External and Internal respiration

Question

Explain the direction of O2 and CO2 flow seen in external and internal respiration

Answer 1

Both O2 and CO2 move down their concentration gradient, primarily in the capillary beds located within metabolically active tissues (internal respiration) and in the lungs (external respiration).

The reason we breath is simply to transfer O2 from the atmosphere/air into the blood, which then travels to aerobically active tissues that consume oxygen during a process called cellular respiration aka oxidative phosphorylation (where the concentration of O2 is lower than in the blood which is lower than at the lungs/atmosphere).

Conversely, catabolic reactions like those of the TCA cycle generate CO2 (highest in the tisues), so it diffuses into the blood where it is picked up and transported to the lungs where it moves into the lungs to be breathed off/expired.