Ventilation, commonly known as breathing, is the process of moving air between the atmosphere and the lungs. This continuous movement of air is fundamental for human life, bringing oxygen into the body and expelling carbon dioxide. Air naturally flows from areas of higher pressure to areas of lower pressure, and muscular actions create these necessary pressure differences within the respiratory system to facilitate breathing.
The Process of Inhaling
Inhalation is an active process driven by muscle contraction. The primary muscles involved in quiet, resting inhalation are the diaphragm and the external intercostal muscles. The diaphragm, a dome-shaped muscle separating the chest and abdomen, contracts and flattens, moving downward into the abdominal cavity. Simultaneously, the external intercostal muscles, located between the ribs, contract to pull the rib cage upward and outward.
These coordinated muscle contractions increase the volume of the thoracic cavity. According to Boyle’s Law, an increase in thoracic volume leads to a decrease in the pressure inside the lungs, known as intrapulmonary pressure. When intrapulmonary pressure falls below the atmospheric pressure outside the body, air flows from the higher external pressure into the lungs until the pressures equalize. During forced inhalation, such as during exercise, accessory muscles like the sternocleidomastoid, scalenes, pectoralis minor, and serratus anterior also contract, further expanding the rib cage and increasing lung volume.
The Process of Exhaling
Exhalation during quiet breathing is primarily a passive process, relying on the relaxation of the inspiratory muscles and the natural elastic recoil of the lungs and chest wall. As the diaphragm and external intercostal muscles relax, the thoracic cavity volume decreases. The diaphragm returns to its dome shape, and the rib cage moves downward and inward.
This reduction in thoracic cavity volume compresses the lungs, leading to an increase in intrapulmonary pressure above atmospheric pressure. This pressure gradient then forces air out of the lungs until the internal and external pressures become equal. When more air needs to be expelled, such as during exercise or a cough, forced exhalation becomes an active process, engaging additional muscles like the internal intercostals and abdominal muscles (rectus abdominis, transverse abdominis, external oblique, and internal oblique). These muscles actively pull the rib cage downward and inward or push the abdominal organs upward against the diaphragm, further decreasing thoracic volume and expelling more air.
Factors Affecting Airflow
The ease with which air moves into and out of the lungs is influenced by several physical properties of the respiratory system. Airway resistance is one factor, referring to the opposition to airflow caused by friction within the airways. The diameter of the airways is a major determinant of resistance; a smaller diameter increases resistance. Halving the radius of an airway can increase its resistance sixteen-fold.
Lung compliance is another property that affects airflow, describing the “stretchiness” or expandability of the lungs and chest wall. High compliance means the lungs can expand easily with a small change in pressure, requiring less effort to inflate. Conversely, low compliance indicates a stiff lung that is harder to inflate. Surfactant, a substance produced by cells in the lungs, helps maintain compliance by reducing surface tension within the small air sacs, preventing their collapse.
Regulation of Breathing
Breathing is largely an automatic process, controlled subconsciously by respiratory centers located in the brainstem. These centers generate the basic rhythm of breathing and receive continuous input from various sensors throughout the body. While breathing is usually involuntary, conscious control is possible for activities like speaking or holding one’s breath, though the brain’s automatic control eventually overrides prolonged voluntary suppression.
Chemoreceptors play a role in adjusting breathing rate and depth by monitoring blood gas levels. Central chemoreceptors are sensitive to changes in carbon dioxide levels and the resulting pH of the cerebrospinal fluid. An increase in carbon dioxide in the blood leads to a decrease in pH, which stimulates these receptors to increase ventilation, helping to expel excess carbon dioxide. Peripheral chemoreceptors are found in the carotid arteries and aorta, monitoring levels of oxygen, carbon dioxide, and pH in the arterial blood. These receptors also respond to changes in oxygen levels, as well as carbon dioxide and pH.