Respiratory System in Humans and Animals, revision notes

Overview

Animals need a continuous, reliable way to take in oxygen for cellular respiration and to remove the carbon dioxide this process produces. This chapter covers the human breathing system, from the nose down to the alveoli, the muscular mechanism that moves air in and out of the lungs, how oxygen and carbon dioxide are exchanged at the alveolus, and the health problems most often linked to the respiratory system.

Together, these five ideas form the foundation that later comparison and application questions on this topic build on.

Respiratory systems in other animals, such as fish and insects, are compared with the human system to show how the structure of a gas-exchange surface suits the habitat and lifestyle of the organism that owns it. Fish gills work underwater; insect tracheae work in air but bypass the blood system entirely.

Breathing rate and depth are not fixed. A control centre in the brain called the medulla oblongata monitors the carbon dioxide concentration of the blood through chemoreceptors and adjusts the rate and depth of breathing to keep this concentration stable, which is why breathing speeds up sharply during exercise.

Understanding this feedback loop, rather than memorising it as an isolated fact, makes several exam questions on breathing control far easier to answer.

Because the alveolus, the breathing mechanism and the effects of smoking are all popular sources of examination questions, this chapter pairs each idea with a labelled diagram, a worked question and a set of common mistakes so a student can check understanding rather than simply memorise facts.

Types of respiratory system across animals (Content Standard 8.1)

Content Standard 8.1 looks at how different animals take in oxygen and remove carbon dioxide, and shows that the structure of a gas-exchange surface suits the habitat and size of the organism. Every gas-exchange surface shares the same basic features: it is thin, moist, has a large surface area and lies close to a transport medium such as blood, because these features all speed up diffusion.

A single-celled organism such as Amoeba exchanges gases across its whole cell surface membrane, which works because its surface area to volume ratio is large and the diffusion distance is tiny. An earthworm uses its thin, moist skin, with a network of capillaries just beneath.

A fish uses gills made of thin filaments, with water usually flowing over them in the opposite direction to the blood (a countercurrent flow) to keep the concentration gradient steep. An insect carries air directly to its tissues through a branching system of tracheae that open at the body surface through spiracles, bypassing the blood entirely.

A human uses millions of alveoli in the lungs.

The table below sets these surfaces side by side, and the key exam skill is to link each feature to a faster rate of diffusion rather than simply naming the structure.

Human respiratory structures and the pathway of air

Air breathed in passes through the nose, where it is warmed, moistened and filtered, then down the trachea (windpipe). The trachea is held open by rings of cartilage and is lined with cilia and mucus-producing cells that trap dust and sweep it upward, away from the lungs.

The trachea divides into two bronchi, one to each lung.

Each bronchus branches repeatedly into narrower bronchioles, which end in clusters of millions of tiny air sacs called alveoli. Gas exchange between the air and the blood takes place across the alveolus walls, so the alveoli are the functional end of the whole pathway.

Being able to state the pathway in order, nose → trachea → bronchi → bronchioles → alveoli, is needed both for labelling a diagram and for explaining how air reaches the exchange surface. A full answer also notes the job done at each point, such as warming and filtering in the nose and trachea.

Mechanism of breathing (Content Standard 8.2)

Breathing moves air in and out of the lungs by changing the volume and therefore the pressure of the thoracic cavity. During inhalation the external intercostal muscles contract to raise the ribcage up and out, and the diaphragm contracts and flattens.

Both actions increase the volume of the thorax, which lowers the air pressure inside it below atmospheric pressure, so air flows in from the higher-pressure atmosphere.

During quiet exhalation these muscles relax: the ribcage moves down and in and the diaphragm domes upward, so the volume of the thorax decreases and the pressure rises above atmospheric, pushing air out. In forced exhalation the internal intercostal muscles contract to pull the ribcage down more strongly.

The lungs themselves contain no muscle; they are passive and are moved by these pressure changes.

The table below summarises the muscle, volume and pressure changes, and a precise answer always gives the correct muscle name together with the direction of each change.

Gaseous exchange at the alveolus (Content Standard 8.3)

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.

Both gases move down their own concentration gradient, so no energy is used; gas exchange is diffusion, not pumping.

The alveolus is adapted in four ways that all speed up diffusion. Its wall is only one cell thick, giving a short diffusion distance.

Millions of alveoli together give the lungs a very large total surface area. The inner lining is moist, so gases dissolve before they cross the membrane.

A dense network of capillaries keeps carrying the gases away and bringing fresh blood, maintaining a steep concentration gradient.

A full-mark answer states each adaptation and links it to the rate of diffusion, for example 'a wall one cell thick gives a short diffusion distance, so gases cross quickly.' Naming the adaptation without the effect gains only part of the marks.

Control of breathing rate and the effect of exercise

Breathing rate is not fixed, and it is controlled mainly by the concentration of carbon dioxide in the blood rather than by the oxygen level. Chemoreceptors detect a rise in carbon dioxide and send signals to the medulla oblongata in the brain, which sends nerve impulses to the intercostal muscles and diaphragm to increase the rate and depth of breathing until the carbon dioxide concentration returns to normal.

During exercise the 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 through the chemoreceptors 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.

When exercise stops, breathing gradually returns to its resting level as the carbon dioxide concentration falls.

Understanding this as a feedback loop, rather than memorising it as an isolated fact, lets a student answer 'explain' questions in full. The controlling factor to name is carbon dioxide concentration, and the control centre to name is the medulla oblongata.

Health issues related to the respiratory system (Content Standard 8.4)

Cigarette smoke contains tar and other chemicals that paralyse and eventually destroy the cilia lining the airways. Once the cilia no longer sweep mucus and trapped particles away, mucus and infection build up, leading to chronic bronchitis.

The same chemicals can break down the walls between alveoli, causing emphysema, which merges small alveoli into larger spaces and reduces the surface area available for gas exchange, so the person becomes breathless. Other chemicals in the smoke can trigger the uncontrolled cell division that leads to lung cancer.

Smoking is not the only cause of respiratory problems. Asthma narrows the airways through inflammation and excess mucus, making breathing difficult; tuberculosis is a bacterial infection that damages lung tissue; and air pollution or occupational dust can cause or worsen similar damage even in a person who has never smoked.

In the exam, a strong answer links each disease to the specific structure that is damaged: the cilia and airways in bronchitis, the alveolus walls in emphysema, and the lung cells in cancer. Much of this damage builds up gradually over years and cannot be fully reversed, which is why clean air and a smoke-free lifestyle are emphasised.

Key concepts to master

• 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.
• Common respiratory health problems, 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.

Quick recall checklist

1. Can you define and explain Human respiratory structures?
2. Can you define and explain Mechanism of breathing?
3. Can you define and explain Gaseous exchange?
4. Can you define and explain Alveolus adaptations?
5. Can you define and explain Respiratory systems in animals?
6. Can you define and explain Health issues?
7. Can you define and explain Control of breathing rate?
8. Can you define and explain Effect of exercise on breathing?
9. Can you define and explain Common respiratory health problems?

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