Full Answer Section

“Both internal and external respiration depends on diffusion.” This statement highlights the fundamental passive process driving gas exchange in the respiratory system. External respiration occurs in the lungs. Oxygen (O2) from the inhaled air in the alveoli, where its concentration is high, diffuses across the thin alveolar walls and the adjacent capillary walls into the blood within the pulmonary capillaries, where the O2 concentration is lower. Simultaneously, carbon dioxide (CO2), a waste product of cellular metabolism, moves in the opposite direction. Its concentration is higher in the blood within the capillaries returning from the body’s tissues, so it diffuses into the alveoli, where the CO2 concentration is lower, to be exhaled. Internal respiration occurs at the tissue level. Oxygen-rich blood in the systemic capillaries surrounding body tissues has a high O2 concentration. This O2 diffuses into the tissue cells, where its concentration is lower due to cellular respiration. Conversely, CO2, produced by the cells and thus at a higher concentration within them, diffuses into the systemic capillaries to be carried back to the lungs for removal. In both scenarios, the movement of O2 and CO2 across membranes is driven by the concentration gradient, a key principle of diffusion.