IGCSE Biology

Cambridge IGCSE™

Biology STUDENT’S BOOK

Mike Smith, Sue Kearsey, Jackie Clegg, Gareth Price

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Our lungs are the organs that allow the body to take in oxygen from the air and expel carbon dioxide that is produced in cells. We breathe in and out about 500 ml of air during every breath.

Oxygen from this air passes into tiny air sacs in the lungs, which are called alveoli, and diffuses into the capillaries that surround them.

From here, the oxygen-rich blood is passed to the heart, where it is pumped around the rest of the body, before being passed back to the lungs to offload the carbon dioxide it has collected and pick up a fresh supply of oxygen.

STARTING POINTS 1. What is gas exchange and where does it happen in humans?

2. What happens when we breathe?

3. How is breathing affected by physical activity, and why?

SYLLABUS SECTIONS COVERED 11.1 Gas exchange in humans

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11

Gas exchange in humans

∆ Gas exchange takes place in the alveoli, located in the lungs.

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Gas exchange in humans INTRODUCTION

Respiration uses oxygen from the air and produces carbon dioxide that is returned to the environment. These gases must get into and out of the body fast enough to support the rate at which respiration needs to work.

For single-celled organisms this isn’t a problem. They have a large surface area to volume ratio, and diffusion across the cell membrane can supply and remove the gases ∆ Fig. 11.1 The lungs are the site of gas exchange in humans. at a fast enough rate. Larger organisms cannot do this. Not only do they have a much smaller external surface area to volume ratio, which slows the rate of diffusion, but many of them also live on land, where the delicate surface required for gas exchange would dry out if it was directly exposed to the external environment. Different groups of organisms have different solutions to these problems but all involve structures with a large surface area. Plants exchange gases inside the leaf; insects have internal tubes (a tracheal system) inside the body where they exchange gases; fish have gills; and many vertebrates, including humans, have lungs.

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KNOWLEDGE CHECK ✓ Animals use oxygen from the air inspired and give out the carbon dioxide they produce in the air they expire.

Humans use lungs for breathing.

LEARNING OBJECTIVES ✓ Describe the features of gas exchange surfaces in humans, limited to: large surface area, thin surface, good blood supply and good ventilation with air. ✓ Identify in diagrams and images the following parts of the breathing system: lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries. ✓ SUPPLEMENT Identify in diagrams and images the internal and external intercostal muscles. ✓ SUPPLEMENT State the function of cartilage in the trachea. ✓ SUPPLEMENT Explain the role of the ribs, the internal and external intercostal muscles and the diaphragm in producing volume and pressure changes in the thorax leading to the ventilation of the lungs. ✓ Investigate the differences in composition between inspired and expired air using limewater as a test for carbon dioxide. ✓ Describe the differences in composition between inspired and expired air, limited to: oxygen, carbon dioxide and water vapour.

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✓ SUPPLEMENT Explain the differences in composition between inspired and expired air. ✓ Investigate and describe the effects of physical activity on the rate and depth of breathing. ✓ SUPPLEMENT Explain the link between physical activity and the rate and depth of breathing in terms of: an increased carbon dioxide concentration in the blood, which is detected by the brain, leading to an increased rate and greater depth of breathing. ✓ SUPPLEMENT Explain the role of goblet cells, mucus and ciliated cells in protecting the breathing system from pathogens and particles.

Gas exchange Animals need to exchange gases with the environment, to supply oxygen for respiration in cells and to remove the waste product of respiration – carbon dioxide. These gases are exchanged at surfaces by diffusion. So gas exchange surfaces, such as in the human lungs, need adaptations to maximise the rate at which diffusion occurs.

An effective gas exchange surface has: • a large surface area • a short distance over which substances have to diffuse, so cells across which diffusion occurs are usually thin • a good blood supply and good ventilation to deliver oxygen and remove carbon dioxide from the body rapidly – this maintains high concentration gradients for both gases.

The human breathing system Breathing is the way that oxygen is taken into our bodies and carbon dioxide is removed. When we breathe, air is moved into and out of our lungs. This involves different parts of the breathing system within the chest.

When we breathe in, air enters though the nose and mouth. In the nose the air is moistened and warmed. The air passes over the larynx, where it may be used to make sounds, for example when we talk. The air travels down the trachea to the lungs. The air enters the lungs through the bronchi (singular: bronchus), which branch and divide to form a network of bronchioles.

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Bands of cartilage surround the trachea and bronchi. These support the tubes and keep them open during breathing, as otherwise they might collapse when air pressure inside them is reduced.

Cells called goblet cells in the lining of the trachea, bronchi and bronchioles secrete mucus, which is a slimy liquid. This traps microorganisms and dust particles that are breathed in. The lining of the trachea and bronchi are covered in tiny hairs called cilia, which are found on the surface of ciliated cells. The cilia sweep in a combined motion to move the mucus up from the lungs, up the trachea to the back of the mouth, where it can be swallowed and pass to the stomach.

The combined action of mucus and cilia helps to prevent dirt and pathogens entering the lungs and causing damage and infection.

GAS EXCHANGE IN HUMANS

SUPPLEMENT

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cilia goblet cells

∆ Fig. 11.2 Section through the trachea lining, showing goblet cells, which secrete mucus, and cilia, which sweep the mucus along the surface.

At the end of the bronchioles are air sacs. The bulges on an air sac are called alveoli (singular: alveolus). The alveoli are covered in tiny blood capillaries. This is where oxygen and carbon dioxide are exchanged between the blood and the air in the lungs. This is called gas exchange. The movement of air across the alveolar surface is called ventilation.

air sacs and their blood supply larynx

bronchiole (air in and out)

blood away from air sac

trachea

blood to air sac

lung

alveolus

blood capillary

bronchus

intercostal muscle bronchiole

heart

SUPPLEMENT internal intercostal muscle SUPPLEMENT external intercostal muscle

rib diaphragm

air sac with alveoli

∆ Fig. 11.3 The human breathing system.

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rib

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The alveoli are where oxygen and carbon dioxide diffuse into and out of the blood. For this reason the alveoli are described as the gas exchange surface.

The alveoli are adapted for efficiency in exchanging gases by diffusion.

They have: • thin permeable walls, which keep the distance over which diffusion of gases takes place between the air and blood to a minimum • a moist lining, in which the gases dissolve before they diffuse across the cell membranes • a large surface area – there are hundreds of millions of alveoli in a human lung, giving a surface area of around 70 m2 for diffusion • high concentration gradients for the gases, because the blood is continually flowing past the air sacs, delivering excess carbon dioxide and taking on additional oxygen, and because of ventilation of the lungs, which refreshes the air in the air sacs. blood entering (low O2 concentration, high CO2 concentration)

air in and out

alveolar wall

moist mucus lining high O2

capillary

lo

w

O

low CO2

blood leaving (high O2 concentration, low CO2 concentration)

2

hig h C

O2

∆ Fig. 11.4 Gas exchange in an air-filled alveolus.

REMEMBER

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Be careful how you describe the process of gas exchange between the air in the lungs and the blood. Remember that diffusion is a passive process that only occurs while there is a concentration gradient. Also, do not make the mistake of stating that all the oxygen in the air we breathe in is replaced by carbon dioxide in the air we breathe out.

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QUESTIONS 1. Explain as fully as you can how the lungs are adapted for a rapid rate of diffusion for gas exchange.

2. Give the structures of the human breathing system and, for each structure, explain its role in breathing.

3. Sketch a diagram of an alveolus and annotate it to show how it is adapted for efficient gas exchange. (Hint: remember to refer to diffusion.)

4. SUPPLEMENT Explain the roles of the cilia and mucus in the human breathing system.

Breathing in and out Breathing in is known as inhalation or inspiration, and breathing out as exhalation or expiration. Both happen because of changes in the volume of the chest cavity. These changes are caused by movements of: • the ribs which are joined together by intercostal muscles • the diaphragm which is a domed sheet of muscle below the lungs.

SUPPLEMENT

Breathing in Air is breathed into the lungs in a large breath as follows.

1. The diaphragm muscle contracts so it flattens in shape.

2. The external intercostal muscles contract, making the ribs move upwards and outwards.

3. These changes cause the volume of the thorax to increase.

4. This causes the air pressure in the thorax to decrease.

5. This decrease in pressure causes air from outside to enter the lungs.

Rings of cartilage in the trachea and bronchi keep the air passages open and prevent them from collapsing when the air pressure decreases and from bursting when air pressure increases.

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The change in volume of the chest cavity or thorax causes pressure changes, which in turn cause air to enter or leave the lungs. There are two sets of intercostals: the internal intercostal muscles and the external intercostal muscles. The ribs, intercostal muscles and diaphragm work together to bring about breathing or ventilation of the lungs.

In gentle breathing, only the diaphragm may be involved. In deeper breathing, such as during exercise, or if we think about it, the ribs and intercostal muscles become involved.

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air enters

air leaves ribcage moves up and out

ribcage moves down and in

lungs get smaller

lungs expand

Inhalation

diaphragm moves down

Exhalation

diaphragm moves up

∆ Fig. 11.5 Left: breathing in. Right: breathing out.

Breathing out Air is breathed out from the lungs as follows.

1. The diaphragm muscle relaxes and returns to its domed shape, pushed up by the liver and stomach. This means it pushes up on the lungs.

2. The external intercostal muscles relax, allowing the ribs to drop back down. This also presses on the lungs. If you are breathing hard the internal intercostal muscles also contract, helping the ribs to move down.

3. These changes cause the volume of the thorax to decrease.

4. This causes the air pressure in the thorax to increase.

5. This causes air to leave the lungs.

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Explain breathing in terms of comparing the pressure inside the lungs and external air pressure. Air moves from an area of higher pressure to an area of lower pressure.

• During inhalation, air enters because the air pressure inside the lungs is lower than the air pressure outside the body.

• During exhalation, air leaves the lungs because the air pressure inside is higher than the air pressure outside the body.

GAS EXCHANGE IN HUMANS

REMEMBER

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● SCIENCE IN CONTEXT

VENTILATION BY MACHINE

Sometimes accident or illness can damage a person’s ability to breathe.

As exchange of gases is essential for respiration, and so for life, it is crucial that this process is continued artificially until the patient is sufficiently recovered to be able to do it independently again.

In the past, the patient was sometimes placed inside a large machine, called an iron lung. The machine was sealed from the air, and changes in pressure inside the machine caused changes in thoracic volume, which resulted in air being drawn in or forced out of the patient’s lungs.

Today, a sealed mask is placed over the patient’s mouth and nose, and air is forced into the lungs by increasing the air pressure. The air is naturally breathed out again as the stretched muscles relax.

∆ Fig. 11.6 A ventilator mask forces air into the patient’s lungs by increasing air pressure, and allows air out of the patient’s lungs by decreasing the air pressure in the mask.

Challenge Question: Mountain climbers at very high altitudes may also need to use ventilator masks. Suggest why it can be difficult to breathe normally at very high altitudes.

SUPPLEMENT

QUESTIONS 1. Describe what happens during inhalation in terms of a) movement of structures in the breathing system and b) pressure and volume changes in the thorax.

2. Describe what happens during exhalation in terms of a) movement of structures in the breathing system and

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b) pressure and volume changes in the thorax.

Inspired air and expired air The air we breathe in and out contains many gases. Oxygen is taken into the blood from the air we breathe in. Carbon dioxide and water vapour are added to the air we breathe out. The other gases in the air we breathe in are breathed out almost unchanged, except for being warmer. oxygen carbon dioxide water

In inspired air 21% 0.04% variable

In expired air 16% 4.5% high

∆ Table 11.1 Differences in composition of inspired and expired air.

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We can compare the carbon dioxide in inspired air and expired air using the apparatus shown in Fig. 11.7. Limewater reacts with carbon dioxide and turns cloudy, so this is a test for carbon dioxide. breathe in and out through this tube

inspired air can only enter through this tube

expired air can only leave through this tube

∆ Fig. 11.7 Limewater in the tubes show that expired air contains much more carbon dioxide than inspired air.

SUPPLEMENT

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Investigating the effects of exercise on breathing There are two aspects of breathing that can change during exercise – the rate of breathing and the depth of breathing (the volume of a breath).

• Rate of breathing is usually counted as number of breaths per minute.

• The volume of a breath can be measured in dm3 using a spirometer. A simple spirometer can be made using a 2-litre plastic bottle that has been marked down the side with volumes of water. (This can be done by adding 500 cm3 of water at a time, and marking the volume on the side of the bottle with a waterproof marker.) When the bottle is full of water, turn it upside-down into a water trough without allowing

GAS EXCHANGE IN HUMANS

The composition of inspired and expired air changes because: • oxygen is removed from blood by respiring cells and used for respiration (see Section 12), so blood returning to the lungs has a lower concentration of oxygen than blood leaving the lungs • carbon dioxide is produced by respiration (see Section 12) and diffuses into the blood from respiring cells; the blood transports the carbon dioxide to the lungs, where it diffuses into the alveoli • water vapour concentration increases because water evaporates from the moist linings of the alveoli into the expired air as a result of the warmth of the body.

Other gases in the air we breathe in and out remain unaffected because they are not used or produced by the body.

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