carotid and aortic bodies and the respiration center, which ultimately cause a higher rate of respiration.
During rest, the level of carbon dioxide is lower, so breathing rate is lower. This ensures an appropriate amount of oxygen is delivered to the muscles and other organs. It is important to reiterate that it is the buildup of carbon dioxide making the blood acidic that elicits the desperation for a breath much more than lack of oxygen.
It is not possible for a healthy person to voluntarily stop breathing indefinitely. If we do not inhale, the level of carbon dioxide builds up in our blood, and we experience overwhelming air hunger. This irrepressible reflex is not surprising given that without breathing, the body's internal oxygen levels drop dangerously low within minutes, leading to permanent brain damage followed eventually by death. However, there have been instances where people have survived for as long as two hours without air; this is only possible when submerged in cold water, as this triggers the mammalian diving reflex.
If a healthy person were to voluntarily stop breathing (i.e. hold his or her breath) for a long enough amount of time, he or she would lose consciousness, and the body would resume breathing on its own. Because of this one cannot suffocate oneself with this method, unless one's breathing was also restricted by something else (e.g. water, see drowning)
Hyperventilating causes a drop in CO2 below normal levels, lowering blood acidity to trick the brain into thinking it has more oxygen than is actually present. Hyperventilating can cause your blood oxygen levels to go to dangerous levels.
 Relationship to death
Breath is sometimes used as a metaphor for life itself, and often "last breath" is the most obvious sign that death has occurred. The association between the end of life and breathing is not absolute, as scientists have discovered the brain/mind can continue to function for many minutes without the continuation of oxygen and/or it's properties. Though thought to "restart" breathing, cardiopulmonary resuscitation (CPR) only circulates blood through the body.
 Composition of air
The air we inhale is roughly 78% nitrogen, 21% oxygen, 0.96% argon and 0.04% carbon dioxide, helium, water, and other gases. (% by volume)
The permanent gases in air we exhale are roughly 78% nitrogen, 15% to 18% oxygen, 4% to 5% carbon dioxide and 0.96% argon (% by volume). Additionally vapors and trace gases are present: 5% water vapor, several parts per million (ppm) of hydrogen and carbon monoxide, 1 part per million (ppm) of ammonia and less than 1 ppm of acetone, methanol, ethanol and other volatile organic compounds.
Not all of the oxygen breathed in is replaced by carbon dioxide; around 15% to 18% of what we breathe out is still oxygen. The exact amount of exhaled oxygen and carbon dioxide varies according to the fitness, energy expenditure and diet of that particular person. Also our reliance on this relatively small amount of oxygen can cause overactivity or euphoria in pure or oxygen rich environments.
 Cultural significance
In Tai Chi Chuan, aerobic training is combined with breathing to exercise the diaphram muscles, and to train effective posture, which both make better use of the body's energy. In music, breath is used to play wind instruments and many aerophones. Laughter, physically, is simply repeated sharp breaths. Hiccups and yawns are other breath-related phenomena.
Ancients commonly linked the breath to a life force. The Hebrew Bible refers to God breathing the breath of life into clay to make Adam a living soul (nephesh, roughly "breather"). It also refers to the breath as returning to God when a mortal dies. The terms "spirit," "qi," and "psyche" are related to the concept of breath.
^ Ramey CA, Ramey DN, Hayward JS. Dive response of children in relation to cold-water near drowning. J Appl Physiol 2001;62(2):665-8.Source: Diana Hacker (Boston: Bedford/St. Martin’s, 2002).Adapted from Victoria E. McMillan (Boston: Bedford/St. Martin’s, 2001). See it cited here
^ psych-, psycho-, -psyche, -psychic, -psychical, -psychically + (Greek: mind, spirit, consciousness; mental processes; the human soul; breath of life)
Parkes M (2006). "Breath-holding and its breakpoint.". Exp Physiol 91 (1): 1-15. PMID 16272264. Full text
 See also
· 1 decade ago