The Respiratory System & Breathing

The respiratory system includes the lungs and airways as well as the mechanics of breathing, respiratory volumes and VO2 max.

The function of the human respiratory system is to transport air into the lungs and to facilitate the diffusion of Oxygen into the blood stream. Its also receives waste Carbon Dioxide from the blood and exhales it.

The respiratory system consists of the upper and lower respiratory tracts. The upper respiratory tract includes the mouth, nose, nasal cavity, pharynx (wind pipe and food pipe) and larynx or voice box. Each has a specific function to aid the flow of air into the body.

The action of breathing in and out is due to changes of pressure within the thorax, in comparison with the outside. This action is also known as external respiration. When we inhale the intercostal muscles (between the ribs) and diaphragm contract to expand the chest cavity. The diaphragm flattens and moves downwards and the intercostal muscles move the rib cage upwards and out.

This increase in size decreases the internal air pressure and so air from the outside (at a now higher pressure that inside the thorax) rushes into the lungs to equalise the pressures.

The main function of the respiratory system is gaseous exchange. This refers to the process of Oxygen and Carbon Dioxide moving between the lungs and blood. Diffusion occurs when molecules move from an area of high concentration (of that molecule) to an area of low concentration. This occurs during gaseous exchange as the blood in the capillaries surrounding the alveoli has a lower oxygen concentration of Oxygen than the air in the alveoli which has just been inhaled.

Respiratory volumes are the amount of air inhaled, exhaled and stored within the lungs at any given time.

Tidal Volume: The amount of air which enters the lungs during normal inhalation at rest. The average tidal volume is 500ml. The same amount leaves the lungs during exhalation.

Inspiratory Reserve Volume: The amount of extra air inhaled (above tidal volume) during a deep breath. This can be as high as 3000ml.

VO2 max is the measure of the peak volume of Oxygen (VO2) you can consume and use in a minute. It is measured in ml/kg/min and so you can see that it is also relative to body weight. As we already know, Oxygen is needed to produce energy. The harder you exercise the more Oxygen you use in order to produce sufficient energy. However, everybody has a maximum level (their VO2 Max), where Oxygen utilisation is at its peak.

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