The respiratory cycle is the continuous sequence of breathing. Each cycle consists of one inhalation, where air is drawn into the lungs, and one exhalation, where air is expelled. This rhythmic pattern is largely automatic, proceeding without conscious thought. On average, a resting adult completes 12 to 16 of these cycles per minute. The process ensures a constant supply of oxygen to the body and the removal of carbon dioxide.
The Two Phases of Breathing
Breathing has two phases, inspiration (inhaling) and expiration (exhaling), governed by pressure differences. Air flows from an area of higher pressure to an area of lower pressure. Inspiration is an active process that begins when the pressure inside the lungs is made lower than the pressure of the atmosphere, causing air to rush in.
This pressure change is achieved by increasing the volume of the chest cavity. Think of it like a bellows: as you pull the handles apart, the internal volume increases, creating a vacuum that sucks air in. When the chest expands, the lungs expand with it, lowering the internal air pressure and drawing air inward. About 500 ml of air is moved with each normal breath, a measure known as the tidal volume.
Quiet expiration, on the other hand, is a passive process. It relies on the natural elasticity of the lungs and chest wall. As the muscles used for inspiration relax, the chest cavity returns to its smaller, resting size. This decrease in volume increases the pressure inside the lungs to a level greater than the atmospheric pressure, forcing air out. A brief pause may occur after exhalation before the next cycle starts.
Muscular Action Driving the Cycle
The mechanical work of breathing is performed by specialized muscles. The primary muscle of respiration is the diaphragm, a large, dome-shaped sheet of muscle located at the base of the chest cavity. During inspiration, the diaphragm contracts and flattens, moving downward toward the abdomen. This action increases the vertical dimension of the thoracic cavity, creating more space for the lungs.
Simultaneously, another set of muscles, the external intercostal muscles located between the ribs, also contract. The contraction of these muscles pulls the rib cage upward and outward, expanding the diameter of the chest. This expansion directly causes the drop in internal lung pressure that drives inhalation.
During quiet exhalation, these processes reverse without effort. The diaphragm and external intercostal muscles relax. The diaphragm domes upward, and the rib cage moves down and inward. This decreases the volume of the chest cavity, compressing the lungs and pushing the air out. Only during forceful exhalation, such as during exercise, do other muscles in the abdomen and ribs actively contract to expel air more rapidly.
The Role of Gas Exchange
The purpose of moving air into and out of the lungs is to facilitate gas exchange. This event occurs deep within the lungs in tiny, balloon-like air sacs called alveoli. The lungs contain millions of these microscopic structures, which are surrounded by a dense network of blood vessels known as capillaries. This is where the respiratory system meets the circulatory system.
The exchange of gases happens through a process called diffusion. Inhaled air that reaches the alveoli is rich in oxygen. Blood arriving in the capillaries from the body’s tissues is low in oxygen but high in carbon dioxide, a waste product of metabolism. Because gases naturally move from an area of higher concentration to one of lower concentration, oxygen moves from the alveoli into the blood.
At the same time, the high concentration of carbon dioxide in the capillary blood causes it to diffuse out of the blood and into the air within the alveoli. This carbon dioxide is then carried out of the body during exhalation. This exchange ensures that blood leaving the lungs is re-oxygenated and ready to be pumped to the rest of the body.
Regulating the Pace of Respiration
The automatic nature of the respiratory cycle is managed by a control center in the brain. Located in the brainstem, specifically within the medulla oblongata and pons, are groups of neurons that form the respiratory center. This center functions as a pacemaker, setting the basic rhythm and rate of breathing. It sends out regular nerve impulses that trigger the contraction of the diaphragm and intercostal muscles, initiating inspiration.
This automatic system can adjust the rate and depth of breathing. The primary factor that drives these adjustments is the concentration of carbon dioxide in the blood. Sensors called chemoreceptors, located in the brainstem and in major arteries, monitor the blood’s chemical composition, including its pH, which is directly affected by CO2 levels.
When carbon dioxide levels rise, as they do during exercise, the blood becomes more acidic. Chemoreceptors detect this change and signal the respiratory center to increase the frequency and depth of breaths. This response helps to expel CO2 more quickly and restore normal blood chemistry. Other factors, such as emotional state, speech, and conscious will, can also influence the respiratory center.