Breathing, or pulmonary ventilation, is the physical movement of air into and out of the lungs. This continuous cycle is necessary for gas exchange, supplying oxygen and removing carbon dioxide. Breathing occurs in two distinct phases: inhalation, the movement of air into the lungs, and exhalation, the movement of air out of the lungs. Understanding the differences between these actions reveals the sophisticated engineering of the human respiratory system.
The Mechanics of Inhalation
Inhalation is an active process requiring the contraction of specific muscles to draw air inward. The primary muscle is the diaphragm, a dome-shaped sheet of muscle beneath the lungs. When the diaphragm contracts, it flattens and moves downward, increasing the vertical dimension of the chest cavity. Simultaneously, the external intercostal muscles between the ribs contract, lifting the rib cage upward and outward.
This combined action significantly expands the volume of the thoracic cavity. According to Boyle’s Law, this increase in volume causes a decrease in pressure within the lungs, known as intra-alveolar pressure. This pressure drop creates a gradient, causing air to flow naturally from the higher atmospheric pressure outside the body into the lower pressure environment of the lungs. Air continues to rush in until the pressure inside the lungs equalizes with the atmospheric pressure.
The Mechanics of Exhalation
Exhalation involves mechanics that differ significantly based on the intensity of the breath. During quiet, relaxed breathing, exhalation is largely a passive process that does not require muscle contraction. It relies on the natural tendency of the expanded lung and chest wall tissues to return to their resting state, known as elastic recoil.
As the diaphragm and external intercostal muscles relax, the chest cavity volume decreases naturally. This reduction in volume compresses the air inside the lungs, causing the intra-alveolar pressure to rise above atmospheric pressure. The resulting pressure gradient forces the air out.
When a person needs to expel air forcefully, such as during exercise or speaking loudly, the process becomes active, requiring muscular effort. This forced exhalation involves the contraction of the internal intercostal muscles, which pull the rib cage inward, and the abdominal muscles, which push the diaphragm higher into the thoracic cavity. These active contractions drastically decrease the thoracic volume further, creating a much higher internal pressure to rapidly push a larger volume of air out.
Differences in Gas Composition
A distinction between inhalation and exhalation lies in the chemical composition of the air being moved. Inhaled air closely mirrors the atmosphere, consisting largely of nitrogen (approximately 78%) and oxygen (about 21%). Carbon dioxide makes up only a minimal fraction, around 0.04%.
The air expelled during exhalation reflects the gas exchange that occurred within the alveoli. The percentage of oxygen drops from 21% to approximately 16% or 17% because a portion is absorbed into the bloodstream for cellular use. Conversely, the concentration of carbon dioxide increases dramatically, rising from 0.04% to about 4% to 4.4%, as it is released from the blood as a metabolic waste product.
Nitrogen remains largely unchanged in both inhaled and exhaled air, as it is not utilized by the body. Exhaled air also contains significantly more water vapor than inhaled air because the air is humidified as it passes through the warm, moist respiratory passages and lungs.
Control of the Breathing Cycle
The rhythm and depth of the breathing cycle are managed by a regulatory system that operates mostly without conscious effort. The primary control center is located in the brainstem, specifically the medulla oblongata, which sets the basic, involuntary rhythm of inspiration and exhalation. This center sends signals to the respiratory muscles to ensure continuous ventilation.
The body constantly monitors the chemical balance of the blood to adjust the breathing rate as needed. Specialized sensory structures called chemoreceptors detect changes in blood chemistry. Central chemoreceptors, situated on the surface of the medulla, are highly sensitive to the concentration of carbon dioxide.
An increase in carbon dioxide leads to a slight decrease in blood pH, which prompts the brainstem to increase the rate and depth of breathing to expel the excess gas. Peripheral chemoreceptors in the carotid arteries and aorta also monitor blood oxygen levels. Carbon dioxide remains the main driver for respiratory adjustments under normal conditions, ensuring gas exchange remains efficient to meet metabolic demands.