The in the alveoli is 40 mm Hg and that of the blood entering the pulmonary capillaries is . This causes to diffuse down its partial pressure gradient from the blood into the alveoli.

The PCO₂ -carbon dioxide partial pressure- in the alveoli is 40 mm Hg and that of the blood entering the pulmonary capillaries is 45 mmHg. This causes carbon dioxide to diffuse down its partial pressure gradient from the blood into the alveoli.

Explanation:

Gas exchange is a physiological process that involves the entry of oxygen into the body and tissues and the exit of carbon dioxide, a product of metabolic reactions.

At the pulmonary level, gas exchange occurs between the alveoli and the alveolar capillary, and the diffusion of gases across the alveolar-capillary barrier is dependent on a pressure gradient due to the partial pressure of gases.

In the case of CO₂ the diffusion goes from where the partial pressure is higher to where it is lower, i.e. from the alveolar capillary, where the PCO₂ is 45 mmHg, to the pulmonary alveolus, where the PCO₂ is 40 mmHg.

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Gas exchange link