Pulmonary ventilation is the process by which air is exchanged between the atmosphere and the lungs. It has three distinct phases -

- Inspiration

- Expiration

- Pause

During inspiration the volume of the thoracic cavity is increased, this is caused by contractions of the diaphragm and intercostal muscles pulling the rib cage upwards and outwards. As a result of the increased volume in the lungs the pressure within the lungs is reduced, this causes air to be drawn into the lungs to try to equalise the lung and atmospheric air pressures. Inspiration is an active process, as it requires an expenditure of energy to produce muscular contraction.

The negative pressure in the thoracic cavity that is created during inspiration is an aid to venous return ( of blood to the heart) and an example of how both systems work together to carry out their functions, it is known as the respiratory pump.

Expiration occurs due to the relaxation of the intercostal and diaphragm muscles and the natural elastic recoil of the lungs. These processes cause the rib cage to move downwards and inwards resulting in decreased volume and higher air pressure within the lungs. This causes air to be forced out of the lungs as the pressure inside the lungs exceeds the atmospheric pressure. Expiration is a passive process, as it does not require the expenditure of energy. After expiration there is a pause before inspiration begins and the cycle repeats itself.

During normal breathing there are roughly 15 respiratory cycles a minute, the volume of air breathed in and out during normal breathing is known as the tidal volume (TV). The various lung volumes and capacities are explained below -

Inspiratory reserve volume (IRV) - This is the extra volume of air that can be inhaled into the lungs during maximal inspiration.

Inspiratory capacity (IC) - This is the volume of air that can be inhaled into the lungs during maximal inspiration, it is the combined total of the tidal volume and the inspiratory reserve volume.

Functional residual capacity (FRC) - This is the volume of air retained in the airways after tidal expiration. This air is mixed with tidal air and so causes relatively small changes in the composition of the air in the alveoli. As blood flow through the pulmonary capillaries is constant this means that the exchange of gases is not stopped between breaths. This prevents changes to the concentrations of gases within the blood and is another example whereby both the cardiovascular and respiratory systems work together to carry out their functions.

Expiratory reserve volume (ERV) - This is the largest volume of air that can be exhaled from the lungs during maximal expiration.

Residual volume (RV) - This is the volume of air left in the lungs after forced expiration, this cannot be measured directly.

Vital Capacity (VC) - This is the maximum amount of air that can be moved in and then out of the lungs and can be calculated as follows

VC = TV + IRV + ERV

Anatomical dead space - During breathing the lungs and air passages are never empty and, as the exchange of gases only takes place in the alveoli, the remaining capcity of the respiratory passages is known as the anatomical dead space.

Alveolar ventilation (AV) - This is the volume of air that moves in and out of the alveoli in one minute. It is calculated as the tidal volume minus the anatomical dead space, multiplied by the respiratory rate.

The apparatus used to measure the volume of air exchanged during breathing is a spirometer or respirometer. The spirogram below is created using a spirometer and illustrates the various lung volumes and capacities.

The exchange of oxygen and carbon dioxide between the blood is brought about by diffusion (the passage of gases from areas of high concentration to areas of low concentration) and is influenced by two factors.

The first is the thickness of the barrier that the blood has to move through. In order to diffuse in or out of the bloodstream the blood has to pass through the membrane of the alveoli and the wall of the blood vessel. The cells in these membranes are flattened and the blood is therefore in close proximity to the lung gases, the capillaries that surround the alveoli are also specifically adapted to allow diffusion and this will be discussed later.

The second factor is the pressure gradient of the different gases in the blood and alveoli. The exchange of gases depends upon the individual pressures exerted by each gas, this is known as the gases partial pressure. Partial pressure can be defined as the pressure an individual gas exerts within a mixture of gases. Partial pressures are determined by the relative amounts of each gas and the sum of the partial pressures must be equal to the pressure exerted by the total gas mixture. Within the alveoli the partial pressures are the forces that promote the diffusion of gases.

During inspiration there is a greater proportion of oxygen in the alveoli than in the blood, this means that the oxygen in the alveoli exerts a greater partial pressure than the oxygen in the blood. This creates a pressure gradient and following the rules of diffusion the oxygen passes down this pressure gradient from the area of high pressure (concentration) to the area of low pressure, i.e. from the alveoli to the blood.

The opposite is true of carbon dioxide where it has a greater partial in the blood when compared with that of the alveoli. This means that the carbon dioxide passes down the pressure gradient from the blood to the lungs where it can be expelled through expiration.