Breathing is the continuous process of exchanging gases between the body and the outside world. This process, known as external respiration, involves taking air into the lungs and then expelling it. The purpose of this exchange is to supply the body with gases for metabolism while simultaneously removing a gaseous waste product. Analyzing the chemistry of inhaled versus exhaled air reveals the precise chemical transaction occurring internally.
Composition of Air We Inhale
The air we inhale has a stable chemical makeup. The largest component of inhaled air is nitrogen, which accounts for approximately 78% of the total volume. Nitrogen is a largely inert gas in the respiratory process, meaning it enters and exits the lungs without being chemically altered. Oxygen makes up about 21% of the air we draw into our lungs. The remaining 1% is composed of various trace gases, including argon, as well as a small amount of carbon dioxide, present at roughly 0.04%.
Primary Gases in Exhaled Air
The air we exhale shows a distinct change in composition compared to the air we inhaled, reflecting the body’s internal gas exchange. The percentage of oxygen drops significantly, decreasing from the inhaled 21% to about 16% in the expelled air. This reduction shows the amount of oxygen transferred from the lungs into the bloodstream. Conversely, carbon dioxide sees a massive proportional increase, rising from the atmospheric 0.04% to around 4% in the exhaled breath. Nitrogen content remains virtually unchanged at approximately 78%, confirming its role as an inactive participant in respiration.
Another major difference is the water vapor content, which is significantly higher in exhaled air. The air we breathe out is nearly saturated with moisture, which is why breath is visible on a cold day. This moisture is picked up as air passes through the warm, moist environment of the respiratory tract and the lungs. Exhaled air is also warmer than inhaled air because it has been heated to body temperature.
The Metabolic Source of Carbon Dioxide
The elevated concentration of carbon dioxide in exhaled air originates at the cellular level through a process called cellular respiration. This is the mechanism by which the body converts the chemical energy stored in nutrients, primarily glucose, into a usable form of energy called adenosine triphosphate (ATP). The process requires oxygen, which is continually supplied via the bloodstream.
During this series of reactions, glucose and oxygen are chemically transformed, producing ATP, water, and carbon dioxide as a waste product. The carbon atoms from the broken-down nutrient molecules combine with oxygen atoms, forming carbon dioxide. This gas then diffuses out of the cells and into the surrounding capillaries, where it is picked up by the blood.
The circulatory system is responsible for transporting this gaseous waste from all tissues back to the lungs. Most of the carbon dioxide does not travel as a dissolved gas but is instead chemically converted within red blood cells and carried as bicarbonate ions. Once the blood reaches the tiny air sacs in the lungs, called the alveoli, the process reverses, allowing the carbon dioxide to diffuse into the air space for removal during exhalation.