Explain the mechanism of breathing in human beings?

Humans breathe by expanding the chest to draw air in and contracting it to push air out.

Inhalation (inspiration): The diaphragm contracts and moves downward while external intercostal muscles lift the ribs up and out. This increases thoracic volume and decreases pressure inside the lungs. Air flows from the outside (higher pressure) into the lungs (lower pressure). Alveoli fill with air, making gas exchange possible.

Exhalation (expiration): The diaphragm relaxes and moves upward. The ribs move down and in as intercostal muscles relax. Thoracic volume decreases and pressure inside the lungs rises, pushing air out. At rest, exhalation is mostly passive due to elastic recoil; during exercise, abdominal and internal intercostal muscles help.

Gas exchange and transport

• Alveoli: Tiny air sacs with thin walls and rich capillary supply for oxygen uptake and carbon dioxide release.
• Oxygen transport: Mostly as oxyhemoglobin in red blood cells.
• Carbon dioxide transport: Mainly as bicarbonate ions in plasma; some binds to hemoglobin.

Control and terms

• Control: Medulla and pons regulate breathing rate. Chemoreceptors sense CO₂, O₂, and pH.
• Tidal volume: Normal breath volume (~500 mL in adults).
• Ventilation vs respiration: Ventilation is air movement; respiration includes cellular energy release.