Question

Define external respiration and explain the mechanism of external respiration (be sure to explain it in terms of partial pressure) and describe the structure of the respiratory membrane and how its structure facilitates external respiration. Which gas (O2 or CO2) is easier to exchange in the alveoli? Explain why.

Answer 1

External respiration : Gas exchange occurs by diffusion. ... External respiration occurs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses into the alveolar air.

Machanism:

Surface Area

The alveoli have a very high surface area to volume ratio that allows for efficient gas exchange. The alveoli are covered with a high density of capillaries that provide many sites for gas exchange.

The walls of the alveolar membrane are thin and covered with a fluid, extra-cellular matrix that provides a surface for gas molecules in the air of the lungs to diffuse into, from which they can then diffuse into the capillaries.

Partial Pressure Gradients

Partial pressure gradients (differences in partial pressure) allow the loading of oxygen into the bloodstream and the unloading of carbon dioxide out of the bloodstream. These two processes occur at the same time.

Gas exchange in the alveolus: External respiration is a result of partial pressure gradients, alveolar surface area, and ventilation and perfusion matching.

Oxygen has a partial pressure gradient of about 60 mmHg (100 mmHg in alveolar air and 40 mmHg in deoxygenated blood ) and diffuses rapidly from the alveolar air into the capillary.

Equilibrium between the alveolar air and capillaries is reached quickly, within the first third of the length of the capillary within a third of a second. The partial pressure of oxygen in the oxygenated blood of the capillary after oxygen loading is about 100 mmHg.

The process is similar in carbon dioxide. The partial pressure gradient for carbon dioxide is much smaller compared to oxygen, being only 5 mmHg (45 mmHg in deoxygenated blood and 40 mmHg in alveolar air).

Based on Henry’s law, the greater solubility of carbon dioxide in blood compared to oxygen means that diffusion will still occur very rapidly despite the lower partial pressure gradient. Equilibrium between the alveolar air and the capillaries for carbon dioxide is reached within the first half of the length of the capillaries within half a second. The partial pressure of carbon dioxide in the blood leaving the capillaries is 40 mmHg.