Breathing is the mechanical process through which air is moved in and out of the lungs. Physiological respiration is the actual exchange of gases inside the lungs at the cellular level. Biochemical respiration refers to the metabolic process which combines oxygen and glucose to produce water, carbon dioxide, and adenosine triphosphate (ATP), the basic molecular unit of cellular energy transfer. As it is a purely biochemical reaction, the ATP energy equation will not be covered further in this article.
In human beings, breathing is governed by the diaphragm and intercostal muscles. When the diaphragm contracts, it increases the space and thus volume of the thorax. When the diaphragm is relaxed, it reduces the thoracic volume. Since the lungs reside in the thoracic cavity, their volume expands and contracts together with the rest of the cavity.
The intercostal muscles connected to the rib cage also work in conjunction with the diaphragm to further expand or contract thoracic volume. When the diaphragm contracts, the intercostal muscles also contract, lifting the rib cage to expand thoracic volume. When the diaphragm is relaxed, the intercostal muscles also relax.
In accordance with the ideal gas law, air pressure increases as volume decreases. Compression of the lungs thus forces air out of the alveoli of the lungs through the bronchial passages and into the trachea and mouth/nose, while expansion of the lungs draws air in.
This process is known as negative pressure breathing, because incoming air is following its natural pressure gradient to the partial vacuum created in the lungs. Amphibians and some other creatures use positive pressure breathing instead: in which the air is first drawn into the mouth, then forced down into the lungs.
Now that fresh air is inside the lungs, oxygen must be selectively exchanged for carbon dioxide at the cellular level. In human beings, the physiological respiratory process consists of four steps: ventilation, pulmonary gas exchange, gas transport, and peripheral gas exchange.
Ventilation has already been covered under physical breathing. As it is controlled by voluntary muscles, it is the only part of the respiratory cycle which can be voluntarily controlled to a limited extent.
Pulmonary gas exchange is governed by changing partial pressures for oxygen and carbon dioxide, such that the oxygen tends to flow into the alveoli along the natural pressure gradient and the carbon dioxide is drawn out of the alveoli the same way. The partial pressure of oxygen drops significantly along the various parts of the respiratory tract, in part because of the higher levels of water vapour. At higher altitudes, the pressure differential relative to atmospheric pressure is lower: making effective breathing much more difficult.
Gas transport occurs in the haemoglobin of red blood cells, which is specialised for oxygen transport. Without haemoglobin, blood plasma could not carry enough dissolved oxygen even to meet resting metabolic needs. The blood of high altitude dwellers compensates for the relatively poor pulmonary gas exchange with much higher levels of red blood cells than average. Blood doping makes use of the same technique by artificially increasing the levels of red blood cells prior to a race.
Gas transport in the opposite direction uses a different mechanism to move carbon dioxide to the alveoli. Carbon dioxide does not bind with haemoglobin in the same way or as readily as oxygen does: so only about 40% of carbon dioxide enters the blood this way. Most of the remainder is dissolved directly in the plasma in the form of dissolved bicarbonate. At the same time, reduced blood pH reduces oxygen affinity for haemoglobin.
Finally, oxygen reaches body tissues through peripheral gas exchange in the capillaries. Gas exchange in these microvessels is made much easier by the vessel walls being only a single cell thick. Capillary walls distend during physical exercise, when higher levels of gas exchange are needed.
The accumulation of carbon dioxide in the blood and corresponding rise of carbon dioxide tension is what stimulates the respiratory centre in the brain. The limiting factor in holding one's breath thus is not the amount of oxygen in the blood and tissues but the amount of carbon dioxide in the blood.