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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In humans and mammals, respiratory gas exchange is carried out by mechanisms of the heart and lungs. Ventilation is the process of air movement into and out of the lungs. Once air enters the lungs, diffusion of O2 and CO2 occurs in the alveoli. The oxygenated blood is then perfused throughout the body where gas exchange occurs in the capillary beds.[1]
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 entrance of the nares, a partial pressure of 150 mmHg (20 kPa) in the trachea due to the effect of partial pressure of water vapor, and an estimated pO2 of 100 mmHg (13 kPa) 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 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 to 40 mmHg (6.0 to 5.3 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.
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.
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 haemoglobin 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).
The hemoglobin molecule is a protein with four subunits. Each subunit is made up of a heme (a molecular group) with a polypeptide attached. Heme contains one
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.
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