Breathing (WJEC GCSE Biology)
Revision Note
Inspiration & Expiration
The diaphragm is a thin sheet of muscle that separates the chest cavity from the abdomen; it is ultimately responsible for controlling ventilation in the lungs
Inspiration (inhalation):
When the diaphragm contracts it flattens and this increases the volume of the chest cavity (thorax), which consequently leads to a decrease in air pressure inside the lungs relative to outside the body, drawing air in
Expiration (exhalation):
When the diaphragm relaxes it moves upwards back into its domed shape and this decreases the volume of the chest cavity (thorax), which consequently leads to an increase in air pressure inside the lungs relative to outside the body, forcing air out
The intercostal muscles between the ribs contribute to the mechanism of inspiration and expiration
Inspiration:
The intercostal muscles contract to pull the ribs up and out
This also increases the volume of the chest cavity (thorax), decreasing air pressure which draws air in
Expiration:
The intercostal muscles relax so the ribs drop down and in
This decreases the volume of the chest cavity (thorax) increasing air pressure which forces air out
Inspiration and expiration (front view) diagram
The mechanisms of inspiration (inhalation) and expiration (exhalation) viewed from the front
Inspiration and expiration (side view) diagram
The mechanisms of inspiration and expiration viewed from the side
Examiner Tips and Tricks
You may see the terms inhalation OR inspiration (breathing in), and exhalation OR expiration (breathing out). Both sets of terms mean exactly the same thing, so don’t let them confuse you!
This sequence of events is a common exam question and you should be able to explain in detail what is happening to the external and internal intercostal muscles, the rib cage, the diaphragm, the volume and the pressure-volume of the lungs when breathing in and out. Remember, if you learn one, the other is almost exactly the opposite.
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