Inspiration, or inhaling, is the initial and active phase of breathing, where air is drawn into the lungs. Breathing is a continuous, involuntary process fundamental to human life, ensuring the body receives oxygen and removes carbon dioxide. This process occurs without conscious thought for most of the day, involving a complex sequence of physical and neurological events. Inspiration is an active process because it requires muscle contraction to bring air into the respiratory system.
The Physics of Air Intake
The movement of air into the lungs during inspiration relies on the principles of pressure and volume. Air naturally flows from higher to lower pressure, creating a pressure gradient. To draw air into the lungs, the pressure inside the lungs must become lower than the atmospheric pressure outside the body.
This pressure difference is achieved through changes in the volume of the thoracic cavity, the space containing the lungs. According to Boyle’s Law, when the volume of a gas increases, its pressure decreases. Conversely, decreasing the volume increases the pressure. During inspiration, the thoracic cavity expands, which increases lung volume and reduces intrapulmonary pressure.
The lungs are held close to the inner wall of the thoracic cavity by a fluid-filled space called the pleural cavity. As the thoracic cavity expands, the lungs are pulled outward and expand with it. This expansion causes the pressure inside the lungs to drop below atmospheric pressure, creating the gradient for air to fill the lungs.
Muscles Driving Inhalation
The primary muscles for quiet inspiration are the diaphragm and the external intercostal muscles. The diaphragm, a large, dome-shaped muscle at the base of the chest cavity, is the primary muscle for quiet breathing. When it contracts, the diaphragm flattens and moves downward, significantly increasing the vertical dimension of the thoracic cavity. This action alone accounts for about 75% of the increase in thoracic volume during quiet inspiration.
Simultaneously, the external intercostal muscles, located between the ribs, contract and pull the rib cage upward and outward. This movement expands the thoracic cavity in the anterior-posterior and lateral dimensions. The combined action of these muscles effectively increases the overall volume within the chest, drawing air into the lungs.
During forced or deeper inspiration, additional accessory muscles are recruited to further increase thoracic volume. These include the sternocleidomastoid and scalene muscles in the neck, which elevate the sternum and the first two ribs. Other muscles like the pectoralis major and minor, serratus anterior, and latissimus dorsi can also assist by pulling the ribs outwards and upwards.
Brain Control of Breathing
The act of inspiration, though often unconscious, is precisely regulated by the nervous system, primarily within the brainstem. The medulla oblongata and the pons contain specialized respiratory centers that generate and maintain the basic rhythm of breathing.
In the medulla, the dorsal respiratory group (DRG) is mainly responsible for stimulating the inspiratory muscles, setting the pace for normal, quiet breathing. The ventral respiratory group (VRG), also in the medulla, is usually inactive during quiet breathing but becomes active during forced inspiration or expiration, controlling accessory muscles. The pontine respiratory group in the pons helps smooth the transition between inspiration and expiration, contributing to the rhythm and depth of breaths.
The rate and depth of inspiration are also influenced by chemoreceptors, sensory organs that monitor chemical levels in the blood and cerebrospinal fluid. Central chemoreceptors in the brainstem are sensitive to changes in carbon dioxide levels and blood pH. Peripheral chemoreceptors in the carotid arteries and aorta also detect changes in oxygen, carbon dioxide, and pH, sending signals to the respiratory centers to adjust breathing as needed.