Describe the mechanism of respiration in man?

Human respiration happens in two main stages — inspiration (inhalation) and expiration (exhalation). These processes are controlled by the diaphragm and intercostal muscles, which contract and relax to move air in and out of the lungs.

Inspiration (Inhalation)

1. Muscle Contraction: The diaphragm contracts and moves downward, while the external intercostal muscles (between the ribs) contract to lift the rib cage upward and outward.

2. Volume and Pressure Change: This movement enlarges the thoracic cavity, increasing its volume. As the volume increases, the air pressure inside the lungs decreases, becoming lower than the atmospheric pressure.

3. Air Intake: Since air naturally moves from areas of high pressure to low pressure, it flows into the lungs through the respiratory tract to equalize the pressure difference.

Expiration (Exhalation)

1. Muscle Relaxation: The diaphragm relaxes and returns to its dome shape, moving upward. The external intercostal muscles also relax, allowing the rib cage to move downward and inward.

2. Volume and Pressure Change: As the thoracic cavity’s volume decreases, the air pressure inside the lungs rises above the external pressure.

3. Air Exit: The higher pressure pushes air rich in carbon dioxide out of the lungs and into the atmosphere.

Gas Exchange

Once air enters the lungs, it travels to the alveoli — tiny air sacs surrounded by capillaries.

1. External Respiration: Oxygen from the inhaled air passes through the thin alveolar walls into the blood. At the same time, carbon dioxide moves from the blood into the alveoli to be exhaled.

2. Internal Respiration: The oxygenated blood circulates through the body, delivering oxygen to tissues for cellular respiration, which produces energy. The carbon dioxide formed as a waste product is then carried back to the lungs for removal.