The act of breathing is fundamental to life, a continuous process that sustains every cell. This exchange involves taking in air and releasing gases. Inhalation brings in necessary components, while exhalation serves as a mechanism for expelling waste products and maintaining physiological balance. Understanding exhaled breath composition provides insights into the body’s internal workings.
Primary Gases Released
Exhaled breath consists of four main gases: nitrogen, oxygen, carbon dioxide, and water vapor. Nitrogen makes up the largest portion, approximately 78-79% of exhaled air, similar to its concentration in inhaled air because the body does not use it. Oxygen, vital for cellular function, decreases significantly from about 21% in inhaled air to roughly 13.6-16% in exhaled air, as a portion is absorbed by the body.
Carbon dioxide, a metabolic byproduct, shows a notable increase, rising from a negligible 0.03-0.04% in inhaled air to about 4-5.3% in exhaled breath. This jump highlights its role as a waste gas. Water vapor is also a substantial component, typically saturating the breath at body temperature, accounting for 5-6.3% by volume. The precise amount can vary depending on external humidity and individual factors.
How These Gases Are Produced
Carbon dioxide production results from cellular respiration, a process where cells convert nutrients like glucose, fats, and proteins into energy. This reaction occurs within the mitochondria of nearly every cell, using oxygen to break down fuel and releasing carbon dioxide as a byproduct. The generated carbon dioxide then dissolves into the bloodstream or binds to hemoglobin for transport.
Once in the blood, carbon dioxide travels to the lungs for removal. Gas exchange, where oxygen and carbon dioxide move between the bloodstream and the lungs, takes place in tiny air sacs called alveoli. Carbon dioxide diffuses from capillaries, rich in the gas, into the alveoli, where its concentration is lower. Simultaneously, oxygen from the inhaled air diffuses from the alveoli into the oxygen-depleted blood in the capillaries. This exchange ensures the body receives adequate oxygen while efficiently expelling carbon dioxide.
Other Substances in Exhaled Breath
Beyond primary gases, exhaled breath contains other substances, often in trace amounts, which can provide insights into a person’s health. These include volatile organic compounds (VOCs), organic chemicals with high vapor pressure that easily become gases at room temperature. Hundreds of VOCs have been identified in exhaled breath, originating from various metabolic processes.
Acetone is one VOC present in breath, often as a byproduct of fat metabolism. Its levels can increase in conditions where the body breaks down fats for energy, such as during fasting or in uncontrolled diabetes. Other trace gases, including methanol, isoprene, ethanol, and various hydrocarbons, are also part of the exhaled mixture. These compounds can be produced endogenously by the body’s cells, or be exogenous, absorbed from the environment or produced by microbial activity, such as in the gut.