Gas exchange

Gas exchange or respiration takes place at a respiratory surface—a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is governed by Fick's law, which determines that respiratory surfaces must have:
 * a large surface area
 * a thin permeable surface
 * a moist exchange surface.

Many also have a mechanism to maximise the diffusion gradient by replenishing the source and/or sink.

Control of respiration is due to rhythmical breathing generated by the phrenic nerve in order to stimulate contraction and relaxation of the diaphragm during inspiration and expiration. Ventilation is controlled by partial pressures of oxygen and carbon dioxide and the concentration of hydrogen ions. The control of respiration can vary in certain circumstances such as during exercise.

Gas exchange in humans and mammals
In humans and mammals, respiratory gas exchange or ventilation is carried out by mechanisms of the heart and lungs. The blood is subjected to a transient electric field (QRS waves of the EKG) in the heart, which dissociates molecules of different charge. The blood, being a polar fluid, aligns dipoles with the electric field, is released, and then oscillates in a damped driven oscillation to form J or Osborn Waves, T, U, and V waves. The electric field exposure and subsequent damped driven oscillation dissociate gas from hemoglobin, primarily CO2, but more important, BPG, which has a higher affinity for hemoglobin than does oxygen, due in part to its opposite charge. Completely-dissociated hemoglobin (which will even effervesce if the electric field is too strong — the reason defibrillation joules are limited, to avoid bubble emboli that may clog vessels in the lung) enters the lung in red blood cells ready to be oxygenated.

Convection occurs over the majority of the transport pathway. Diffusion occurs only over very short distances. The primary force applied in the respiratory tract is supplied by atmospheric pressure. Total atmospheric pressure at sea level is 760 mmHg (101 kPa), with oxygen (O2) providing a partial pressure (pO2) of 160 mmHg, 21% by volume, at the entrance of the nares, a partial pressure of 150 mmHg in the trachea due to the effect of partial pressure of water vapor, and an estimated pO2 of 100 mmHg in the alveoli sac, pressure drop due to conduction loss as oxygen travels along the transport passageway. Atmospheric pressure decreases as altitude increases, making effective breathing more difficult at higher altitudes. Higher BPG levels in the blood are also seen at higher elevations, as well.

In similar manner, CO2, which is a result of tissue cellular respiration, is also exchanged. The pCO2 changes from 45 mmHg to 40 mmHg in the alveoli. The concentration of this gas in the breath can be measured using a capnograph. As a secondary measurement, respiration rate can be derived from a CO2 breath waveform.

Gas exchange occurs only at pulmonary and systemic capillary beds, but anyone can perform simple experiments with electrodes in blood on the bench-top to observe electric field-stimulated effervescence. Trace gases present in breath at levels lower than a part per million are ammonia, acetone, isoprene. These can be measured using selected ion flow tube mass spectrometry.

Diffusion
Blood carries oxygen, carbon dioxide, and hydrogen ions between tissues and the lungs. The majority of CO2 transported in the blood is dissolved in plasma (primarily as dissolved bicarbonate; 60%). A smaller fraction is transported in red blood cells combined with the globin portion of hemoglobin as carbaminohaemoglobin. This is the chemical portion of the red blood cell that aids in the transport of oxygen around the body, but, this time, it is carbon dioxide that is transported back to the lung.

As CO2 diffuses into the blood stream, it is absorbed by red blood cells before the majority is converted into H2CO3 by carbonic anhydrase, an enzyme that is not present in the plasma. The H2CO3 dissociates into H+ and HCO3−. The HCO3− moves out of the red blood cells in exchange for Cl− (chloride shift). The hydrogen ions are removed by buffers in the blood (Hb).