Respiration is the continuous exchange of gases fundamental to sustaining human life. Every breath draws air into the body, allowing us to absorb elements necessary for cellular function. The composition of the air we exhale is significantly altered compared to the air we inhale, reflecting the body’s internal biological activity and how much oxygen remains after gas exchange.
The Composition of Inhaled and Exhaled Air
The air we inhale is composed primarily of nitrogen, making up approximately 78% of the volume. Oxygen constitutes about 21% of the inhaled mixture. The remaining fraction consists of argon, water vapor, and a small amount of carbon dioxide, usually around 0.04% of the total.
After gas exchange occurs within the lungs, the composition of the air we exhale changes noticeably. The percentage of nitrogen remains largely stable at about 78%, as the body does not metabolize this gas. The most significant changes occur in oxygen and carbon dioxide, the two gases involved in metabolism.
The air that leaves the body still contains a considerable amount of oxygen, measuring roughly 16% of the exhaled volume. This reduction from the initial 21% means only a fraction of the inhaled oxygen is absorbed and used. Conversely, the concentration of carbon dioxide increases substantially, rising from a trace amount to about 4% to 4.4% of the total exhaled air.
How the Body Utilizes Oxygen
The drop in oxygen percentage begins in the lungs’ air sacs, called alveoli. Oxygen from the inhaled air crosses the thin barrier of the alveolar walls and enters the surrounding capillaries through diffusion. This movement is driven by the difference in concentration, or partial pressure, of oxygen between the air in the alveoli and the deoxygenated blood arriving from the body.
Once in the bloodstream, oxygen molecules are picked up by hemoglobin, a protein contained within red blood cells. Hemoglobin acts as a transport mechanism, binding to oxygen to efficiently carry it away from the lungs and circulate it throughout the body. The heart pumps this oxygen-rich blood to all tissues and organs.
At the level of the body’s tissues, oxygen diffuses out of the blood and into the individual cells. Here, oxygen is used in aerobic cellular respiration, the process by which cells convert nutrients, such as glucose, into usable energy (adenosine triphosphate or ATP). This conversion is the reason the body constantly absorbs oxygen. The production of ATP provides the energy required for every function, from muscle contraction to brain activity.
The Role of Other Gases in Exhalation
While oxygen is consumed by the body, nitrogen and carbon dioxide play distinct roles in exhalation. Nitrogen is an inert gas in human respiration because the body does not metabolize it. Consequently, the nitrogen we breathe in travels into the lungs, dissolves slightly in the blood, and is then expelled almost entirely unchanged in concentration.
Carbon dioxide sees a dramatic increase in exhaled air because it is the primary waste product of cellular respiration. As cells produce energy using oxygen, they generate carbon dioxide as a byproduct, which is released into the bloodstream. The blood carries this carbon dioxide back to the lungs, where it diffuses from the capillaries into the alveoli, driven by the concentration gradient.
The expulsion of carbon dioxide is necessary for the body’s internal balance, as its concentration in the blood helps regulate blood acidity (pH). If too much carbon dioxide accumulates, the blood becomes too acidic, disrupting numerous biological functions. The respiratory system manages this exchange, ensuring a continuous supply of oxygen and the removal of metabolic waste.