Respiratory Movements

In air breathing vertebrates, gaseous exchange occurs in lungs (external respiratory organs). These are suspended in the thoracic cavity, which is separated from the abdominal cavity by the diaphragm, a muscular sheet. The lungs are surrounded and protected by the rib cage, and enclosed within a double membraned sac (pleura) , which is both airtight and waterproof. The set up enables air breathing vertebrates to inhale fresh air from their external environment and exhale used air from their bodies.

    Mammals breathe by moving the diaphragm and the ribcage. These respiratory movements bring about inspiration (inhalation) and expiration (exhalation).

  Note : External and internal intercoastal muscles (antagonistic pair) connect adjacent ribs. Their activity moves the bones of the ribcage.

Inspiration : During inspiration, the thoracic cavity is enlarged , causing air to enter the lungs from the external surroundings. This is done in the following way:

• The external intercoastal muscles contract and the internal ones relax. Moving the ribs diagonally upwards and outwards away from the backbone. This causes the sternum to move upwards and outwards also (the position of the backbone remains fixed).

• Simultaneously, the dome snapped diaphragm contracts , flattening downwards and pushing out the ribs.

These movements enlarge the rib cage, increasing the volume of the thoracic cavity. This creates a partial vacuum, I.e. a low pressure, in the thoracic cavity which is filled at once by the air from outside rushing into the lungs to inflate them.

Expiration: During expiration, the thoracic cavity goes back to its normal size, causing air from the lungs to flow out to the external environment. This is done in the following way:

• The internal intercoastal muscles contract and the external ones relax, moving the ribs downwards and inwards.

• Simultaneously, the diaphragm relaxes and curves upwards to assume its normal dome like shape.

These movements reduce the volume of the thoracic cavity, creating a higher pressure there than in the atmosphere. This forces the air out of the lungs into the external environment