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Respiratory System in Humans and Animals, worked answers

Fully worked answers for Respiratory System in Humans and Animals, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Explaining the pressure and volume changes in the thorax during inhalation and exhalation, using the correct muscle names.
Listing all four alveolus adaptations and linking each one to a faster rate of diffusion.
Comparing the gas-exchange surfaces of humans, fish and insects and explaining why each suits its habitat.
Explaining how carbon dioxide concentration, detected by chemoreceptors, controls breathing rate through the medulla oblongata.
Linking smoking to specific respiratory diseases by naming the structure that is damaged in each case.
Interpreting a graph of breathing rate or depth against exercise intensity or time.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Describe what happens to the ribcage, the diaphragm, the volume of the thorax and the air pressure during inhalation.

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During inhalation the external intercostal muscles contract, raising the ribcage up and out, while the diaphragm contracts and flattens from its dome shape. Both actions increase the volume of the thoracic cavity. The increase in volume lowers the air pressure inside the thorax below atmospheric pressure. Because the outside pressure is now higher, air flows in through the nose and trachea into the lungs until the pressures equalise.

external intercostal muscles contractdiaphragm contracts and flattensvolume increasespressure fallsair flows in

State the four adaptations of the alveolus and explain how each one increases the rate of gaseous exchange.

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The alveolus wall is only one cell thick, which gives a short diffusion distance so gases cross quickly. There are millions of alveoli, giving a very large total surface area so more gas can diffuse at once. The inner lining is moist, so gases dissolve before diffusing across the membrane. A dense network of capillaries constantly carries gases away and brings fresh blood, keeping the concentration gradient steep. Each feature therefore raises the rate of diffusion.

one cell thicklarge surface areamoist liningdense capillary networksteep concentration gradient

Compare the way a human and a fish carry out gaseous exchange.

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A human exchanges gases across the alveoli in the lungs, taking oxygen from air, while a fish exchanges gases across gills made of thin filaments, taking dissolved oxygen from water. In both, oxygen diffuses into the blood and carbon dioxide diffuses out across a thin, moist surface with a good blood supply. In the fish, water usually flows over the gills in the opposite direction to the blood, a countercurrent flow that keeps the concentration gradient steep along the whole gill.

alveoligillsdissolved oxygendiffusioncountercurrent flow

Explain how a rise in the carbon dioxide concentration of the blood leads to faster breathing.

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Chemoreceptors detect the rise in carbon dioxide concentration in the blood and send signals to the medulla oblongata in the brain. The medulla oblongata sends nerve impulses to the intercostal muscles and the diaphragm, which increase the rate and depth of breathing. This brings in more oxygen and removes carbon dioxide faster, so the carbon dioxide concentration falls back to normal. Breathing rate is controlled mainly by carbon dioxide concentration, not by oxygen level.

chemoreceptorscarbon dioxide concentrationmedulla oblongatarate and depth increasenerve impulses

Explain how smoking can lead to emphysema and why this makes a person breathless.

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Chemicals in cigarette smoke break down the thin walls between the alveoli. As the walls are destroyed, small alveoli merge into fewer, larger air spaces, which reduces the total surface area available for gas exchange. With a smaller surface area, less oxygen can diffuse into the blood for the same breath, so the person cannot take in enough oxygen and becomes breathless, especially during activity. This damage is usually permanent and cannot be fully reversed.

alveolus walls broken downreduced surface arealess oxygen diffusesemphysemabreathless

Explain why both the rate and the depth of breathing increase during vigorous exercise and return to normal afterwards.

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During exercise the muscles respire faster and produce carbon dioxide more quickly, so its concentration in the blood rises. Chemoreceptors detect this and signal the medulla oblongata, which increases both the rate and the depth of breathing. This brings in more oxygen to meet the higher energy demand and removes carbon dioxide faster. When exercise stops, the muscles respire more slowly and produce less carbon dioxide, so its blood concentration falls and the medulla oblongata allows breathing to return gradually to its resting level.

carbon dioxide riseschemoreceptorsmedulla oblongatarate and depth increasereturns to resting level

Describe how an insect carries out gaseous exchange and state one way it differs from a human.

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An insect takes air in through openings called spiracles on the surface of its body. The air travels through a branching network of tubes called tracheae that carry it directly to the tissues, where oxygen diffuses into the cells and carbon dioxide diffuses out. Unlike a human, an insect does not use blood to transport respiratory gases; the air is delivered straight to the tissues by the tracheae, bypassing the blood system.

spiraclestracheaeair direct to tissuesbypasses the blooddiffusion

Phrasing that earns marks

Human respiratory structures: Air breathed in passes through the nose, where it is warmed, moistened and filtered, then down the trachea, which splits into two bronchi, one to each lung. Each bronchus branches repeatedly into narrower bronchioles that end in millions of tiny air sacs called alveoli, where gas exchange with the blood takes place.
Mechanism of breathing: During inhalation, the external intercostal muscles contract to raise the ribcage and the diaphragm contracts and flattens, both of which increase the volume of the thoracic cavity and lower the air pressure inside it, so air flows in from the higher-pressure atmosphere. Exhalation reverses these muscle actions, shrinking the thoracic cavity and pushing air out.
Gaseous exchange: At the alveolus, oxygen diffuses from the air inside the alveolus, where its concentration is high, into the blood in the surrounding capillaries, where its concentration is lower. Carbon dioxide diffuses in the opposite direction, from the blood into the alveolar air, because both gases move down their own concentration gradient.
Alveolus adaptations: Each alveolus has a wall only one cell thick, which keeps the diffusion distance short, an extremely large total surface area created by millions of alveoli packed into both lungs, a moist inner lining that lets gases dissolve before crossing the membrane, and a dense network of capillaries that keeps carrying gases away, maintaining a steep concentration gradient.
Respiratory systems in animals: Fish extract dissolved oxygen from water using gills made of thin, blood-rich filaments, with water usually flowing over the gills opposite to the direction of blood flow to keep the concentration gradient favourable. Insects instead carry air directly to their tissues through branching tubes called tracheae, which open to the body surface through spiracles.
Health issues: Cigarette smoke contains tar and other chemicals that paralyse and eventually destroy the cilia lining the airways, so mucus and trapped particles are no longer swept away, which leads to chronic bronchitis. The same chemicals can break down alveolus walls, causing emphysema, which reduces the surface area available for gas exchange, and can trigger the uncontrolled cell division that leads to lung cancer. These effects are usually permanent and cannot be fully reversed, which is why prevention is emphasised over treatment.
Control of breathing rate: Breathing rate is not constant; it is controlled mainly by the carbon dioxide concentration in the blood rather than by oxygen level. Chemoreceptors detect a rise in carbon dioxide and send signals to the medulla oblongata in the brain, which increases the rate and depth of breathing through nerve impulses to the intercostal muscles and diaphragm until the concentration returns to normal. This feedback mechanism keeps the body supplied with enough oxygen without requiring any conscious control from the person breathing.
Effect of exercise on breathing: During exercise, muscles respire faster and produce carbon dioxide more quickly, so its concentration in the blood rises sooner and by a larger amount. The medulla oblongata detects this change and increases both the rate and the depth of breathing, bringing in more oxygen and removing carbon dioxide faster to match the higher energy demand of active muscles. Once exercise stops, breathing rate gradually returns to its resting level as the carbon dioxide concentration falls back to normal.

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.