何謂氣體交換?

氣體交換

氣體在肺泡處的交換是通過肺泡和毛細血管壁進行的。氣體在組織內的交換也通過毛細血管壁進行。氣體是怎樣進行交換的呢?


肺泡是唯一可供氣體交換及氣體擴散的場所，人體肺臟可供擴散的表面積約 60 -80平方公尺，相當半面網球場般的大小。影響肺泡換氣的因素，包括分壓、擴散路徑的距離、擴散面積、紅血球與血紅素量等。女性的肺擴散能量比男性稍低些；運動員比一般人有較大的肺擴散能量。肺擴散能量的增加是因訓練增加肺容量，提供更多肺泡與微血管的接觸面積。
氣體分子不論在氣體狀態或溶解在體液中，都在不斷地運動，具有擴散性。一種氣體總是由多的地方向少的地方擴散，即總是由濃度高的地方向濃度低的地方擴散，直到平衡為止。氣體的濃度與壓力有關，濃度高，壓力也大；濃度低，壓力也小。因此也可以說，氣體是由壓力高的地方向壓力低的地方擴散的。氣體在肺泡和在組織內的交換，都是通過這種擴散作用實現的。

空氣由氧、二氧化碳、氮等組成。各種氣體都有一定的壓力，空氣中各種氣體壓力的總和即是大氣壓，其中每種氣體的壓力，即是該氣體的分壓。由於氣體總是由壓力高的地方向壓力低的地方擴散的，因此，某種氣體分子也總是從分壓高的部位擴散到分壓低的部位。

由於肺泡氣、血液和組織中的氧和二氧化碳的分壓不同(表6)，因此，在血液流經肺部毛細血管和組織細胞時，就可以進行氣體交換。

表6 氧和二氧化碳的分壓(kP)


肺泡氣
靜脈血
動脈血
組織

氧氣
13.6
5.3
13.33
4

二氧化碳
5.33
6.13
5.33
6.67




具體地說，當靜脈血流經肺部毛細血管時，由於肺泡氣中的氧分壓高於靜脈血中的氧分壓，而二氧化碳分壓則低於靜脈血中的二氧化碳分壓，因此，氧由肺泡向靜脈血擴散，而二氧化碳則由靜脈血向肺泡擴散。這就是肺泡內的氣體交換(圖11)。經氣體交換後，靜脈血變成動脈血。由於外界空氣不斷地進入肺內，肺泡氣的成分保持相對恒定，因此，肺泡內的氧分壓總是比靜脈血中的高，二氧化碳分壓則總是比靜脈血中的低，於是氧氣總是不斷地由肺泡向血液擴散，二氧化碳總是不斷地由靜脈血向肺泡內擴散。當動脈血流經組織時，由於組織內氧分壓低於動脈血中的氧分壓，而二氧化碳分壓則高於動脈血中的二氧化碳分壓，因此，氧由動脈血向組織擴散，而二氧化碳則由組織向血液擴散。這就是組織內的氣體交換。經過氣體交換，動脈血就變成了靜脈血。由於組織細胞在代謝過程中不斷地消耗氧和產生二氧化碳，因此，組織內的氧分壓總是低於動脈血中的氧分壓，而二氧化碳分壓總是高於動脈血中的二氧化碳分壓，於是氧氣總是不斷地由動脈血向組織擴散，二氧化碳總是由組織向血液擴散。

從氣體交換過程得知，氣體分壓差是氣體交換的動力。

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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 simply the cell membrane, but for large organisms it usually is carried out in respiratory systems.

This name can cause problems - in biology the word "respiration" can mean cellular respiration or metabolism (ATP generation inside cells), however sometimes (such as here) it can also refer to breathing (which is how the word is most often used by non-biologists).

Gases cross the respiratory surface by diffusion, so from Fick's law we can predict 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 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 other mammals, respiratory gas exchange or ventilation is carried out by mechanisms of the lungs. The actual exchange of gases occurs in the alveoli.

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 mm Hg, with oxygen (O2) providing a partial pressure (pO2) of 160 mm Hg, 21% by volume, at the entrance of the nares, and an estimated pO2 of 100 mm Hg 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.

Gas exchange occurs only at pulmonary and systemic capillary beds.

CO2 is a result of cellular respiration. 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.

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.


Transporting of oxygen, carbon dioxide, and hydrogen ions
Blood carries oxygen, carbon dioxide and hydrogen ions between tissues and the lungs.

The majority (70%) of CO2 transported in the blood is dissolved in plasma (primarily as dissolved bicarbonate; 60%). A smaller fraction (30%) is transported in red blood cells combined with the globin portion of hemoglobin as carbaminohemoglobin.

picture in the following link

http://upload.wikimedia.org/wikipedia/commons/thumb/8/8b/Alveoli.svg/300px-Alveoli.svg.png