Mechanisms of breathing 1

Mechanisms of breathing

What you will learn about in this topic:

1. The function of the respiratory system

2. The mechanisms of breathing

3. The role of the lungs

4. Alveoli

5. Composition of air inhaled and exhaled

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Mechanisms of breathing 3

Learning objectives

By the end of this presentation you should be able to:

• Understand the function of the respiratory system

• Describe the effects of exercise on breathing

• Explain what happens when you breathe

The function of the respiratory system

The function of the respiratory system is to get oxygen into the body and carbon dioxide and waste products from metabolism out of the body.

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This happens through the act of breathing. Breathing in (inhalation) gets the oxygen in, so it can be used by the body to release energy.

Breathing out (exhalation) removes the carbon dioxide so it does not build up and poison the body.

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The following are all parts of the respiratory system:

• Air passages

• Lungs

• Diaphragm

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Composition of inspired and expired air

The air we breathe in is very different from the air we breathe out.

The parts that make up inhaled and exhaled air are called its composition.

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Percentage of air inhaled:

Key■ Nitrogen■ Oxygen■ Carbon dioxide■ Other

0.5% 0.5%

20.0%

79.0%

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Percentage of air exhaled:

Key■ Nitrogen■ Oxygen■ Carbon dioxide■ Other

4.0%1.0%

16.0%

79.0%

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The link between respiration and circulation

Oxygen goes from the respiratory system to the circulatory system and then back to the respiratory system.

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Oxygen breathed in goes through the mouth or nose, down the trachea, into the lungs and into the alveoli.

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Oxygen then passes through the alveoli walls into the red blood cells, via the capillaries.

Oxygen joins with haemoglobin to make oxyhaemoglobin.

The oxyhaemoglobin is used by the working body and is transported by the circulatory system to cells needing to release energy.

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Carbon dioxide is produced as a waste product. This is converted into a gas and passes back through the alveoli walls, via the capillaries into the blood plasma.

The carbon dioxide passes through the capillary and alveoli walls into the alveoli. It is then exhaled from the body.

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Parts of the respiratory system

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Expiration

The mechanism of breathing

Lungs are not muscle, cannot move of their own accord and are not controlled by the central nervous system.

The key to breathing is the diaphragm and the intercostal muscles between the ribs.

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When we breathe in (inspiration) the following happens:

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• The diaphragm pulls down

• The intercostal muscles contract

• Air pressure is reduced

• Air is sucked through the tubes into the lungs

• The chest expands.

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When we breathe out (expiration) the opposite happens:

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• The diaphragm relaxes into its dome position

• The intercostal muscles relax

• The chest becomes smaller

• Pressure increases in the lungs

• Air is forced out

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Task 1

In groups of four, write 13 statements on 13 pieces of card (one on each card) about the route of oxygen and carbon dioxide around the body.

Put your cards in order, starting and ending with a card marked ‘Oxygen is inhaled’.

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Possible answers to Task 1

• Oxygen passes through the larynx and trachea.

• Oxygen arrives in the lungs via the bronchi and bronchioles.

• Oxygen passes to the alveoli and transfers to the haemoglobin through the plural membrane

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• The haemoglobin in the red blood cells binds with the oxygen to become oxyhaemoglobin.

• The blood travels through the pulmonary vein to the left side of the heart.

• The heart pumps the blood through the aorta to the arteries.

• The arteries carry the oxygenated blood around the body.

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• The haemoglobin releases the oxygen at the muscles and collects the carbon dioxide; the blood is now considered deoxygenated.

• The veins carry the deoxygenated blood.

• The deoxygenated blood arrives at the heart.

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• The heart pumps the deoxygenated blood back to the lungs via the pulmonary artery.

• The carbon dioxide passes across the pleural membrane and into the alveoli.

• The carbon dioxide passes through the trachea and is exhaled.

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The role of the lungs

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The lungs are positioned inside the chest cavity and are protected by the ribcage.

The action of breathing means that the lungs are constantly moving in and out. To protect them from any friction due to the movement, the pleural membrane forms a complete lining around them.

The pleural membrane is smooth and has a moist, slimy mucus.

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Alveoli

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Alveoli are tiny air sacs at the end of the bronchioles.

There are millions of them that allow gases to exchange inside them.

Oxygen passes across the pleural membrane and into the blood. Carbon dioxide passes from blood to the alveoli.

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Here is a simplified version of what happens:

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• Oxygen from the air breathed in enters the circulatory system to be used by the working muscles.

• Carbon dioxide, which is toxic to the system, transfers from the used blood, out of the circulatory system, back into the alveoli, to be breathed out along with oxygen, water and nitrogen.

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• The capillaries, covering the surface of the alveoli, link the respiratory system with the circulatory system.

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• Regular exercise conditions the lungs to excrete more of the carbon dioxide from the body.

Tidal volume is the amount of air breathed in and out of the body during normal breathing.

During exercise, the volume is forced to change. It is then called forced breathing.

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Vital capacity is the largest amount or volume of air that can be exhaled after the largest possible inhalation.

Residual volume is the amount of air that, even after as much air as possible has been exhaled, is left in the lungs.

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Task 2

In groups of four, describe the action of breathing using the following words to help you:

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• Nose• Mouth• Trachea• Lungs

• Ribs• Intercostal

muscles• Diaphragm

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Possible answers to Task 2

From the nose or mouth, air enters the trachea and moves towards the lungs.

The trachea divides into two branches called the bronchi. These sub-divide into smaller tubes called bronchioles.

During heavy exercise the body uses the intercostal muscles between the ribs to aid the diaphragm in the exhalation of air from the lungs.

At the end of the bronchioles are alveoli.

Alveoli are air sacs with many tiny blood vessels called capillaries running from them.

Exam questions

1. Explain the process of inspiration.

2. Explain the process of expiration.

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What you have learnt about in this topic:

1. The function of the respiratory system

2. The mechanisms of breathing

3. The role of the lungs

4. Alveoli

5. Composition of air inhaled and exhaled

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Learning objectives

You should now be able to:

• Understand the function of the respiratory system

• Describe the effects of exercise on breathing

• Explain what happens when you breathe