The act of inhalation is a continuous, involuntary process that sustains life by drawing in the atmosphere surrounding us. This intake of air is the first step in a complex biological operation that powers every cell in the human body. The air we breathe is a mixture of several gases, which is processed by the respiratory system to extract what is needed and expel waste.
The Composition of Ambient Air
The air we inhale is a consistent blend of gases that makes up the Earth’s lower atmosphere. The largest component is nitrogen gas, comprising approximately 78% of the total volume. Nitrogen is an inert gas that passes through the body without being used.
Oxygen is the second most abundant gas, making up close to 21% of the air we draw in. Oxygen is the crucial element the body seeks to extract for metabolic processes. Argon, an inert noble gas, is the third most common component at about 0.93% of the volume.
The remaining fraction consists of trace gases, including carbon dioxide, which is present at a concentration of approximately 0.04%. Water vapor is another component, but its concentration varies widely depending on the environment, ranging from nearly 0% in dry climates up to 4% in humid conditions.
The Mechanism of Breathing and Gas Exchange
Drawing air into the lungs begins with the contraction of specialized muscles. The primary muscle of inhalation is the diaphragm, a dome-shaped sheet of muscle situated beneath the lungs that flattens and moves downward. Simultaneously, the external intercostal muscles between the ribs contract to pull the rib cage upward and outward. This coordinated muscular action dramatically increases the volume inside the thoracic cavity.
The expansion of the chest creates a negative pressure, or vacuum, inside the lungs relative to the air outside the body. This pressure difference causes the atmospheric air mixture to rush in through the mouth or nose, down the trachea, and into the branching network of airways. The air ultimately reaches millions of tiny air sacs called alveoli, which are surrounded by a dense mesh of capillaries.
Gas exchange occurs passively across the thin membrane separating the air in the alveoli from the blood in the capillaries. Oxygen molecules move from the high concentration area in the alveolar air into the low concentration area of the blood. At the same moment, carbon dioxide molecules move in the opposite direction, passing from the blood into the alveolar air to be expelled.
The Difference Between Inhaled and Exhaled Air
Exhaled air represents the output of the body’s energy production cycle, showing a distinct change in composition from the air that was inhaled. While the nitrogen percentage remains unchanged, the oxygen content drops significantly. Inhaled air is approximately 21% oxygen, but the air leaving the body is reduced to about 16% oxygen, reflecting the amount transferred into the bloodstream.
The most noticeable difference is the increase in carbon dioxide, the primary waste product of cellular respiration. Cells use oxygen and glucose to create usable energy, generating carbon dioxide as a byproduct that must be removed. Consequently, the concentration of carbon dioxide rises from the initial 0.04% in ambient air to about 4% in exhaled air.
Exhaled air is also much warmer and contains a much higher concentration of water vapor than the air that was initially inhaled. As the dry atmospheric air travels through the moist airways and lungs, it becomes fully saturated with water vapor from the body’s internal environment.