Breathing takes oxygen in and carbon dioxide out of the body. Aerobic organisms require oxygen to create energy via respiration, in the form of the metabolism of energy-rich molecules such as glucose. The medical term for normal relaxed breathing is eupnea.
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.
In mammals, breathing in, or inhaling, is usually an active movement, with the contraction of the diaphragm muscle. This is known as negative pressure breathing. Normally, the diaphragm's relaxed position recoiled (decreasing the thoracic volume) whereas in the contracted position it is pulled downwards (increasing the thoracic volume). This process works in conjunction with the intercostal muscles connected to the rib cage. Contraction of these muscles lifts the rib cage, thus aiding in increasing the thoracic volume. Relaxation of the diaphragm compresses the lungs, effectively decreasing their volume while increasing the pressure inside them. The intercostal muscles simultaneously relax, further decreasing the volume of the lungs. With a pathway to the mouth or nose clear, this increased pressure forces air out of the lungs. 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 pressure 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.
This automatic control of respiration can be impaired in premature babies, or by drugs or disease.
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.
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.
The air we inhale is roughly 78% by volume nitrogen, 21% oxygen, 0.96% argon and 0.04% carbon dioxide, helium, water, and other gases. In addition to air, underwater divers often breathe oxygen-rich or helium-rich gas mixes. Oxygen and analgesic gases are sometimes given to patients under medical care. The atmosphere in space suits is pure oxygen.
The permanent gases in gas we exhale are roughly 4% to 5% carbon dioxide and 4% to 5% less oxygen than was inhaled. 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. 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.
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.