Role of the Intercostal Muscles and Diaphragm

Section 1 of 7The muscles involved

Muscles can only pull

• Muscles can only pull A muscle can contract to pull on a bone but cannot push,
  • so movements in two directions need two sets of muscles working as an antagonistic pair.

External intercostal muscles

• External intercostal muscles They lie on the outside of the rib cage.
  • When they contract, they pull the ribs up and out.

Internal intercostal muscles

• Internal intercostal muscles They lie on the inside of the rib cage.
  • They contract to pull the ribs down and in, but mainly during forced exhalation.

Diaphragm

• Diaphragm A dome-shaped sheet of muscle below the lungs.
  • It flattens when it contracts and returns to a dome when it relaxes.

Section 2 of 7Inhalation (breathing in)

1Muscles contract

• Muscles contract The diaphragm contracts and flattens, and the external intercostal muscles contract, raising the ribs up and out.

2Volume increases

• Volume increases The volume of the thorax becomes larger.

3Pressure falls

• Pressure falls The pressure inside the lungs drops below atmospheric pressure outside the body.

4Air flows in

• Air flows in Air moves from the higher pressure outside to the lower pressure inside the lungs.

Section 3 of 7Normal exhalation (breathing out)

1Muscles relax

• Muscles relax The diaphragm relaxes and rises back into a dome, and the external intercostal muscles relax so the ribs drop down and in.

2Volume decreases

• Volume decreases The thorax becomes smaller.

3Pressure rises

• Pressure rises The pressure in the lungs rises above the pressure outside.

4Air flows out

• Air flows out Air is pushed out of the lungs.

Section 4 of 7Comparing the three types of breathing

Stage	Diaphragm	Intercostal muscles	Thorax volume	Lung pressure
Inhalation	Contracts, flattens	External contract	Increases	Decreases
Normal exhalation	Relaxes, domed	External relax	Decreases	Increases
Forced exhalation	Relaxes	Internal contract and pull the ribs down and in	Decreases further	Increases more

Section 5 of 7Why forced exhalation matters

During strenuous exercise

• During strenuous exercise Muscles respire faster and produce more carbon dioxide, which must be removed quickly.
• The internal intercostal muscles help by pulling the ribs down more strongly, forcing air out faster so a larger volume of air is exchanged each minute.

Section 6 of 7Key terms

Term	Meaning
Inhalation	Breathing in; also called inspiration.
Exhalation	Breathing out; also called expiration.
Antagonistic muscles	A pair of muscles that work in opposite directions, one relaxing as the other contracts.
Intercostal muscles	The muscles between the ribs that move the rib cage.
Forced exhalation	Active breathing out using the internal intercostal muscles, which empties the lungs more quickly.

Section 7 of 7Exam technique

1

Saying air is 'sucked' in because the lungs expand on their own. The lungs have no muscle; changes in thorax volume and pressure move the air.

2

Mixing up the diaphragm positionsit flattens on inhalation and is domed on exhalation.

3

Forgetting to mention pressure. A full answer links volume change to pressure change to air movement.

4

Stating that normal exhalation uses the internal intercostals. They are used for forced exhalation only.