Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs. In unicellular organisms the respiratory surface and gas-exchange is governed by Fick's law.
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[edit] Pulmonary physics
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 kPa), 21% by volume, at the alveolar sac. The pressure drop is due to conduction loss as oxygen travels along the transport passageway. Atmospheric pressure de\well.
[edit] Interchange
In similar manner, CO2, which is a result of tissue cellular respiration, is also exchanged. The pCO2 changes from 45 to 35 mmHg (6.0 to 4.7 kPa) 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.
[edit] Trace gases
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.
[edit] 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 and nutrients 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 HCO−
3. The HCO−
3 moves out of the red blood cells in exchange for Cl− (chloride shift). The hydrogen ions are removed by buffers in the blood (Hb).
[edit] Control of respiration
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.
[edit] See also
[edit] External links
- Human Physiology Respiration at eku.edu
- MeSH Pulmonary+Gas+Exchange
- RT Corner, educational website for RT's and nurses
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