Form 4 · Common mistakes

Respiratory System in Humans and Animals, common mistakes

The mistakes SPM students make on Respiratory System in Humans and Animals, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying the lungs actively pull air in.	The lungs contain no muscle and cannot contract, so they cannot pull air in by themselves.	State that air is pushed in by the higher atmospheric pressure after the diaphragm and intercostal muscles lower the pressure inside the thorax.
Confusing breathing with respiration.	Breathing is a physical movement of air, while respiration is a chemical reaction in cells, so using one for the other answers the wrong question.	State that breathing (ventilation) moves air in and out of the lungs, while respiration releases energy from glucose inside cells.
Writing that oxygen is 'pumped' into the blood at the alveolus.	Gas exchange uses diffusion down a concentration gradient, which needs no energy or pumping.	State that oxygen diffuses from the alveolus into the blood down its concentration gradient.
Forgetting that the alveolus lining is moist.	A moist lining is a required adaptation because gases must dissolve before they can diffuse across the wall.	Include the moist lining as one of the four alveolus adaptations and link it to gases dissolving before diffusion.
Mixing up the intercostal muscles used in inhalation and exhalation.	The external and internal intercostal muscles act at different times, so swapping them describes the opposite movement.	State that the external intercostal muscles contract during inhalation, and the internal intercostal muscles contract during forced exhalation.
Assuming insects breathe through a mouth or nose like humans.	Insects have no lungs; air enters through openings on the body surface, so this describes the wrong structure.	State that insects take in air through spiracles leading into a network of tracheae that reach the tissues directly.
Saying breathing rate is controlled mainly by the oxygen level in the blood.	Chemoreceptors respond far more sensitively to carbon dioxide than to oxygen, so the main control is carbon dioxide.	State that breathing rate is controlled mainly by the carbon dioxide concentration in the blood, detected by chemoreceptors.
Treating bronchitis and emphysema as the same condition.	They damage different structures, so giving the same description for both loses the distinction marks.	State that bronchitis is inflammation and mucus build-up in the airways, while emphysema is the breakdown of alveolus walls.
Saying the diaphragm relaxes during inhalation.	The diaphragm contracts and flattens to enlarge the thorax during inhalation; it relaxes during exhalation.	State that the diaphragm contracts and flattens during inhalation and relaxes, doming upward, during exhalation.
Describing the alveolus wall as thick or more than one cell thick.	A thick wall would lengthen the diffusion distance and slow gas exchange, which is the opposite of the adaptation.	State that the alveolus wall is only one cell thick to give a short diffusion distance.
Confusing the trachea (human windpipe) with the tracheae of an insect.	They share a similar name but are different structures in different animals, so mixing them up describes the wrong system.	Use 'trachea' for the single human windpipe and 'tracheae' for the branching air tubes of an insect.
Saying smoking damage can be fully reversed by stopping.	Destroyed alveolus walls and lost surface area do not regrow, so the structural damage is permanent.	State that stopping smoking slows further damage, but damage such as emphysema is usually permanent.

How to avoid these mistakes

Label a diagram of the breathing system and describe the path air takes from the nose to the alveolus.
Learn the muscle, volume and pressure changes for inhalation and exhalation, and be ready to name the muscles involved.
List the four alveolus adaptations and explain how each one speeds up diffusion.
Practise explaining, in order, how a rise in blood carbon dioxide leads to faster and deeper breathing.
Build a comparison table of the gas-exchange surfaces used by humans, fish and insects.
Review each smoking-related disease and state exactly which lung structure it damages.

Frequently asked questions

What happens to the ribcage and diaphragm during inhalation?

During inhalation, the external intercostal muscles contract to raise the ribcage outward and upward, while the diaphragm contracts and flattens from its usual dome shape. Together these actions increase the volume of the thoracic cavity, which lowers the air pressure inside it below atmospheric pressure, so air flows in from the higher-pressure atmosphere until the pressures equalise. This step-by-step understanding lets a student answer 'explain' questions on breathing fully rather than partially.

Why is the alveolus well suited to gas exchange?

The alveolus has a wall only one cell thick, giving a short diffusion distance, and millions of alveoli together give the lungs an enormous total surface area. Its lining is moist, so gases dissolve before crossing, and a dense network of capillaries constantly carries gases away, keeping the concentration gradient steep and diffusion fast.

How is gas exchange in fish different from humans?

Fish exchange gases across gills instead of alveoli. Water flows over thin, blood-rich gill filaments, usually in the opposite direction to blood flow, so oxygen diffuses from the water into the blood and carbon dioxide diffuses out along the whole length of the gill. This countercurrent arrangement keeps the concentration gradient favourable for almost the entire gill surface.