Respiratory System in Humans and Animals, practice questions

B, Air passes from the nose down the trachea, into the bronchi, then the bronchioles, and finally into the alveoli where gas exchange occurs.

B, The diaphragm contracts and flattens during inhalation, which increases the volume of the thorax and lowers the pressure so air flows in.

B, Oxygen and carbon dioxide move by diffusion down their own concentration gradients, so no energy is used at the gas-exchange surface.

D, A thick, dry coat would lengthen the diffusion distance and stop gases dissolving; the other three features all speed diffusion up.

B, Water and blood flow in opposite directions, so oxygen can diffuse from water into blood along the whole length of the gill.

C, Insects take air in through spiracles on the body surface, and the tracheae carry it directly to the tissues, bypassing the blood.

B, Chemoreceptors detect a rise in carbon dioxide and signal the medulla oblongata to increase breathing, which makes carbon dioxide the main control.

C, The medulla oblongata receives signals from chemoreceptors and sends impulses to the breathing muscles to adjust the rate and depth.

B, Emphysema breaks down the walls between alveoli, merging them into larger spaces and reducing the surface area for gas exchange.

B, When tar paralyses and destroys the cilia, mucus and trapped particles build up in the airways, leading to infection and chronic bronchitis.

• Pathway: nose → trachea → bronchi → bronchioles → alveoli.
• Job 1: the air is warmed and moistened in the nose.
• Job 2: dust and particles are filtered out and trapped by mucus and cilia in the nose and trachea.
• Gas exchange takes place in the alveolus.
• Adaptation: the wall is only one cell thick, giving a short diffusion distance (accept large surface area, moist lining, or dense capillary network).
• Each adaptation increases the rate of diffusion of gases.

• Manipulated variable: the activity level (before, during, after exercise), or time.
• Responding variable: the breathing rate (breaths per minute).
• During the run the breathing rate increases.
• Explanation: the muscles respire faster and produce more carbon dioxide, which chemoreceptors detect, so the medulla oblongata increases the rate and depth of breathing.
• After the run the breathing rate gradually falls back towards the resting level.
• Explanation: the carbon dioxide concentration in the blood falls as the muscles respire more slowly, so the medulla oblongata reduces the breathing rate.

• Tar paralyses and destroys the cilia lining the airways.
• Mucus and trapped particles are no longer swept away, so they build up and cause infection, leading to chronic bronchitis.
• Chemicals in the smoke break down the walls between the alveoli.
• The alveoli merge into larger spaces, reducing the total surface area for gas exchange (emphysema).
• With less surface area, less oxygen diffuses into the blood, so the person becomes breathless.
• Another condition linked to smoking is lung cancer.

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.

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.

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.

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.

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.

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.

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.

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.

Besides bronchitis, emphysema and lung cancer caused by smoking, asthma narrows the airways through inflammation and excess mucus, making breathing difficult, while tuberculosis is a bacterial infection that damages lung tissue. Air pollution and occupational dust can worsen or cause similar damage even in non-smokers, which is why clean air matters for lung health. Most of these problems can be avoided or reduced through clean air and a smoke-free lifestyle.