Respiratory System

The respiratory system is the body system that carries oxygen from the air into the blood and removes carbon dioxide (CO₂) from it. The main organs are the nose, pharynx, larynx, trachea, bronchi, and lungs. The principal muscle of respiration is the diaphragm, which separates the chest from the abdomen.

The system is divided into two parts: the airways and the site of gas exchange. Air terminates in small sacs called alveoli, whose thin walls are in close contact with blood capillaries. At this point oxygen and CO₂ are exchanged through diffusion, a process driven by the difference in gas concentrations.

Breathing

Inhalation begins when the diaphragm and intercostal muscles contract, the chest expands, and atmospheric pressure drops. Air then enters. Normal exhalation is passive: the muscles relax, and the lungs' elasticity pushes air outward. The rate and depth of breathing are controlled by centres in the brainstem, which monitor CO₂ levels and blood pH.

Gas Transport

Most oxygen is carried by haemoglobin, a protein in red blood cells. CO₂ is transported in multiple forms: as bicarbonate, bound to proteins, and dissolved in plasma. CO₂ levels directly affect blood pH, so respiration is one of the body's mechanisms for maintaining acid-base balance.

Defence and Clearance

The airways contain mucus-secreting tissue and small cilia that sweep mucus and dust backward. The nose warms and humidifies air. Coughing is a rapid reflex that clears the airways. The lungs also host immune cells called macrophages that engulf bacteria.

Related Disorders

Common diseases include asthma (airway inflammation), chronic obstructive pulmonary disease (COPD), pneumonia, influenza, and lung cancer. Smoking, air pollution, and occupational exposures (dust, chemicals) are recognized risk factors.