Breathing is an involuntary process: it happens without the individual having to think about it. It is controlled by the medulla in the brain, which is connected to the autonomic nervous system that controls all functions that happen unconsciously. Breathing can be divided into two components:

• Inhalation (breathing in or inspiration) happens due to the diaphragm – the big muscle that sits at the base of the chest cavity – and the intercostal muscles – located between the ribs – contracting. This increases space inside the chest (also known as the thorax), while reducing the air pressure inside. In order to balance the pressure, air is pulled in and the lungs inflate.

• Exhalation (breathing out or expiration) occurs when the diaphragm and intercostal muscles relax, reducing the space while increasing the pressure inside the thorax. This causes air to leave the lungs.

The whole process is controlled by stretch receptors in the bronchioles and alveoli that talk to the brain. It is possible to control breathing – for example, when someone chooses to hold their breath – but once the carbon dioxide in the blood reaches a certain level, the medulla forces the autonomic system to take over. It is vital – literally – that the body has enough oxygen available, as it is needed for the production of adenosine triphosphate (ATP), the main source of energy for most cells.

By controlling the amount of oxygen and carbon dioxide in the blood, the respiratory system also maintains the pH of the blood. This, in turn, keeps the amount of proteins and enzymes in the body at the correct levels.

Fast facts!

• Total lung capacity for a healthy adult male is five to six litres
• When sitting down, only around 500ml of air goes in and out of the lungs – this is the tidal volume
• The amount of air that always stays in the lungs – around one litre – is known as the residual volume.