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Respiratory System. Ventilation 18.4 page 492. Ventilation. Ventilation. VENTILATION

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respiratory system

Respiratory System

Ventilation 18.4 page 492



The thoracic cavity, in which the lungs are located, is a closed box supported externally by the ribs. It is separated from the abdominal cavity by the dome shaped diaphragm. Changes in the volume of the thorax immediately cause changes in the pressure of the air in the lungs. This results in the flow of air into and out of them.

  • Active movement of air into and out of the lungs is termed ventilation.
  • Movement of air into the lungs is called inspiration
  • Movement of air out of the lungs is expiration
  • During normal breathing, the respiratory movements occur about sixteen times per minute in young human adults. The total capacity of the human lung is about 5.5 litres, but even with the most forced expiration only about 4 litres of air can be expelled.
lung volumes
Lung Volumes
  • Thus a certain residual volume of air remains in the lungs at all times.
  • The maximum amount of air that can be ventilated during forced breathing is termed the vital capacity. This capacity depends very much upon fitness and is greater in athletes than in non-active people.
  • The volume of air moving in and out of the lungs during normal breathing is known as the tidal volume and is approximately 500 mL.
control of human ventilation
  • During normal ventilation, the breathing rate is controlled by two centres in the medulla of the hindbrain. Stimulation of the inspiration centre sends nerve impulses which bring about the contraction of the external intercostal and diaphragm muscles. The lungs inflate.
The most important factor in the control of ventilation is the concentration of respiratory gases in the blood.
  • Several centres (chemoreceptors) for detecting gas levels exist:
  • • pH and oxygen sensors of the blood in the carotid bodies—nodules of tissue in the carotid arteries leading to the brain
  • • pH and oxygen sensors of the blood in the aortic body—nodules of tissue in the aorta
  • • pH sensors of tissue fluids in centres of the medulla oblongata (hindbrain) adjacent to the inspiration and expiration centres.