Pulmonary Ventilation

Pulmonary ventilation, commonly known as ‘breathing’ is the process of moving air into and out of the lungs.

Anatomy of the lungs

Air enters the respiratory system through the mouth and the nasal cavity, passing through the pharynx then larynx (where sounds are produced for speech) and finally the trachea which enters the chest cavity.

These passages (the nasal cavity in particular) are lined with cilia (tiny hair like structures) that act as a filter, blocking germs from entering the respiratory system as well as warming and moistening the air. 

At the top of the oesophagus is a plate of flexible elastic cartilage called the epiglottis which closes over the larynx as we swallow food to ensure the food travels to the stomach and not into the lungs.

In the chest cavity the trachea splits into two smaller tubes called the left and right bronchi.

These further divide into secondary bronchi which lead directly into the lungs, where they divide into many smaller tubes called bronchioles. 

The bronchioles finally connect to tiny structures called alveoli where gas exchange occurs, as can be seen in the adjacent diagram.

Respiratory muscles and their role in respiration

When we breathe in, air flows into the lungs down what is known as a ‘pressure gradient’ from an area of high pressure to an area of low pressure.  The pressure gradient within the lungs is controlled by a muscle called the diaphragm.

When contracted the diaphragm flattens and pushes downwards resulting in an enlargement of the chest cavity (area within which the lungs sit) and a decrease of pressure in the lungs.  This reduction in pressure causes air to be sucked into the lungs and we breathe in, as seen in the following image.

As we breathe out the opposite happens as the diaphragm relaxes and pushes upwards, as seen in the right image above.  This reduces the size of the chest cavity making the pressure within the lungs higher and results in air being exhaled.  In turn the pressure gradient within the body returns to its original state and the process starts again.

During exercise when inspiration increases, the external intercostal muscles are recruited to help with the increase in ventilation rate.  They work to lift the ribs up and outwards, further increasing the chest cavity and enabling more air to be inspired, as seen on the image below.

Expiration is generally a passive process that does not require muscle contraction.  As the muscles of inspiration relax and return to their resting positions air is forced out of the lungs as the pressure within them increases.

However, when needed, active and or forced expiration can be completed by voluntary contraction of the abdominals and internal intercostal muscles, as seen in the above image.  This further reduces the volume within the chest cavity and more air is exhaled.

Pulmonary ventilation summarised

The following images and table provides a usefull summary of pulmonary ventilation or 'breathing'.