Inhalation, also known as inspiration, is the active phase of breathing that draws air from the atmosphere into the respiratory system. This continuous, rhythmic action is generally regulated automatically by the nervous system. The process brings oxygen-rich air into the lungs for biological functions. The complete breathing cycle involves this air intake followed by exhalation, which removes metabolic waste gases.
Key Anatomical Components
The air pathway begins in the nasal cavity or the mouth, where the air is filtered, warmed, and moistened. Air then passes through the pharynx and enters the larynx. It moves into the trachea, a strong tube reinforced with C-shaped cartilage rings that prevent collapse.
The trachea descends into the chest cavity and branches into the two main bronchi, one leading to each lung. These tubes divide repeatedly, becoming smaller airways called bronchioles. The smallest bronchioles terminate in tiny, specialized air sacs known as alveoli, where gas exchange occurs.
The entire system is housed within the thoracic cavity, enclosed by the rib cage and the diaphragm muscle. The lungs and the inside of the chest wall are lined by pleural membranes. Fluid between these membranes creates a seal, ensuring the lungs expand when the chest wall and diaphragm move.
The Mechanics of Air Intake
Air moves into the lungs based on the principle that air flows from higher pressure to lower pressure. Inhalation is initiated by the contraction of two primary muscle groups that increase the volume of the chest cavity. The diaphragm contracts and flattens downward.
Simultaneously, the external intercostal muscles between the ribs contract, pulling the ribs upward and outward. The combined action of these muscles significantly expands the size of the thoracic cavity. Due to the pleural seal, the lungs are forced to expand along with the chest wall.
This increase in lung volume causes the pressure inside the lungs, known as intra-alveolar pressure, to drop below the atmospheric pressure. This pressure difference creates a vacuum effect. Air rushes in through the conducting airways until the pressure inside the lungs equalizes with the outside pressure, completing inhalation.
The Purpose: Gas Exchange
The purpose of drawing air into the lungs is to facilitate the exchange of gases necessary for cellular function and waste removal. This exchange occurs across the respiratory membrane within the alveoli, which are enveloped by a dense network of pulmonary capillaries.
The movement of gases across this membrane is accomplished by simple diffusion. Inhaled air has a high partial pressure of oxygen within the alveoli. Conversely, the blood arriving at the capillaries is oxygen-poor and carbon dioxide-rich.
This concentration difference causes oxygen molecules to move rapidly from the alveoli into the bloodstream, attaching to hemoglobin in red blood cells. Simultaneously, carbon dioxide diffuses out of the capillaries and into the alveolar air. The oxygenated blood is then circulated back to the heart for distribution.