UNIT 4: RESPIRATORY SYSTEM
The respiratory system works in close partnership with the cardiovascular system to facilitate gas exchange, ensuring that oxygen is extracted from the atmosphere and delivered to the blood, while carbon dioxide is removed from the body. During physical activity, the respiratory system undergoes immediate and significant adjustments to maintain homeostasis and meet the elevated metabolic demands of contracting muscles. Understanding this system requires examining the air pathway, the mechanics of breathing, and how specific respiratory volumes respond to exercise.
The process of respiration begins with pulmonary ventilation, commonly known as breathing, which is the mechanical movement of air into and out of the lungs. This movement is governed by Boyle’s Law, which states that pressure and volume are inversely proportional. During inspiration (breathing in) at rest, the diaphragm contracts and flattens downward, while the external intercostal muscles contract to lift the ribcage upward and outward. This expansion increases the volume of the thoracic cavity, causing the air pressure inside the lungs to drop below atmospheric pressure. Air naturally rushes down this pressure gradient into the lungs. During exercise, this process is intensified by recruiting additional inspiratory muscles, such as the sternocleidomastoid, scalenes, and pectoralis minor, to expand the chest cavity even further and maximise air intake.
Conversely, expiration (breathing out) at rest is a passive process that relies on the elastic recoil of the lung tissue. The diaphragm and external intercostal muscles relax, reducing thoracic volume, increasing internal air pressure above atmospheric pressure, and forcing air out. However, during exercise, expiration becomes an active process because air must be expelled quickly to allow for the next breath. The internal intercostal muscles contract forcefully to pull the rib cage down and inward, while the abdominal muscles contract to push the diaphragm upward. This rapid compression drastically reduces thoracic volume, accelerating the removal of carbon dioxide-rich air from the lungs.
