Breathing muscles: function at rest
On inspiration the diaphragm contracts and flattens and the external intercostal muscles contract, pulling the ribs up and out. Both raise the thoracic cavity volume, so pressure falls and air is drawn IN. Expiration at rest is passive: both muscles relax, so volume falls, pressure rises and air is pushed OUT. 0413 tests the FUNCTION (what the muscle does and the volume change), not just the visible movement.
Breathing volumes and the VE equation
Four measures describe how much air moves. Tidal volume = air in one normal breath (~0.5 dm³ at rest). Vital capacity = the maximum air forced out after a maximum breath in (a fixed capacity). Residual volume = air that stays in the lungs after a maximum breath out. Minute ventilation = total air per minute: VE = tidal volume x breathing rate, e.g. 0.5 x 14 = 7 dm³/min. The unit is always a volume PER MINUTE (dm³/min), never a volume alone.
Effect of exercise on the four measures
Exercise affects the four measures differently. Tidal volume and breathing rate both increase; because VE = tidal volume x breathing rate and both rise together, minute ventilation rises sharply. Vital capacity does NOT change in a single bout (it grows only slowly over months of training); residual volume stays about constant. So the exercise story is TV, breathing rate and VE, not VC or RV.
Drawn from real examiner reports.
Stroke volume is not a breathing volume
Because "volume" appears in both, candidates sometimes list stroke volume as a fourth breathing volume. It is not: stroke volume is the blood pumped from the heart per beat (Q = SV x HR), not a volume of air. The measures here describe air in the lungs. If a "volume" relates to the heart or beats, it is cardiac, not respiratory.
s22 P11 Q9b-c; w22 P11 Q4c; s22 P12 Q9b
Tidal volume vs vital capacity
Tidal volume is the air in a normal, relaxed breath; vital capacity is the maximum air forced out after a maximum breath in -- a much larger figure. Examiners flag vital capacity as the least secure named volume: it gets muddled with tidal or residual volume, or with minute ventilation. Lock it to the word "maximum".
s23 P13 Q11a(i)-(iii); w23 P12 Q1e
Movement described, not function
Asked for the function of the diaphragm or intercostal muscles, weaker answers describe the movement seen ("the ribs go up") without saying what the muscle DOES. A function answer says the muscle contracts (or relaxes), names the shape change, and links it to the thoracic volume change. A second slip is not naming the muscles at all.
s22 P11 Q9b-c; w22 P11 Q4c; s22 P12 Q9b
No unit on minute ventilation
Minute ventilation calculations are usually numerically correct, but the unit is regularly dropped, especially on multi-part questions where it is easy to forget after the arithmetic. A bare number loses a mark even when the value is right. Always finish with dm³/min (or ml/min) -- the unit is part of the mark, not optional.
s23 P13 Q11a(i)-(iii); w23 P12 Q1e
VE "increases" with no mechanism
Asked how exercise changes minute ventilation, many write only that it "increases", with no mechanism. It rises because BOTH the rate (breaths/min) AND the depth (tidal volume per breath) increase together, so their product climbs more steeply than either alone. Name both changes and link them through VE = tidal volume x breathing rate.
s23 P13 Q11a(i)-(iii); w23 P12 Q1e
VC and RV do not rise during exercise
A common wrong assumption is that vital capacity or residual volume rise during exercise. They do not change acutely in a single bout: both are fixed capacities set by lung and rib-cage size. Only minute ventilation, tidal volume and breathing rate rise; vital capacity grows only slowly over months of training.
Explain VE with a linked chain
"Explain" questions on minute ventilation reward a chain, not a one-word claim. Name BOTH changes -- breathing rate up AND tidal volume up -- then link them through VE = tidal volume x breathing rate, so their PRODUCT rises steeply. Give the mechanism, not just the direction.
Always write the unit dm³/min
Whenever you quote a ventilation figure, write the unit: dm³/min (or ml/min). Examiners treat a missing unit as a lost mark even when the arithmetic is right, so make writing the unit the automatic last step of every calculation on this topic.
Give function, not just movement
On a "function" question, say what the muscle DOES -- contracts or relaxes -- and the thoracic volume change, not just the movement you see. "The ribs move up" merely describes; name the contraction and the volume change instead.
Apply the answer to the named activity
When the question names a sport or performer, apply your answer: a distance runner needs the higher minute ventilation to supply the leg muscles with more oxygen and clear carbon dioxide faster. A generic answer with no named activity is capped below full marks.
The diaphragm (a dome-shaped sheet of muscle beneath the lungs) and the intercostal muscles (between the ribs) change the volume of the thoracic cavity to draw air in and push it out.
