26.16
外呼吸と内呼吸
外呼吸は肺で行われ、酸素が体内に取り込まれる最初のステップです。息を吸うと、酸素は肺に入り、薄い肺胞膜を透過します。肺胞は空気で満たされた小さな袋で、ガス交換のための広い表面積を提供します。肺胞内の酸素は、隣接する肺毛細血管 (40 mmHg) よりも高い分圧 (105 mmHg) を持ち、圧力勾配…
体外呼吸は、肺胞と肺の毛細血管との間で酸素と二酸化炭素を交換するものです。
このガス交換は、常に分圧の高い領域から低い領域へ、またはPの高からPの低へ発生します。
酸素は、PO2 が 105 mmHg である肺胞空気から、PO2 が 40 mmHg である肺毛細血管内の血液に拡散します。
同様に、二酸化炭素は、PCO2 が 45 mmHg の血液から PCO2 が 40 mmHg の肺胞空気に駆動されます。
内呼吸では、酸素は全身の毛細血管から全身の組織細胞に移動しますが、CO2は反対方向に移動します。
全身性毛細血管は、細胞内でのATP産生に酸素が消費され、毛細血管から細胞内に酸素が拡散するため、組織細胞よりもPO2が高くなります。
逆に、活発な代謝によるCO2産生は、全身の毛細血管よりも細胞内のPCO2が高くなり、CO2が細胞からこれらの毛細血管に拡散する原因となります。
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Q1: What is the difference between external and internal respiration?
External respiration occurs in the lungs, where oxygen diffuses from alveoli into pulmonary capillaries and carbon dioxide moves from blood into alveoli. Internal respiration happens at body tissues, where oxygen transfers from systemic capillaries to tissue cells for ATP production, while CO2 produced by cellular metabolism diffuses from cells back into capillaries. Both processes follow partial pressure gradients.
Q2: How do partial pressure gradients drive gas exchange in the lungs?
Gas always moves from higher to lower partial pressure. In external respiration, alveolar oxygen at 105 mmHg diffuses into pulmonary capillaries at 40 mmHg. Simultaneously, blood carbon dioxide at 45 mmHg moves into alveolar air at 40 mmHg. These pressure differences create the driving force for gas exchange across the alveolar membrane.
Q3: Why does oxygen diffuse from capillaries into tissue cells during internal respiration?
Tissue cells consume oxygen rapidly during ATP production through cellular respiration, lowering their oxygen partial pressure below that of systemic capillaries. This pressure gradient causes oxygen to diffuse from capillaries into cells. Conversely, CO2 accumulates in cells from metabolism, creating a higher PCO2 in cells than capillaries, driving CO2 diffusion outward.
Q4: What factors reduce the rate of gas exchange in external respiration?
Decreased partial pressure gradients, reduced surface area, and increased diffusion distance all slow gas exchange. Conditions like emphysema reduce alveolar surface area, while pulmonary edema extends diffusion distance by accumulating interstitial fluid. Drugs such as morphine slow respiratory rate, decreasing oxygen and carbon dioxide exchange rates between alveoli and blood.
Q5: How does altitude affect oxygen diffusion into the blood?
At higher altitudes, both total atmospheric pressure and oxygen partial pressure decline, reducing the pressure gradient between alveolar air and pulmonary capillaries. This slower oxygen diffusion results in decreased blood oxygen levels, causing high-altitude sickness symptoms including breathlessness, headache, fatigue, insomnia, nausea, and dizziness.
Q6: Why is the large surface area of alveoli important for gas exchange?
The vast surface area provided by alveoli, combined with abundant surrounding capillaries, enables efficient gas exchange between air and blood. This extensive interface maximizes the opportunity for oxygen and carbon dioxide molecules to cross the thin respiratory membrane. Conditions reducing this surface area, such as emphysema, significantly impair gas exchange efficiency.
Q7: How does the thin respiratory membrane facilitate gas exchange?
The thin respiratory membrane minimizes diffusion distance, allowing oxygen and carbon dioxide to cross quickly between alveolar air and blood. Red blood cells pass single-file through narrow capillaries, further reducing diffusion distance and maximizing contact time with the alveolar membrane. This structural efficiency enables rapid, effective gas exchange during both external and internal respiration.