Breathing, also known as ventilation, is a fundamental biological process that moves air into and out of the lungs. This rhythmic movement ensures the body receives oxygen and expels carbon dioxide. It involves two phases: inspiration (inhaling), which draws air into the lungs, and expiration (exhaling), which releases air.
Defining Active and Passive Biological Processes
In biology, processes are categorized as either active or passive based on their energy requirements. An active biological process demands direct energy input, often in the form of adenosine triphosphate (ATP), for a specific function. This energy expenditure typically powers cellular work, such as muscle contraction or the movement of substances against a concentration gradient.
Conversely, a passive biological process occurs without the direct expenditure of cellular energy. These processes often rely on natural physical forces, gradients, or the inherent properties of tissues and structures. Examples include diffusion, where molecules move from higher to lower concentration, or the elastic recoil of a stretched material returning to its original shape.
The Mechanics of Normal Expiration
Normal, quiet expiration is primarily a passive process, requiring no direct muscle contraction or energy expenditure. This phase begins with the relaxation of the inspiratory muscles active during inhalation. The diaphragm, a dome-shaped muscle that flattens during inspiration, relaxes and returns to its upward, resting position. Simultaneously, the external intercostal muscles, located between the ribs, also relax.
The relaxation of these muscles leads to a decrease in the volume of the thoracic cavity. A significant factor in this passive air expulsion is the elastic recoil of the lung tissue and the chest wall. The lungs, having been stretched during inspiration, naturally rebound to their smaller, original size. This reduction in lung volume increases the pressure within the lungs, known as intrapulmonary pressure, higher than atmospheric pressure. Air then flows out of the lungs, down this pressure gradient, until the pressures inside and outside the lungs equalize.
Forced Expiration: An Active Process
While normal expiration is typically passive, breathing out can become an active process under certain conditions, requiring muscle contraction and energy. Situations that necessitate forced expiration include strenuous exercise, coughing, sneezing, blowing out candles, or speaking loudly.
During forced expiration, specific muscles contract to further decrease the volume of the thoracic cavity, expelling more air than during quiet breathing. The internal intercostal muscles, located beneath the external intercostals, contract to pull the ribs downward and inward. Additionally, the abdominal muscles, including the rectus abdominis, external obliques, internal obliques, and transversus abdominis, contract forcefully. This contraction pushes the diaphragm further upward into the thoracic cavity, significantly reducing its volume and increasing intrapulmonary pressure to forcefully expel air.