26.16
External and Internal Respiration
External respiration occurs in the lungs, and it is the first step in the journey of oxygen inside the body. When we inhale, oxygen enters our lungs a…
External respiration involves the exchange of oxygen and carbon dioxide between alveoli and pulmonary capillaries in the lungs.
This gas exchange always occurs from an area of higher partial pressure to lower or from Phigh to Plow .
Oxygen diffuses from the alveolar air, where PO2 is 105 mmHg, into the blood within pulmonary capillaries, where PO2 is 40 mmHg.
Similarly, carbon dioxide is driven from blood at a PCO2 of 45 mmHg to alveolar air with a PCO2 of 40 mmHg.
In internal respiration, oxygen is transferred from systemic capillaries to tissue cells throughout the body, while CO2 moves in the opposite direction.
Systemic capillaries have a higher PO2 than tissue cells because oxygen is spent for ATP production in cells, prompting oxygen to diffuse from the capillaries into the cells.
Conversely, CO2 production by active metabolism results in higher PCO2 in cells than in systemic capillaries, causing CO2 to diffuse from cells into these capillaries.
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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.