Respiratory system

Constituted with the trachea, the bronchi and the lungs, the respiratory system allows exchanging essentially oxygen O2 and carbon dioxide CO2 between air and blood.

Mechanical ventilation

The mechanical ventilation is a sequence of inspirations and expirations phases from 12 cycles/min at rest to 25 cycles/min during an intensive effort.

Diaphragm and intercostal muscles produce inspiration.

Lung elasticity at rest and abdomen muscles when forced produce expiration.

The lung volumes are defined by 4 main volumes:

  • The current volume VT (about 0.5 l)
  Volume moving in and out at rest.

  • The reserve inspiration volume VRI (about 2.5 l)
  Biggest volume we can inspire after a normal inspiration at rest.

  • The reserve expiration volume VRE (about 1.5 l)
  Biggest volume we can expire after a normal expiration at rest.

  • The residual volume VR (about 1.5 l)
  Volume which stays in lungs after total expiration.

The vital capacity is the sum of three first volumes VT + VRI + VRE (about 4.5 l).

The total capacity is the sum of all volumes VT + VRI + VRE + VR (about 6 l).

It is important to note that the gas exchange is done only in the alveoli. The space of the conductive ways is named anatomical dead space (about 0.15 l).

Ventilation increases always brutally at the beginning of exercise (ventilation hooking), and fall brutally at the end of exercise (ventilation unhooking).

The ventilation flow is equal to the current volume multiplied by the ventilation frequency: D = VT . FV

Respiratory ratio

To analyze an effort, it is possible de compare the produced carbon dioxide VCO2 with the consumed oxygen VO2.

The respiratory exchange ratio is then defined by the formula: QR = VCO2 / VO2

When the ratio is about:

  • 0.70 the organism uses mainly lipids.
  • 0.85 the organism uses a mix of lipids-glucids (50%-50%).
  • 1.00 the organism uses only glucose.

Respiratory efficiency

For a non-trained person, the respiratory system can generally supply more oxygen than the cardio-vascular system can transport or than the muscular structure can consume. In this case, it is not a limitation of the performance.

At the opposite, for a trained person, the respiratory system can supply not enough oxygen. In this case, it is a limitation of the performance.