26.16
外部和内部呼吸
外部呼吸发生在肺部，这是氧气在体内旅程的第一步。当我们吸气时，氧气进入肺部并扩散穿过薄薄的肺泡膜。肺泡是微小的充满空气的囊，为气体交换提供了巨大的表面积。肺泡中的氧气分压（105 毫米汞柱）高于相邻的肺毛细血管中的氧气分压（40 毫米汞柱），从而形成压力梯度。因此，氧分子从肺泡进入血液，与红细胞中的…
外呼吸涉及肺部肺泡与肺毛细血管之间氧气和二氧化碳的交换。
这种气体交换总是从较高分压区域向较低分压区域进行，即从 Phigh 到 Plow。
氧气从肺泡气中（PO2 为105 mmHg）扩散至肺毛细血管内的血液中（PO2为40 mmHg）。
同样，二氧化碳会从 PCO2 为 45 mmHg 的血液中扩散至 PCO2 为 40 mmHg 的肺泡气中。
在内呼吸过程中，氧气从体循环毛细血管转移至全身各处的组织细胞，而二氧化碳则从组织细胞转移至血液中。2 向相反方向移动。
体循环毛细血管的氧分压（PO2）高于组织细胞，因为细胞在产生ATP过程中消耗氧气，促使氧气从毛细血管扩散进入细胞。
相反，CO2 由活跃代谢产生的二氧化碳导致更高的PCO₂2 在细胞中的浓度高于全身毛细血管，导致CO2 从细胞扩散到这些毛细血管中。
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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.