Understand the Respiratory System

Whether we're awake or asleep, we don't have to think about breathing - it's so vital to life that it happens automatically. Each day, we breathe about 20,000 times, and by the time we're 70 years old, we will have taken at least 600 million breaths.

Breathing is the general function of the respiratory system.  This system is composed of:

  • the oral cavity and nasal passage, which are the entrance and exit points in the body for air
  • the pharynx, larynx and trachea (windpipe) through which air travels from and to the mouth and nose

  • the bronchial tree, a series of branching tubes that get smaller and smaller as they deliver and remove oxygen from the lungs

  • the lungs themselves, which contain alveolar sacs which in turn house alveoli, small hollow cavities where cells come into direct contact with capillary beds.

The purpose of breathing is two-fold, to draw oxygen into the body, where it enters the circulation and is transported bound to haemoglobin in erythrocytes to cells that take it up and use it to produce energy from nutrients and to collect carbon dioxide, the waste product of cellular respiration and energy production, and return it to the lungs so it can be breathed back out into the atmosphere.

The Physiology of Breathing

Breathing is not the sucking in and blowing out of air.  It is actually a more complex process regulated by the relative pressure of the atmosphere outside the body and the pressure inside the thoracic cavity.  The breathing in and out of air is known as pulmonary ventilation.  This process is regulated in turn by the diaphragm and results in air being pushed into the lungs when we breathe in and pushed into the atmosphere when we breathe out.  The movement of air is caused because the natural state is equilibrium, and air moves from areas of where it is under high pressure, to areas where it is under low pressure.

Before you continue, the following terminology is worth learning:


The state where a system is stable.  This can mean all the influences on the system are static (unmoving) but it can also mean that all the influences are in a state of constant flux the results of which cancel each other out resulting in no net change.


The force applied on one object by another object in direct contact with it.  The formula for calculating pressure is:

Pressure (P) = Force (F) divided by Area (A)

Pressure is measured in units called Pascals (newtons per square metre), or atmospheres, where 1 atmosphere (atm) = 1 x 105 Pascals.  The air pressure (atmospheric pressure) at sea level is 1atm.  It gets lower as the altitude increases.    

Inspiration is breathing in.  When we breathe in our diaphragm will contract, moving forcefully downward.  As it is the bottom boundary of the thoracic cavity, its downward motion actually enlarges the thoracic cavity.  The intercostal muscles also contract, lifting the rib cage up.  Following the laws of physics, increasing an area, without increasing, in this case, the gas content, the pressure will decrease (there are the same number of molecules in a larger space).  This means the air pressure in the lungs is now lower than atmospheric air pressure.  The effect of this is that air rushes into the lungs.  As the lungs fill the space in the thoracic cavity decreases (as the lungs expand and take up more space) and the free space in the lungs decreases.  This increases the pressure in the lungs and when it is equal to the atmospheric pressure the rush of air into the lungs stops.  You now have lungs full of oxygen rich air.

After the body has used the oxygen, the waste product carbon dioxide is returned to the lungs.  This is discussed in the following lesson.  In order to remove the carbon dioxide rich, oxygen poor air from the lungs, the diaphragm muscle relaxes, reducing the area in the thoracic cavity and thereby decreasing the air pressure compared to the atmospheric pressure.  This pushes the air out of the lungs and back into the atmosphere.