The respiratory system draws air from the environment to transfer oxygen to the bloodstream. This process is necessary because every cell in the body requires a constant supply of oxygen to produce the energy needed for life. The lungs facilitate this movement, ensuring the body is continually refreshed with atmospheric gas through the interaction between inhaled air and circulating blood.
The Exchange Location: Alveoli and the Respiratory Membrane
Oxygen moves into the blood within millions of microscopic air sacs called alveoli. These cup-shaped cavities are surrounded by a dense network of tiny blood vessels known as pulmonary capillaries. The close proximity of the air inside the alveoli to the blood in the capillaries makes gas exchange possible.
The respiratory membrane separates the air from the blood and is extremely thin to promote rapid gas transfer. This membrane is composed of the alveolar wall, the capillary wall, and a fused basement membrane between them. The total thickness of this barrier is typically less than one micrometer.
The lungs contain an immense number of alveoli, collectively providing a massive surface area for gas exchange comparable to the size of a tennis court. This large area, combined with the membrane’s minimal thickness, creates ideal conditions for rapid oxygen movement into the circulating blood. The minimal diffusion distance and large surface area maximize the total amount of gas that can be transferred at any given moment.
The Driving Force: Partial Pressure and Gas Diffusion
Oxygen moves across the respiratory membrane via simple diffusion, which requires no active energy from the body. Gases move passively from an area of high concentration to an area of low concentration. In a gas mixture like air, this concentration is measured using a value called partial pressure.
Partial pressure represents the proportion of the total pressure that a single gas contributes to the mixture. In the air-filled alveoli, the partial pressure of oxygen (\(P_{O2}\)) is high, typically around 104 millimeters of mercury (mmHg) at sea level. Conversely, the deoxygenated blood returning to the lungs has a lower oxygen partial pressure, usually about 40 mmHg.
This substantial difference in partial pressure between the alveolar air (high \(P_{O2}\)) and the incoming capillary blood (low \(P_{O2}\)) creates a steep pressure gradient. This gradient acts as the driving force, causing oxygen molecules to diffuse from the alveoli, across the respiratory membrane, and into the blood plasma. The movement continues until the oxygen partial pressure in the blood equilibrates with the partial pressure in the alveoli, resulting in oxygenated blood with a \(P_{O2}\) of approximately 100 mmHg.
Oxygen’s Journey: Transport via Hemoglobin
Once oxygen diffuses into the blood plasma, it must be transported to the body’s tissues. Oxygen is not highly soluble in plasma, so only about 1.5% of the total remains dissolved. The vast majority of oxygen, approximately 98.5%, is carried by a specialized protein called hemoglobin.
Hemoglobin is found inside red blood cells, and each molecule can bind up to four oxygen molecules. Binding oxygen to hemoglobin as the blood passes through the lungs maintains the partial pressure gradient. This prevents the dissolved oxygen from quickly reaching equilibrium with the alveolar air, allowing for maximum oxygen transfer.
When oxygen binds to hemoglobin, the resulting complex is called oxyhemoglobin, which significantly increases the blood’s total oxygen-carrying capacity. Without this mechanism, the heart would need to pump a much larger volume of blood to meet the body’s metabolic demands. The oxygen-loaded red blood cells exit the lungs and circulate, ready to release their cargo at tissues where the \(P_{O2}\) is lower.