Full Answer Section

The lower respiratory tract consists of the trachea, bronchi, bronchioles, alveolar ducts, and alveoli. Structurally, it is dedicated to gas exchange. The trachea and bronchi are cartilaginous tubes that further conduct air into the lungs, branching into smaller bronchioles. The bronchioles eventually lead to the alveoli, tiny air sacs surrounded by a dense network of capillaries. The thin walls of the alveoli and the capillaries facilitate efficient gas exchange.

The sentence, “Both internal and external respiration depends on diffusion,” highlights a fundamental principle of gas exchange. External respiration, the exchange of gases between the lungs and the blood, occurs in the alveoli. Oxygen (O2) inhaled into the alveoli has a higher partial pressure than in the deoxygenated blood within the surrounding capillaries. This pressure gradient drives the oxygen to diffuse across the thin alveolar and capillary walls into the bloodstream, where it binds to hemoglobin in red blood cells. Simultaneously, carbon dioxide (CO2), a waste product of cellular metabolism, has a higher partial pressure in the blood within the capillaries than in the alveoli. Consequently, CO2 diffuses from the blood into the alveoli to be exhaled.

Internal respiration, the exchange of gases between the blood and the body’s tissues, also relies on diffusion. Oxygenated blood, now rich in oxygen, travels through the capillaries to the tissues where cellular respiration has consumed oxygen, resulting in a lower partial pressure of oxygen in the tissue fluid. This gradient causes oxygen to diffuse out of the capillaries and into the tissue cells. Conversely, the metabolic activity of tissue cells produces CO2, leading to a higher partial pressure of CO2 in the tissue fluid compared to the blood in the capillaries. Therefore, CO2 diffuses from the tissues into the capillaries to be transported back to the lungs for elimination. In both processes, the movement of oxygen and CO2 is passive, driven by the concentration gradients across the thin membranes of the alveoli and tissue capillaries.